National Water-Quality Program

National Water-Quality Assessment Project

U.S. Department of the Interior

U.S. Geological Survey

Circular 1433

Agriculture—A River Runs Through It—The Connections 

Between Agriculture and Water Quality

The Quality of Our Nation’s Waters
Cover: A conservation buffer along a stream, Iowa. Photograph taken by Lynn Betts, 

U.S. Department of Agriculture, Natural Resources Conservation Service photo gallery, 

NRCSIA99058, 1999.
The Quality of Our Nation’s Waters

Agriculture—A River Runs Through It—

The Connections Between Agriculture and Water Quality

By Paul D. Capel, Kathleen A. McCarthy, Richard H. Coupe, Katia M. Grey, 

Sheila E. Amenumey, Nancy T. Baker, and Richard L. Johnson

National Water-Quality Program

National Water-Quality Assessment Project

Circular 1433

U.S. Department of the Interior

U.S. Geological Survey
U.S. Department of the Interior

RYAN K. ZINKE, Secretary

U.S. Geological Survey

James F. Reilly II, Director

U.S. Geological Survey, Reston, Virginia: 2018

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Although this information product, for the most part, is in the public domain, it also may contain copyrighted materials 

as noted in the text. Permission to reproduce copyrighted items must be secured from the copyright owner.

Suggested citation:

Capel, P.D., McCarthy, K.A., Coupe, R.H., Grey, K.M., Amenumey, S.E., Baker, N.T., and Johnson, R.L., 2018, 

Agriculture—A River runs through it—The connections between agriculture and water quality: U.S. Geological 

Survey Circular 1433, 201 p., https://doi.org/10.3133/cir1433.

ISSN 2330-5703 (online)
iii

Foreword

Sustaining the quality of the Nation’s water resources and the health of our diverse ecosystems 

depends on the availability of sound water-resources data and information to develop effective, 

science-based policies. Effective management of water resources also brings more certainty and 

efficiency to important economic sectors. Taken together, these actions lead to immediate and longterm economic, social, and environmental benefits that make a difference to the lives of the almost 400 

million people projected to live in the United States by 2050.

In 1991, Congress established the U.S. Geological Survey National Water-Quality Assessment 

(NAWQA) to address where, when, why, and how the Nation’s water quality has changed, or is likely to 

change in the future, in response to human activities and natural factors. Since then, NAWQA has been 

a leading source of scientific data and knowledge used by national, regional, state, and local agencies 

to develop science-based policies and management strategies to improve and protect water resources 

used for drinking water, recreation, irrigation, energy development, and ecosystem needs. Plans for the 

third decade of NAWQA (2013–23) address priority water-quality issues and science needs identified 

by NAWQA stakeholders, such as the Advisory Committee on Water Information and the National 

Research Council, and are designed to meet increasing challenges related to population growth, 

increasing needs for clean water, and changing land-use and weather patterns.

This report is one of a series of publications, The Quality of Our Nation’s Waters, which describes major 

findings of the NAWQA Project on water-quality issues of regional and national concern and provides 

science-based information for assessing and managing the quality of our groundwater resources. Other 

reports in this series focus on occurrence and distribution of nutrients, pesticides, and volatile organic 

compounds in streams and groundwater, the effects of contaminants and stream-flow alteration on the 

condition of aquatic communities in streams, and on the quality of groundwater from private domestic 

and public supply wells. Each reports builds toward a more comprehensive understanding of the quality 

of regional and national water resources. All NAWQA reports are available online (https://water.usgs.

gov/nawqa/bib/).

We hope this publication will provide you with insights and information to meet your water-resource 

needs and will foster increased citizen awareness and involvement in the protection and restoration of 

our Nation’s waters. The information in this report is intended primarily for those interested or involved 

in resource management and protection, conservation, regulation, and policymaking at the regional and 

national levels.

Dr. Donald W. Cline

Associate Director for Water

U.S. Geological Survey
iv

Good soil, plentiful sunshine, and sufficient water are needed for successful crop production (Wisconsin). 

Photograph by Paul Capel, U.S. Geological Survey, 2010. 
v

Prologue – Lessons from Slugs and Beetles 

My wife is an avid backyard gardener. Her many flowers are beautiful! For the most part, she understands what her flowers need in 

order to thrive, but at times, her knowledge of the garden ecosystem becomes limiting. Last year she bought some heirloom zinnia 

seeds. She looked forward to planting them and watching them grow and bloom. After a week in the soil, they sprouted and grew a

couple of inches. One morning, the little leaves were full of smooth round holes and some of the seedlings had died. What was the 

problem? Insects? Rabbits? Was an insecticide needed? Was fencing needed?

After some investigation, it was determined that the culprits were slugs. The solution to the problem—Epsom salts scattered on the soil. 

In another part of the garden, she discovered that some of the leaves on her roses had been skeletonized—all of the green leafy

material between the veins had been eaten. Slugs again? Should she get out the Epsom salts? No, slugs generally just eat round holes 

in the leaves; they do not eat the leaves between the veins in a patterned fashion. This culprit turned out to be Japanese beetles. A 

little investigation revealed that Japanese beetles can be controlled in the short term by conventional or botanical insecticides, and, 

perhaps, in the long term by changing plants in the garden or adding a natural, disease-causing bacteria (specific to the grub stage of 

the Japanese beetle) to the soil.

What are the lessons to be learned from the slugs and beetles? First, everything is connected. Cause and effect relations are the basis 

of the changes in the world. (Slugs enjoy eating zinnia seedlings, therefore, the zinnia leaves disappear.) Second, these connections 

are often complex and not always obvious. We need to understand these connections to make informed and useful decisions. (Slugs 

generally eat round holes in leaves, whereas Japanese beetles mine the green material between the veins of the leaves.) Third, if the 

connection is unknown or misunderstood, then the attempted solution, no matter how thorough or costly, may have little or no positive 

effect. (Fencing or insecticides would not keep the slugs from eating zinnias.) 

When the cause and effect relations (scientific principles) for the observations in the garden (or in an agricultural field) are correctly 

understood, the proposed solutions will be better informed and, ultimately, more effective.

Lessons from slugs and beetles are considered in this report as to 

the connections between agricultural activities and the environment. 

Water is one of the most important and dynamic connections between 

agriculture and the broader environment, as well as a vital and highly 

valued component of our world. Water moves freely between agricultural 

and non-agricultural areas. Water is often purposely moved into or out 

of agricultural landscapes to meet the needs of agricultural production. 

It moves through a variety of natural and constructed flowpaths that 

are both seen and unseen. Chemicals and soil move with the water. The 

movement of water, chemicals, and soil out of agricultural landscapes 

causes numerous concerns and effects on people and the environment. A 

continual improvement in our understanding of the connections among the 

agricultural landscape, water flowpaths, chemical and soil movement, and 

the environment is needed in order to make informed agricultural policy and 

useful management decisions. 

Paul D. Capel

Skeletonized morning glory leaf after a visit from 

Japanese beetles (Minnesota). Photograph by Paul 

Capel, U.S. Geological Survey, 2011.

vii

Contents

Foreword........................................................................................................................................................iii

Prologue—Lessons from Slugs and Beetles ..........................................................................................v

The Agricultural Water and Chemical Use Footprint .............................................................................x

Overview.........................................................................................................................................................1

Chapter 1. NAWQA Studies on Agriculture and Water Quality .........................................................23

Chapter 2. Overview of Agriculture and Water Quality........................................................................29

Chapter 3. Changes in the Nation’s Agriculture Over Time .................................................................43

Chapter 4. Terrain, Climate, Soil, and Water..........................................................................................65

Chapter 5. Water on the Pre-Agricultural Landscape..........................................................................83

Chapter 6. Agricultural Water and Soil Management..........................................................................99

Chapter 7. Water on the Modified Agricultural Landscape...............................................................119

Chapter 8. Chemicals in Crop and Animal Agriculture.......................................................................139

Chapter 9. Connections Between Agriculture and Water Quality....................................................155

Final Thoughts............................................................................................................................................180

References Cited .......................................................................................................................................181

Glossary of Terms ......................................................................................................................................196

Glossary of Farm Implements..................................................................................................................198
viii

Conversion Factors

International System of Units to Inch/Pound

Multiply By To obtain

Length

centimeter (cm) 0.3937 inch (in.)

millimeter (mm) 0.03937 inch (in.)

meter (m) 3.281 foot (ft) 

kilometer (km) 0.6214 mile (mi)

meter (m) 1.094 yard (yd) 

Area

hectare (ha) 2.471 acre

square kilometer (km2

) 247.1 acre

hectare (ha) 0.003861 square mile (mi2

) 

square kilometer (km2

) 0.3861 square mile (mi2)

Volume

liter (L) 33.82 ounce, fluid (fl. oz)

liter (L) 0.2642 gallon (gal)

liter per day (L/d) 0.2642 gallon per day (gal/d)

cubic meter (m3

) 0.0002642 million gallons (Mgal) 

Flow rate

cubic meter per second (m3

/s) 35.31 cubic foot per second (ft3/s)

millimeter per day (mm/d) 0.03937 inch per day (in/d)

Mass

kilogram (kg) 2.205 pound avoirdupois (lb)

kilogram per hectare (kg/ha) 0.8922 pound per acre (lb/acre)

kilogram per square kilometer (kg/m2

) 5.7099 pound per square mile (lb/mi2)

megagram (Mg) 1.102 ton, short (2,000 lb)

megagram (Mg) 0.9842 ton, long (2,240 lb)

metric ton per year 1.102 ton per year (ton/yr) 

metric ton 1.102 ton 

nanogram (ng) 3.527 ounce (oz.)

Energy

joule (J) 0.0000002 kilowatt hour (kWh)

Application rate

kilograms per hectare per year 

[(kg/ha)/yr]

0.8921 pounds per acre per year 

[(lb/acre)/yr]
ix

Conversion Factors—Continued

Inch/Pound to International System of Units

Multiply By To obtain

Length

inch (in.) 2.54 centimeter (cm)

foot (ft) 0.3048 meter (m)

mile (mi) 1.609 kilometer (km)

Area

acre 4,047 square meter (m2

)

acre 0.4047 hectare (ha)

acre 0.4047 square hectometer (hm2

) 

acre 0.004047 square kilometer (km2

)

Mass

ton, short (2,000 lb) 0.9072 megagram (Mg) 

Temperature in degrees Fahrenheit (°F) may be converted to degrees Celsius (°C) as 

°C = (°F – 32) / 1.8.

Concentrations of chemical constituents in water are given in either milligrams per liter (mg/L) 

or micrograms per liter (μg/L).

Datum

Horizontal coordinate information is referenced to the North American Datum of 1983 (NAD 83).
x

The Agricultural Water and Chemical Use Footprint

Even though greater than 80 percent of the Nation’s population 

lives in urban areas, everyone has a connection to agriculture. 

Agricultural production in the United States supplies a major 

portion of the Nation’s food, feed, and fiber needs.

Over the last 100 years, agricultural expansion and 

intensification has led to adverse effects, including changes 

in water quantity and quality and the health of stream 

ecosystems. Agricultural chemicals move into and through 

every component of the hydrologic system, including air, soil, 

soil water, streams, wetlands, and groundwater. 

A holistic understanding of water movement coupled with an 

understanding of how different chemicals behave in different 

hydrologic settings will help identify effective conservation 

actions and inform the development of agricultural 

management policies.

Cropland Areas Across the Nation

tac11-0566_Overview_fig01_cropland map

tac11-0566_figOverview3-1

The hydrology in many of our agricultural lands has been altered 

to maximize crop yields. About 490 billion liters of water—almost 

two-thirds of the total water use in the Nation—is used each day to 

irrigate crops. Long-term irrigation for crops can cause significant 

declines in groundwater levels. Artificial drainage—surface ditches 

and subsurface drains—has been installed on about 25 percent 

of the cropland. Water can move quickly through these drains and 

transport agricultural chemicals from the fields to streams.

0

3

6

9

12

1930 1950 1970 1990 2010

Nitrogen fertilizer applied, 

in billions of kilograms as N 

Total pesticides applied, 

in millions of kilograms

0

400

300

200

100

1930 1950 1970 1990 2010

Nitrogen fertilizer use Pesticide use

Considerable increases in fertilizer and pesticide use began in the 1960s. In 2010, about 11 billion kilograms of nitrogen fertilizer and 

300 million kilograms of pesticides were used annually to enhance crop production or control pests. Increased levels of nutrients from 

fertilizers draining into streams can stimulate algal blooms and affect stream health and recreational uses of local streams, downstream 

reservoirs, and estuaries, and increase treatment costs for drinking water. Pesticides that are transported to streams can pose risks for 

aquatic life and fish-eating wildlife and drinking-water supplies.
Agriculture—A River Runs Through It—

The Connections Between Agriculture and Water Quality

By Paul D. Capel1, Kathleen A. McCarthy1, Richard H. Coupe1, Katia M. Grey2, Sheila E. Amenumey2, 

Nancy T. Baker1

, and Richard L. Johnson3

Overview

Over the last 100 years, expansion of agricultural lands, 

modification of the landscape, and technological advances 

in mechanization, chemical use, conventional breeding and, 

more recently, genetic engineering have resulted in increased 

agricultural production in the United States, which supplies 

a major portion of the Nation’s food, feed, and fiber. Today, 

more than 11 billion kilograms of nitrogen fertilizer are applied 

annually to crops and 490 billion liters of water are withdrawn 

annually to irrigate crops in the United States. Typically, 5 to 

50 percent of applied nitrogen moves from fields to streams 

through runoff and through groundwater discharge. The 

cumulative effects of agricultural expansion and intensification 

have led to adverse effects, including changes in water quantity,

water quality, and the health of ecosystems.

Agricultural effects on water quality can occur at local, 

regional, and national scales. For example, increased levels 

Understanding the movement of water—

amounts, timing, and pathways— is fundamental 

for making optimal agricultural management 

and policy decisions to minimize the impacts of 

agriculture on water quality.

of nutrients from agricultural fertilizers can stimulate algal 

blooms and affect the ecology of local streams. Nitrate and 

some herbicides can move through the soil to groundwater 

and, eventually, to local streams. Farther downstream, these 

elevated nutrients can increase costs associated with treating 

the water so that it is suitable for drinking. Ultimately, 

chemicals associated with agricultural activities (such as 

nutrients, pesticides, antimicrobials, and trace elements) and 

sediment (eroded soil) empty into our estuaries and can harm 

valuable commercial and recreational fisheries. Elevated 

nutrient inputs stimulate harmful algal blooms along the 

Nation’s coasts causing negative economic impacts.

A B

In Wisconsin (A), fields are 

contoured and the crops 

are planted in strips to help 

decrease the movement of water 

and sediment (eroded soil) from 

the fields to the steam. 

In Kansas (B), where there is not 

enough natural rainfall for most 

crops, the green circles show 

the widespread prevalence of 

center pivot irrigation used to 

enhance agricultural production. 

Groundwater levels in the High 

Plains aquifer beneath Kansas 

and many other areas of the 

Nation are declining due to 

withdrawals for irrigation.

1

U.S. Geological Survey.

2

University of Minnesota.

3

Oregon Health and Science University.
2 Agriculture—A River Runs Through It—The Connections Between Agriculture and Water Quality

The movement (flowpaths) of water is the most important 

connection between agricultural activities and impacts on the 

quality of groundwater, streams, rivers, estuaries, and oceans. 

Understanding the movement of water—amounts, timing, and 

pathways—is fundamental for making optimal agricultural 

management and policy decisions toward minimizing the 

impacts of agriculture on water quality. 

The U.S. Geological Survey conducted a systematic 

national study in agricultural areas to improve our 

understanding of field-scale and watershed-scale hydrology 

and the environmental behavior of individual chemicals. The 

goals of the study were to understand the connections among 

agricultural activities, hydrology, and chemical transport—

critical information needed to inform management decisions 

and to help set the expectations for the protection and 

improvement of water quality, which can result from changes 

in agricultural management and policy decisions. This study 

was conducted in seven, hydrologically diverse, agricultural 

areas across the Nation (California, Indiana, Iowa, Maryland, 

Mississippi, Nebraska, and Washington).

Decades ago, the landscapes that were naturally too wet for agriculture were drained in the 

upper Midwest. The landscape was dotted with wetlands and shallow water tables (top left). 

These areas were drained by digging surface ditches. From Popular Mechanics, March 1922: 

Big Ditcher Quickly Turns Swamps into Farms “Working steadily with a crew of five men, the 

machine ... can drive a ditch ahead at the rate of about a quarter mile in a working day. .... Work 

is done on a big scale, dozens of teams, workmen, and tractors being employed. ... in short time 

the marshlands have been changed into highly productive farms.” New technologies, such as this 

76-ton monster wheel excavator (bottom), together with government support for the conversion 

of wetlands to farmland, opened up vast expanses of fertile land, particularly in the Midwest. 

(Windsor, 1922). Intensive row crop and animal agriculture now covers the landscape (top right).
Overview 3

Agricultural Landscape Modifications Over the Last Century Have Significantly Altered the Natural 

Flow of Water and Agricultural Chemicals Entering Streams and Aquifers

activities, such as irrigation and (or) drainage, can change 

the availability of water to crops, and the addition of manure 

and other sources of nutrients can increase soil fertility. 

Each agricultural activity and landscape modification has 

an effect on the movement of water and, ultimately, the 

transport of agricultural chemicals and sediment to the 

broader environment. The combined effects of all activities 

and modifications defines the hydrology of the agricultural 

landscape, and thereby, determines the amount, timing, and 

specific flowpaths of water, chemicals, and sediment moving 

through and out of the agricultural landscape.

Understanding how agricultural chemicals move and 

are transformed in each of the components of the hydrologic 

system can provide key insights into which management 

actions might be most effective in reducing transport of 

chemicals off the farm. Agricultural chemicals move into 

and through the various components of the hydrologic 

system, including air, soil, soil water, streams, wetlands, and 

groundwater. These chemicals also can be transformed by 

biological and chemical processes to other chemicals. 

The landscape has been modified to allow crops to be 

grown and thrive in many areas that are naturally too wet, 

too dry, or have low soil fertility for crops. Agricultural 

Corn moldboard 

till

Corn conservation 

till

Soybean no till

Soybean 

chisel till

Center pivot irrigation

Soybean moldboard till

Corn no till

Corn chisel 

till

Corn no till

Corn moldboard till

Corn chisel till Corn moldboard till

Corn moldboard till

Soybean conservation till

Corn chisel 

till Corn no till

CAFO

Pasture Pasture

EXPLANATION

Subsurface drain outfall

Surface drain outfall

Vertical surface drain

Subsurface drain

Drainage channel

Drains

Pasture

Center pivot irrigation

Building

Buffer strip

Bridge

 Base image from National 

Agriculture Imagery 

Program (NAIP), 2009.

Lagoon

0 200 400 600 800 1,000 Feet

0 100 200 300 Meters

N

1 

10 

100 

1,000 

10,000 

100,000 

1,000 10,000 100,000 

Nitrogen outputs from agricultural areas 

through streams, in kilograms per

 square kilometer per year

Nitrogen inputs to agricultural areas, in kilograms per square kilometer per year

Subsurface drainage present at >5 percent of the landscape 

Subsurface drainage present at <5 percent of the landscape 

EXPLANATION

50 percent

5 percent of 

nitrogen lost to 

stream

Parts of north-central Iowa have been extensively modified to accommodate crop and animal agriculture including a network of surface 

and subsurface drainage. The combined effects of these modifications define the hydrology of the agricultural landscape. This area, 

and other agricultural areas with substantial subsurface drainage in their watersheds, has some of the highest stream concentrations 

of nitrate in the Nation. Nitrogen, an essential plant nutrient, is applied to agricultural fields in the forms of chemical fertilizers and 

animal manure. Nitrogen exports in areas with subsurface drained watersheds are slightly more than three times larger than in other 

agricultural streams (graph modified from Dubrovsky and others, 2010).
4 Agriculture—A River Runs Through It—The Connections Between Agriculture and Water Quality

Agricultural activities have modified the pathways and amounts of water and chemicals 

moving through the landscape

Pre-agricultural landscape

Too little soil water

Too much soil water

Mississippi

Iowa

Indiana

Maryland

Nebraska

California

Washington

Wetland forests covered much of the flat landscape. 

Levees were built to stop the annual flooding from the 

Mississippi River, forests were cleared, and the land was 

drained. Streams were straightened and ditches were 

added to move water quickly and efficiently off the 

landscape. Currently, cotton, rice, corn, and soybean 

fields and catfish ponds cover the landscape.

 

The topography, soils, and climate of the Delmarva 

Peninsula make it well suited to agriculture. Forests 

were cleared mid-17th century to make way for crops 

and animals—currently, corn, soybean, dairy, and 

poultry. Forested buffers along streams and scattered 

wetlands and farm ponds are common in the area.

The landscape receives about 18 centimeters (cm) of 

precipitation each year. Prior to irrigation, the land was 

covered with small desert shrubs. Sunnyside Canal, 

completed in 1980, delivers snowmelt water from the 

Cascade Mountains (110 cm) for irrigation to support 

dairies and crops (alfalfa, hops, vegetables, orchards). 

Areas included in this study are shown on the continuum of too much to too little soil water. 
Overview 5

Most historical and current water-quantity and water-quality impacts from 

agriculture are the result of the modification of the natural water flowpaths 

and (or) the use of chemicals.

Agricultural landscape Water-quality effects

Groundwater pumping for irrigation has 

lowered the water table and dried up some 

rivers. Sediment, nutrients, and pesticides are 

exported from agricultural fields to the 

Mississippi River.

Excess nitrogen from fertilizer and manure 

has contaminated the groundwater and 

affected drinking water. The contaminated 

groundwater seeps into local streams and 

rivers and contributes to the eutrophication of 

Chesapeake Bay.

Prior to the 1990s, excess irrigation runoff transported large amounts of sediment and nutrients to streams. 

In the 1990s, changes in irrigation methods reduced the amount of runoff, decreasing the amount of sediment 

transported to streams. As the streams became less turbid, increased light penetration stimulated excessive 

aquatic plant growth in the clear, nutrient-rich stream water.
6 Agriculture—A River Runs Through It—The Connections Between Agriculture and Water Quality

Artificial Drainage—For where there is too much water

About 25 percent of the cropland in the country has artificial drainage—surface (ditches) and subsurface (tiles). Prior to 

European settlement, there were about 1 million square kilometers (km2) of wetlands in the conterminous United States. By 

the mid-1980s, less than half of this wetland area remained; most of the drained areas have been converted to agricultural use. 

Generally, the drained water is moved quickly—hours to weeks—through the surface layers of soil, into the local drainage 

network, and out to the stream. Artificial drainage quickly moves water out of the soil to protect plant roots from excess water 

and to allow farm machinery to operate in the fields. Agricultural chemicals, pesticides, and nitrate can be transported with the 

drained water. Nitrogen export in streams with substantial subsurface drainage in their watersheds is slightly more than three 

times larger than in other agricultural streams in the Nation. 

Artificial Drainage and Cropland 

EXPLANATION

Cropland on soils that

need subsurface drainage

Cropland on soils that 

need surface drainage

Cropland without artificial drainage

Study locations

Surface and subsurface drainage 

areas have turned wetlands into 

productive cropland, particularly 

along the East coast in Maryland, 

North Carolina, South Carolina, 

in the Midwest from Ohio to 

Minnesota, and in California.

Temporary ponds form on the landscape, like this field in Iowa, when rainfall intensity exceeds the soil’s ability to infiltrate water. 

One method that is used to minimize the damage to the crops by ponded water is the installation of surface inlets to the subsurface 

drainage. An orange inlet can be seen near the left edge of the pond. The surface inlets, open conduits to the subsurface drains, 

quickly move water along with sediment and chemicals from the pond to the stream.
Overview 7

Irrigation—For where there is too little water

Irrigation is used to meet agricultural needs in naturally arid areas and in humid areas where precipitation may not come at 

the proper time of year. Irrigation for agriculture constitutes 62 percent of the total water use in the Nation (excluding water used 

for thermoelectric power). Irrigation water can be derived from groundwater or surface water. The selection of the source of 

irrigation water largely depends on the availability of the water resource, cost of moving the water, legal ownership of the water, 

and its degree of salinity. Excess irrigation water can transport agricultural chemicals and sediment directly to streams. Excess 

irrigation water also can increase water recharge and the movement of agricultural chemicals to groundwater. In parts of the arid 

West, irrigation has mobilized naturally occurring trace elements and increased salinity in streams and shallow groundwater. For 

example, irrigation recharge along the eastern San Joaquin Valley in California has mobilized naturally occurring uranium to 

levels that now exceed the drinking-water standard.

Artificial Water Use and Cropland

tion

rigation

EXPLANATION

Surface-water irrigated cropland

Groundwater irrigated cropland

Non-irrigated cropland

Study locations

Water, derived from both 

surface-water and groundwater 

sources, is used to irrigate 

crops and water livestock 

in areas across the Nation, 

particularly in the arid parts 

of the West. Irrigation is used 

in humid areas to supplement 

natural rainfall. 

A center-pivot irrigator sits idle in Wisconsin during winter. Although this area receives adequate rainfall, supplemental 

irrigation is used to maintain good levels of soil moisture in the quickly draining sandy soils. 
8 Agriculture—A River Runs Through It—The Connections Between Agriculture and Water Quality

Tracking How Water Moves Through Agricultural Landscapes Provides Insights on the Movement 

of Agricultural Chemicals

The movement of water in many agricultural areas has been altered to increase crop production. These hydrological 

alterations also can influence the movement of agricultural chemicals to streams and groundwater. The water flowpaths, as well 

as the timing and intensity of precipitation and (or) irrigation, are important in determining the timing and magnitude of the 

movement of chemicals to streams and groundwater.

Water reaches a stream through a combination of fastflow, drainflow, and slowflow pathways. Fast flowpaths (fastflow), 

such as surface runoff, can be high energy and occur over timeframes of hours to days. Slow flowpaths (slowflow), such as 

movement through groundwater to streams, occur over timeframes of months to decades. Drainage flowpaths (drainflow) are 

engineered movement of water through subsurface drains.

Groundwater aquifer

Stream

Slowflow

Fastflow

Drainflow

Water reaches a stream through a 

combination of fastflow, drainflow, and 

slowflow pathways. In many agricultural 

settings, the movement of water has been 

altered to enhance crop production. These 

changes to the hydrologic system also can 

influence the movement of agricultural 

chemicals to groundwater and streams.

Drainflow

(subsurface drains)

Fastflow

(runoff)

Slowflow

(groundwater)

Maryland

California

Nebraska

Iowa

Indiana

Mississippi

Washington

(irrigation

season)

Washington

(nonirrigation

season)

The corners of the triangle represent examples of 

streams that have a single important flowpath. The 

edges of the triangle represent streams that have a 

mixture of two flowpaths. The interior of the triangle 

represents streams that have a mixture of all three 

flowpaths. The squares on the triangle denote the 

flowpaths observed in the study area shown on page 4.

Fastflow—water that moves off the landscape to the 

stream in hours to days following rainfall or irrigation. 

This water moves through surface runoff, overland 

flow, and other pathways. 

Drainflow—water that moves off the landscape 

through artificial subsurface drains in hours to weeks

following rainfall or irrigation. The subsurface drains 

quickly and moves out of the root zone to protect the 

crop roots and to allow machines to enter the fields. 

Slowflow—water that takes months to decades to 

move from the landscape to the stream. This water 

can infiltrate into the soil, recharge groundwater, and 

eventually discharge to the stream. Slowflow water 

also can be stored in wetlands before it reaches 

the stream. 
Overview 9

Excess rainfall and irrigation water move from the landscape to 

streams through a combination of flowpaths—fastflow, slowflow, 

and (or) drainflow (artificial subsurface drainage).

Water in a stream is a mixture of water from various sources that flow through the landscape and streams in small 

watersheds can be categorized by their important flowpath(s). A national assessment of watershed properties, probable locations 

of subsurface drainage, and hydrologic studies of gaged streams provides insights about which flowpaths are important in 

various areas of the Nation. However, care must be taken when using a large-scale map on small scales because all flowpaths are 

local. The flowpaths leaving even two adjacent fields could be markedly different due to topography, soils, land management, 

and (or) the presence/absence of subsurface drainage.

Flowpaths on Agricultural Lands

EXPLANATION

Drainflow

Fastflow

Mixed flow

Slowflow

Non-agricultural land

Study locations
10 Agriculture—A River Runs Through It—The Connections Between Agriculture and Water Quality

Fastflow 

In some watersheds, especially those with steep slopes and (or) clayey soils, surface runoff is an important flowpath. The 

streamflow in these watersheds responds quickly to rain events, usually within minutes to hours. Agricultural chemicals can be 

transported to the stream with the surface runoff, but fastflow is especially important for transporting sediment and sedimentassociated chemicals, such as total phosphorus, pesticides, and some trace elements.

Agricultural Lands Where Fastflow is Important

Episodic Delivery of Chemicals and Sediment to Streams—Hours to Days

0

0.3

0.6

0.9

1.2

0

6

12

18

24

Streamflow, in cubic meters per second

Nitrate concetration, in miligrams per liter

2013

Aug. 31 Sept. 2 Sept. 4 Sept. 6 Sept. 8 Sept. 10

Fastflow transport of water and nitrate 

in the Bogue Phalia, Mississippi

EXPLANATION

Fastflow

Other agricultural land

Non-agricultural land

Study locations

Streamflow

Nitrate concentration

EXPLANATION

Fastflow is an important pathway 

for the delivery of nitrate to this 

stream. Many conservation 

management practices, such as 

buffer strips and conservation 

tillage, are effective in preventing 

sediment and sedimentassociated chemicals from 

reaching the stream. However, 

these practices are less effective 

at controlling dissolved chemicals 

such as nitrate.
Overview 11

Slowflow

In watersheds with gentle slopes and permeable soils, groundwater is generally a major contributor to streamflow. Rain 

water and agricultural chemicals dissolved in the water infiltrate through the soil, recharge groundwater, and eventually 

discharge to the stream. It can take months to decades for the water and chemicals that infiltrate the soil to eventually discharge 

to a stream. This can cause long lag times between when the chemical is applied at the land surface and when it actually appears 

in the stream. Nitrate, herbicides, and herbicide transformation products are often important water-quality concerns in streams 

that receive water from slow flowpaths.

Agricultural Lands Where Slowflow is Important 

Steady Delivery of Dissolved Chemicals to Streams—Months to Decades

Nitrate concetration, in miligrams per liter

Streamflow, in cubic meters per second

2013

June 18 June 19 June 20 June 21 June 22 June 23

Streamflow

Nitrate concentration

EXPLANATION

0

3

6

9

12

15

18

0

3

6

9

Slowflow transport of water and nitrate in 

Chesterville Branch, Maryland

EXPLANATION

Slowflow

Other agricultural land

Non-agricultural land

Study locations

The increase in nitrate 

concentration following the 

peak streamflow comes from 

slowflow groundwater—the 

legacy from nitrogen that 

was applied to agricultural 

fields years to decades ago. 

Thus, benefits of today’s 

conservation actions may not 

be realized in this stream for 

several decades.
12 Agriculture—A River Runs Through It—The Connections Between Agriculture and Water Quality

Subsurface Drainflow

The installation of subsurface drains creates an artificial water flowpath engineered to quickly move water out of the soil 

and into the stream. The drains, located below the root zone, control the groundwater level so it is at or below the level of the 

drains. Water flowing in the subsurface drains is generally fastflow from recent excess rainfall and (or) excess irrigation and 

slowflow that is being skimmed off the top of the groundwater. The flow in the drains can respond to rainfall or irrigation within 

hours. Chemicals that readily dissolve in water can move through the soil root zone into the drains and directly to the stream. 

The chemicals moving through subsurface drains bypass many natural removal processes of the soil. Nitrate, herbicides, and 

herbicide transformation products are often observed in streams that receive water from subsurface flowpaths. 

Agricultural Lands Where Drainflow is Important 

Episodic and Steady Delivery of Dissolved Chemicals to Streams—Hours to Weeks

Nitrate concetration, in miligrams per liter

Nitrate concetration, in miligrams per liter

Streamflow, in cubic meters per second

Streamflow, in cubic meters per second

2011

Apr. 24 Apr. 28 May 2 May 6 May 10

2011

Apr. 1 May 31 July 30 Sept. 28

0

5

10

15

20

0

10

20

30

40

Drainflow transport of water and nitrate in the

South Fork of the Iowa River, Iowa 0

6

12

18

24

0

10

20

30

40

EXPLANATION

Drainflow

Other agricultural land

Non-agricultural land

Study locations

Streamflow

Nitrate concentration

EXPLANATION

Subsurface drainage quickly drains 

wet soils, but this engineered flowpath 

bypasses much of the soil’s capacity to 

remove nitrate. In this watershed, the 

nitrate that moves through the top layers 

of the soil quickly reaches the subsurface 

drain and is routed to the stream.

The nitrate concentration remained 

elevated throughout the entire growing 

season due to the frequent rains and 

the delivery of water from the drained 

landscape.
Overview 13

Nitrate and Metolachlor in Leary-Weber Ditch, Indiana

Numerous surface and subsurface drains quickly move excess water from poorly drained soils to protect plant 

roots and to allow farm machinery to access fields in the Leary-Weber Ditch watershed in central Indiana. This quick 

and efficient drainage of the watershed creates a short circuit. Water and agricultural chemicals bypass the natural 

transformation processes and are quickly transported to the Leary-Weber Ditch, and eventually to the Ohio River and 

Gulf of Mexico. Two commonly used agricultural chemicals 

(nitrate and metolachlor) were detected in every component of 

the hydrologic system in Leary-Weber Ditch; however, these 

agricultural chemicals are transported to the stream through 

different flowpaths. The difference in the important flowpath 

for the two chemicals is partly because of the different times 

of application of the chemicals relative to rainfall and their 

different spatial patterns of application. The water from all 

these components (rain, surface runoff water, subsurface drain 

water, and groundwater) is integrated into the stream, but 

with different transit times, from hours to years. An improved 

understanding of how these chemicals are transported by 

different flowpaths to a stream can help water resource 

managers select and target appropriate conservation actions. Water enters a surface ditch from a subsurface 

drain outlet. 

tac11-0566_fig20A

INDIANA

Indianapolis

Watershed boundary

Subsurface drain

Subsurface drain outlet to stream

Subsurface drain sampling site

Percipitation sampling site

Surface runoff sampling site

Groundwater sampling site

Stream sampling site

EXPLANATION

Base From 2003 National Agricultural Imagery Program, 

County Tiles for Indiana, Hancock, County. 

Universal Transverse Mercator projection, Zone 16, NAD 83 0 0.5 1 Kilometer 

0 0.5 1 Mile

Leary Weber

Ditch gage

and

sampling

site

Mohawk

Sugar Creek

35 ̊

51'

85 ̊51' 85 ̊50'

35 ̊

52'
14 Agriculture—A River Runs Through It—The Connections Between Agriculture and Water Quality

tac11-0566_Overview_fig20

0.1

1

10

1.0

.06

.04

.02

0.0

.08

Nitrate concentration, in milligrams per liter

15

0.05

Surface

runoff

water

GroundwaterStream

water

Subsurface

drain water

Rain

Surface

runoff

water

GroundwaterStream

water

Subsurface

drain water

Rain

EXPLANATION 

50th percentile (median)

25th percentile

75th percentile

Nitrate

Metolachlor

Leary-Weber Ditch, Indiana

Total metolachlor concentration, 

in milligrams per liter

Water transit time

Shorter Longer

Integrator

of water

flowpaths

Nitrate was detected in relatively high 

concentrations in the subsurface drain water 

and stream water. Nitrate also was present, 

although at much lower concentrations, in 

rain, groundwater, and surface runoff. These 

results show that the movement of nitrate 

into this stream is more likely through the 

subsurface drains than through groundwater 

or surface runoff. 

The highest concentrations of total 

metolachlor were detected in the stream 

and the surface runoff water. In deep soils 

and groundwater, however, the long-lived 

metolachlor transformation products were 

detected at higher concentrations than the 

more quickly transformed metolachlor. Over 

the course of a year, surface runoff is the 

important flowpath for metolachlor to reach 

the stream, whereas groundwater discharge 

is the important flowpath for metolachlor 

transformation products to reach the stream. 

The difference in the important flowpath for 

the two chemicals is, in part, because of the 

different times of application of the chemicals 

relative to rainfall and their different spatial 

patterns of application.

The agricultural landscape of central Indiana shown from ground level and from 

the air.
Overview 15

Setting Realistic Expectations for Water-Quality Improvements

Expectations for water-quality improvements due to the implementation of agricultural activities must consider the properties 

of the chemical and sediment and especially the hydrologic flowpaths that transport the chemical to the impacted water body. 

Examples of Agricultural Activities Used for Protection or Improvement of Water Quality

Trapping practices

 Terraces, grassed waterways

 Buffer/filter strips

 Brims at edge of stream

 Cover crops

 Strip cropping

Tillage practices

 Conservation tillage

 No-till tillage

 Contoured plowing

Drainage practices

 Controlled subsurface drainage

 Biofilters on subsurface drains

 Removal of subsurface drains

 Removal of surface inlets to 

 subsurface drains

Irrigation practices

 General decrease in volume and 

 energy of irrigation water

Chemical use practices

 Decrease in chemical use

 Use of chemicals with short 

 environmental half-lives

Set-aside land for conservation

 Conservation reserve programs 

 (Federal and state; for example, 

 USDA CRP)

 Constructed wetlands

The effectiveness of an agricultural activity 

depends on the local hydrologic flowpaths 

within a specific field and the environmental 

behavior of the specific chemical or sediment.
16 Agriculture—A River Runs Through It—The Connections Between Agriculture and Water Quality

Trapping and tillage practices are primarily designed 

and implemented to minimize soil loss and prevent runoff of 

the sediment into the stream by fastflow. These practices are 

effective because they slow down the water moving across 

the fields by fastflow and increase the volume of recharge. 

However, trapping and tillage practices have little effect 

on the removal of water-associated chemicals moving with 

fastflow, and can potentially exacerbate the movement of 

water-associated chemicals to the subsurface by increasing 

the rate of recharge through enhanced drainflow and slowflow. 

Because trapping and tillage practices can increase recharge, 

they can increase the movement of water-associated chemicals 

into the subsurface. Because water-associated chemicals in 

the subsurface are no longer influenced by activities at the land 

surface, only natural transformation processes can decrease their 

concentrations until they are eventually discharged to a stream.

Connecting Water Flowpaths and Chemical Behavior Can 

Inform Agricultural Management Decisions

Understanding the connections among agricultural 

activities, water flowpaths, and behavior of specific chemicals 

and sediment can be combined into a framework to guide 

policy and management decisions to reduce current and 

prevent future impacts on water quality. This framework is 

built on a generalization of stream hydrology—water, whether 

from natural rainfall or irrigation, moves to a stream by a 

combination of slowflow, fastflow, and drainflow and each 

of these flowpaths are associated with a range of timescales. 

The framework generalizes the behavior of chemicals and 

sediment—all chemicals and sediment are distributed on a 

continuum between those completely associated with water 

and those completely associated with sediment. Finally, 

each agricultural activity is assumed to have its expected 

effect on the movement of water and chemicals across the 

landscape, supported by observations in the environment and 

quantified in models. This framework, shown as a decision 

tree, is a gross simplification and it is not intended to be used 

as a primary decision-making tool on a site-specific basis. 

Additional detailed knowledge of the local environment, 

climate, and agricultural production is required to make design 

decisions for the implementation of an agricultural activity for 

a particular location.

The first question in the decision tree concerns the 

environmental behavior of the chemical of concern: Is the 

chemical of concern primarily associated with sediment? The 

second question addresses the presence of permanent or semipermanent agricultural modifications to the landscape that 

are already in place: Is the water moving through subsurface 

drainage? The final question regards the nature of the 

hydrologic setting: Is the water moving through groundwater? 

The answers to these three questions can guide the selection 

and expectation of the effectiveness of implementing an 

agricultural activity.

Although valuable knowledge can be gained from a general 

understanding of how water and chemicals move in an 

agricultural landscape as presented in this conceptual 

framework, local knowledge of hydrology and chemistry are 

required to optimize effective conservation actions on a 

field-by-field and chemical-by-chemical basis.
Overview 17

Is the chemical of concern primarily associated with sediment? 

No

No No

Yes

Yes

Yes No

Yes

Is the water moving through subsurface drainage?

Is the water moving through groundwater? 

Not usually a

water quality

concern*

Not usually a

water quality

concern*

Useful activities: increased set-aside land;

decreased drainage, irrigation, and chemical use

Counterproductive: some increases in trapping

practices and conservation tillage

Yes No

Useful activities: increased trapping practices,

 conservation tillage, and set-aside land;

decreased irrigation and chemical use

Counterproductive: none 

Useful activities: increased set-aside land;

decreased drainage, irrigation, and chemical use

Counterproductive: none 

Useful activities: increased set-aside land;

decreased irrigation and chemical use

Counterproductive: some increases in trapping

practices and conservation tillage

EXPLANATION

Slow flowpaths Fast flowpaths Drain flowpaths

A Simple Decision Tree Connects Water Flowpaths, Chemical Behavior, and the 

Effectiveness of Agricultural Activities

The decision tree can assist with identification of which agricultural activity(s) (see “Understanding the connections among hydrologic 

settings and chemical behaviors can help set realistic expectations for water-quality improvements” in Chapter 9”) could be effective 

in protecting or improving stream-water quality and which agricultural activity(s) could be counterproductive. * Denotes that these 

chemicals do not usually cause a water-quality concern for the fraction of water moving through subsurface flowpaths (for example, 

moving through soil to groundwater or to subsurface drains), but they can cause a concern for the water moving through flowpaths 

across the land surface.
18 Agriculture—A River Runs Through It—The Connections Between Agriculture and Water Quality

Example—Which Agricultural Activity(s) is Most Effective for Reducing Nitrate in Slowflow, 

Chesterville Branch, Maryland? 

Groundwater is a major contributor to streamflow in 

the Chesterville Branch located on the Delmarva Peninsula 

in Maryland. This watershed contains gentle slopes and 

permeable soils, which allow rain water to easily infiltrate 

the soil and recharge the groundwater—key characteristics 

of a slowflow area. Chemicals that are largely associated 

with water, such as nitrate from fertilizers and manure, 

infiltrate into the soils with precipitation and move slowly 

to groundwater. It takes, on average, about 50 years for this 

water to travel from land surface through groundwater and 

to ultimately discharge to Chesterville Branch. On an annual 

basis, about half the streamflow in Chesterfield Branch comes 

from groundwater, so it is an important source of water and 

nitrate to the stream, especially during low-flow periods. 

A coupled understanding of water movement and how 

different chemicals are transformed in different hydrologic 

settings can help identify the most effective management 

practices and inform conservation policies. The heavy black 

line on the decision tree follows the chemical behavior and 

hydrologic setting—nitrate is strongly associated with water, 

there is minimal subsurface drainage in the watershed, and 

groundwater is an important water flowpath to the stream. The 

use of most trapping and tillage practices that are designed 

to reduce soil erosion could be counter-productive, because 

they generally increase groundwater infiltration and can 

increase the movement of nitrate to groundwater. Cover crops 

can be effective at storing nitrogen in the plants and soil, but 

the stored nitrogen has the potential to eventually leach to 

groundwater. In this slowflow area of Chesterville Branch, 

one of the most effective activities is the reduction of fertilizer 

use. Optimizing the amount of irrigation, in addition to the 

fertilizer, could decrease the amount of nitrate that reaches the 

groundwater system.

In slowflow areas, such as the Chesterfield Branch 

watershed, it can take years for a chemical like nitrate to 

move from the land to the groundwater and ultimately to the 

stream. Groundwater nitrate inputs in this stream currently are 

representative of fertilizer applied to crops years or decades 

ago, thus benefits of today’s conservation actions may not be 

realized in this stream for several decades.

The agricultural landscape of 

Maryland’s Eastern Shore shown 

from ground level and from the air. 

Nitrate concentration, in milligrams per liter

100

10

0.1

0.01

10,000

Phosphorus or sediment concentration,

in milligrams per liter

Streamflow, in cubic feet per second

10,000

1,000

100

10

1

0.1

0.01

0.001

1 10 100 1,000

Chesterville Branch near Crumpton, Maryland

Suspended sediment

EXPLANATION

Phosphorus

Nitrate (as nitrogen)

Sediment-associated

Water-associated

Data for water- and sediment-associated chemicals show the complexities of real 

life and reinforce the need to understand the connections between hydrologic 

flowpaths and chemical transport to inform agricultural management decisions. In 

Chesterville Branch, different flowpaths are primarily responsible for the transport 

of water-associated and sediment-associated chemicals. This stream continually 

receives slowflow (groundwater), but it also receives fastflow after periods of rain. The 

fastflow water contains greater concentrations of sediment and sediment-associated 

chemicals, like phosphorus, than does groundwater. Therefore, the concentrations of 

suspended sediment and phosphorus increase with higher streamflow (more fastflow). 

The nitrate concentration in the stream decreases with increasing streamflow because 

groundwater with a higher nitrate concentration is being diluted by fastflow, which has 

a lower concentration. (From Ator and Denver, 2015.)
Overview 19

Is the chemical of concern primarily associated with sediment? 

No

No No

Yes

Yes

Yes No

Yes

Is the water moving through subsurface drainage?

Is the water moving through groundwater? 

Not usually a

Water quality

 concern*

Not usually a

Water quality

 concern*

Useful activities: increased set-aside land;

decreased drainage, irrigation, and chemical use

Counterproductive: some increases in trapping 

practices and conservation tillage

Yes No

Useful activities: increased trapping practices,

conservation tillage, and set-aside land;

decreased irrigation and chemical use

Counterproductive: none 

Useful activities: increased set-aside land;

decreased drainage, irrigation, and chemical use

Counterproductive: none 

Useful activities: increased set-aside land;

decreased irrigation and chemical use

Counterproductive: some increases in trapping

practices and conservation tillage

Nitrate Movement in Slowflow Areas—Chesterville Branch, Maryland 

EXPLANATION

Slow flowpaths Fast flowpaths Drain flowpaths

In slowflow areas, it can take decades for nitrogen fertilizer 

to move from the land to the groundwater and to the 

stream—groundwater nitrate inputs to the stream today 

reflect fertilizer and manure inputs decades ago.
20 Agriculture—A River Runs Through It—The Connections Between Agriculture and Water Quality

Example—Which Agricultural Activity(s) is Most Effective for Reducing Sediment in Fastflow from 

Excess Irrigation in Granger Drain, Washington? 

Excess irrigation runoff is a major contributor to streamflow during the 

growing season to the Granger Drain in central Washington. The naturally arid 

climate requires irrigation to maintain agricultural productivity. Irrigation water 

is relatively abundant from the snowmelt water from the Cascade Mountains. 

Excess irrigation runoff transports large amounts of sediment, as well as 

agricultural chemicals associated with sediment such as total phosphorus. The 

transported sediment creates turbidity in the stream, which can adversely affect 

aquatic organisms. 

Prior to the late 1990s, much of the sediment came from fields that received 

furrow irrigation, an inefficient method of irrigation that can cause large amounts 

of soil erosion. After the late 1990s, irrigation practices shifted from furrow 

to sprinkler and drip methods. This widespread change resulted in dramatic 

improvements in water quality—sediment loads were reduced by more than 90 

percent in some locations.

Sediment-laden waters enter the Yakima 

River from Granger Drain, Washington. 

Photograph by U.S. Geological Survey, 1991.

 

 

 

 

The agricultural landscape of central 

Washington shown from ground level and from 

the air. Sunnyside Canal (top) provides the water 

that irrigates the Granger Drain watershed. 

The heavy black line on the decision tree follows the behavior of 

sediment and sediment-associated chemicals in the excess irrigation 

water (fastflow) from the fields of the central Washington to the streams. 

Since the early days of farming, the streams and rivers of central 

Washington have been affected by sediment from irrigation runoff. Even 

though the slowflow groundwater is an important source of water to 

Granger Drain, the sediment does not move through the soil layers to 

subsurface drains (drainflow) or groundwater (slowflow). 

The use of less irrigation water and efficient irrigation methods were 

effective in controlling sediment in runoff. More recently, other sediment 

control methods, such as the application of polyacrylamide (PAM) and 

even more efficient methods of irrigation have been implemented.

In fastflow environments, such as the excess irrigation water in 

Granger Drain, the benefits of conservation actions—improvement in 

stream-water quality—may be realized within months or a few years. 

In this stream, turbidity and suspended sediment, total phosphorus, and 

DDT concentrations substantially decreased within a few years. Granger 

Drain became clearer as the amount of sediment erosion was reduced. 

Aquatic plant growth increased downstream in the clear water due to 

excess nutrients. This changed the stream ecology and created a different 

impact on water quality that may require additional modification of 

current agricultural practices, such as reducing fertilizer application rates 

to reduce nutrient loading from excess irrigation water.
Overview 21

Is the chemical of concern primarily associated with sediment? 

Sediment Movement in Fastflow From Excess Irrigation—Grainger Drain, Washington

No

No No

Yes

Yes

Yes No

Yes

Is the water moving through subsurface drainage?

Is the water moving through groundwater? 

Not usually a

water quality

 concern*

Not usually a

water quality

 concern*

Useful activities: increased set-aside land;

decreased drainage, irrigation, and chemical use

Counterproductive: some increases in trapping

practices and conservation tillage

Yes No

Useful activities: increased trapping practices,

conservation tillage, and set-aside land;

decreased irrigation and chemical use

Counterproductive: none 

Useful activities: increased set-aside land;

decreased drainage, irrigation and chemical use

Counterproductive: none 

Useful activities: increased set-aside land;

decreased irrigation and chemical use

Counterproductive: some increases in trapping

practices and conservation tillage

EXPLANATION

Slow flowpaths Fast flowpaths Drain flowpaths

The effectiveness of an agricultural activity depends on 

the local hydrologic flowpaths in a specific field and the 

environmental behavior of the specific chemical or sediment.
22 Agriculture—A River Runs Through It—The Connections Between Agriculture and Water Quality
NAWQA Studies on Agriculture and Water Quality 23

—NAWQA Studies on Agriculture and Water Quality 1 Chapter

The U.S. Geological Survey (USGS) has established 

various long-term monitoring programs to assess the quality 

of our Nation’s water. A consistent study design and uniform 

methods of data collection and analysis are used in each 

program to ensure that water-quality data in a specific locality, 

watershed, river basin4, or aquifer can be directly compared 

with data collected in other geographic regions and at different 

time periods. A unique feature of these studies is the ability 

to monitor the movement of water and chemicals in multiple 

components of the hydrologic system at multiple scales. 

Since 1991, the USGS National Water-Quality 

Assessment (NAWQA) Project has implemented 

interdisciplinary assessments in 51 of the Nation’s most 

important river watersheds and aquifers, which represent 

between 60 and 70 percent of total water use in the Nation 

(fig. 1.1; U.S. Geological Survey, 2012). The mission of the 

NAWQA is to assess the status and trends of national water 

quality and to improve our understanding of the various 

factors that affect water quality. Information from NAWQA 

assessments help us understand how water quality varies over 

space and time, and how it is affected by human activities 

and natural factors across the Nation. NAWQA findings 

thereby describe the general health of water resources, 

and help identify current and emerging water issues while 

providing information that is essential for developing practical 

management strategies for protecting and restoring water 

quality. The NAWQA study areas are representative of the 

Nation’s major hydrologic landscapes, priority ecological 

resources, and agricultural, urban, and natural sources of 

contamination. By combining information on water chemistry, 

landscape characteristics, stream habitat, and aquatic life, 

NAWQA provides science-based insights that can be used 

to set priorities among various current and emerging issues 

related to water quality across the Nation. 

EXPLANATION

National Water Quality Assessment

Project (NAWQA) Study Unit basin

High Plains Aquifer

Cropland

NAWQA studies on the connections 

between agriculture and water quality

Figure 1.1. Study areas in the U.S. Geological Survey National Water-Quality Assessment 

Project (study units, major river basins, and High Plains aquifer) compared to areas of 

cropland in the conterminous United States. 

4

Bold words indicate glossary terms. See “Glossary of Terms” at back of report.
24 Agriculture—A River Runs Through It—The Connections Between Agriculture and Water Quality

The NAWQA Project completed a series of topical 

studies that were intended to improve understanding of how 

natural features and human activities affect water quality. 

The specific focus of these studies was to establish links 

between sources of chemicals, the transport of those chemicals 

through the hydrologic system, and the potential effects of 

chemicals on humans and aquatic ecosystems. This report 

focuses on the connections between agriculture and water 

quality in agricultural watersheds; however, the report also 

features findings from other NAWQA studies that examined 

the effects of agricultural activities on the quality of surfacewater and groundwater.

Connections Between Agriculture 

and Water Quality

The primary questions of the study were: “How do 

environmental processes and agricultural activities interact 

to affect the transport and fate of agricultural chemicals 

in the hydrologic system?” and “What are their effects on 

water quality and the implications for management of water 

resources?” These questions were addressed by studying 

multiple components of the hydrological system in agricultural 

areas to improve the understanding of how agricultural 

chemicals are transported into and through streams 

and groundwater.

In this whole-system approach, five components of the 

hydrologic system (atmosphere, surface water, land surface/

root zone, unsaturated zone, and groundwater) and the 

interfaces and flowpaths that connect these components were 

addressed by using a combination of field observations and 

model simulations to provide information on the sources, 

transport, and fate of water and agricultural chemicals. These 

data were coupled with field-scale information on agricultural 

activities (crops, irrigation, drainage, management practices, 

and chemical use) and large-scale (larger than field scale; 

for example, watershed scale) spatial information available 

from national data bases (soils, weather, chemical use, and 

cropping patterns) to provide information across a range of 

spatial scales. The study areas represent major agricultural 

settings, such as irrigated diverse cropping in the West and 

corn and soybean row cropping in the Midwest, Mid-Atlantic, 

and Southeast, and, therefore, findings are relevant throughout 

much of the Nation.

Design of the Study.—This study design led to an 

improved understanding of the many factors that can affect 

the movement of water and chemicals in different agricultural 

settings. The study integrated the collection and analysis of 

field data with numerical modeling to evaluate the sources, 

transport, and fate of water and selected agricultural chemicals 

in a variety of nationally important agricultural settings. 

Agricultural settings were the superposition of hydrologic 

settings and agricultural systems. The hydrologic setting 

was the combination of surface and subsurface hydrologic 

systems, characterized by specific topography, geology, soils, 

and climate. The agricultural systems were defined by the 

classification of cropland; this definition suggests that the 

distribution of crops can be used to characterize regional 

agricultural patterns (Gilliom and Thelin, 1997). 

A unique feature of these studies was the simultaneous 

assessment of agricultural chemicals throughout the 

hydrologic system at many scales. In a small watershed 

within each study area, data were collected on precipitation 

and weather; on streamflow; on water quality in streams, 

runoff, subsurface drains and shallow groundwater; and on 

streambed sediments and agricultural soils. Streamflow and 

water quality also were assessed in the larger river network of 

the study area. A network of wells was sampled to characterize 

the age, movement, and quality of shallow groundwater. In 

each study area, wells along a 1- to 3-kilometer (km)-long 

flow system were used to characterize chemical transport and 

transformation rates in the shallow groundwater that recharged 

within about the last 50 years. The design applied to the study 

area near Indianapolis, Indiana, is shown in figure 1.2. The 

7.2-km2

 drainage area of the Leary-Weber Ditch was nested 

within the Sugar Creek watershed, a 240-km2 agricultural 

watershed nested within the larger, more heterogeneous White 

River watershed (more than 29,000 km2).

This study compared and contrasted the environmental 

processes that control the fate and transport of water, 

agricultural chemicals, and sediment within and among the 

various components of the hydrologic system using consistent 

methodologies and analyses. Environmental observations were 

made and mathematical models applied at a range of scales 

from the field (less than 1 km2

) to large watersheds and aquifer 

systems (greater than 10,000 km2). Particular attention was 

given to the small watershed scale (about 3–15 km2). The data 

necessary to calculate water and chemical mass budgets were 

collected. It was recognized that calculations of precise mass 

budgets for agricultural chemicals generally were not possible 

at this scale, but the approach provided a useful paradigm for 

the field and modeling study designs. Results gained by using 

this approach can add to the knowledge of environmental 

transport and fate processes, and to the ability to extrapolate 

findings to unstudied areas and at different scales. 
NAWQA Studies on Agriculture and Water Quality 25

tac11-0566_fig01-02

EXPLANATION

Leary-Weber Ditch

Sugar Creek

White River

E Fk

Indianapolis

Sugar Creek at New 

Palestine, Indiana 

(downstream of 

agricultural land use)

White River at Hazleton, 

Indiana (downstream of 

heterogeneous land use)

Leary-Weber Ditch at 

Mohawk, Indiana

Leary-Weber

Ditch

Sugar Creek watershed

White River Basin

85°37'30"

85°45'

85°52'30"

39°52'30"

39°45'

Atmosphere (precipitation quality and 

meteorological data)

Groundwater (water level and water quality)

Subsurface drain (discharge and water quality)

Overland flow (discharge and water quality)

Soil (soil moisture, soil and water quality)

Surface water (discharge and water quality)

Groundwater/surface-water interaction 

(water flux and porewater quality)

INDIANA

Figure 1.2. Design of the NAWQA studies on the connections between agriculture and water quality applied to the study area near 

Indianapolis, Indiana, showing locations of various data collection activities.

Characteristics of Study Areas.—The study areas 

represent a range of agricultural settings—with varying 

crop types and agricultural practices related to tillage, 

irrigation, artificial drainage, and chemical use (fig. 1.3, 

table 1.1). A number of land-surface, subsurface, and 

climatic characteristics affect the fate and transport of water 

and chemicals. For agricultural areas, these characteristics 

include watershed area, soil properties, crop types, irrigation 

practices, drainage enhancements, streamflow characteristics, 

and whether the local subsurface flow system exchanges 

water with the deeper, regional groundwater system (deeper 

than the shallow groundwater system). These characteristics 

vary considerably among the study areas, which provided an 

opportunity to compare and contrast these diverse settings. A 

summary of each study area, including the history, climate, 

agricultural crops and animals, and recent water-quality 

studies are presented at the beginning of each of the chapters. 

The results from other NAWQA studies are also highlighted to 

extend the geographical coverage. 

The study areas in California and Washington are 

characterized by semiarid to arid climates and more than 

95 percent of crops in these areas are irrigated. The sources 

of irrigation water in these two areas, however, differ 

substantially. Most irrigation water in the California study area 

is pumped from deep groundwater, whereas in the Washington 

study area an extensive network of water delivery canals 

distributes surface water from the Yakima River. Agriculture 

is comprised of predominantly orchards, vineyards, row crops, 

and dairies in both study areas.

The study areas in Nebraska, Iowa, Indiana, and 

Maryland are characterized by humid climates, each receiving 

70–100 cm of precipitation each year. These four study areas 

are dominated by corn and soybean row cropping, but differ 

in ways that affects the movement of agricultural chemicals. 

Specifically, irrigation is used to augment water needs in the 

Nebraska study area, where 30 percent of the farmland is 

irrigated from deep wells. Subsurface drains, used to move 

excess water from the root zone, are widely used in the Iowa 

and Indiana study areas, where topography is flat and soils 

were relatively high in clay content and impermeable. In 

contrast, soils in the Maryland study area are permeable and 

well drained, and precipitation and natural soil drainage are 

adequate for agricultural activities.
26 Agriculture—A River Runs Through It—The Connections Between Agriculture and Water Quality

tac11-0566_fig01-03

Webster 

City

State: Washington

Yakima River Basin

Study site: DR 2

State: California

San Joaquin River Basin

Study site: Mustang Creek

State: Nebraska

Elkhorn River Basin

Study site: Unnamed tributary

State: Iowa

Iowa River

Study site: South Fork of the 

Iowa River at Blairsburg

State: Mississippi

Bogue Phalia River Basin

Study site: Tommie Bayou

State: Indiana

White River Basin

Study site: Leary-Weber Ditch

State: Maryland

Study site: Morgan Creek

0 250 MILES

0 400 KILOMETERS

State

River basin

Study site

City

High Plains Aquifer

Stream

EXPLANATION

Figure 1.3. Locations of the NAWQA studies on connections between agriculture and water quality in the 

conterminous United States. 

The study area in Mississippi is characterized as a subtropical climate that receives 130–150 cm of precipitation 

each year—much of it in torrential downpours. Approximately 

one-half of the cropland is planted in soybean, with the 

remaining land planted in rice, cotton, and a variety of other 

crops including corn, peanuts, pecans, sugar cane, and sweet 

potatoes. Because of the uneven seasonal distribution of rain, 

irrigation is often necessary to meet crop needs.

NAWQA Studies on Agriculture and Water-Quality 

in Context.—The NAWQA Project is one of numerous 

ongoing studies of the effects of agriculture on water quality. 

Water-quality studies are being conducted by academia, 

Federal, State, and local government agencies, watershed and 

drainage districts, corporations, advocacy groups, and farmers. 

The results of these diverse studies are reported in scientific 

journals, agency reports (for example, U.S. Department of 

Agriculture, National Agricultural Library, 2012a, 2012b; 

U.S. Environmental Protection Agency, 2012e), books, and 

other resources. Multiple agencies within the U.S. Department 

of Agriculture (USDA) have cooperated on the Conservation 

Effects Assessment Project “to quantify the environmental 

effects of conservation practices and programs and develop 

the science base for managing the agricultural landscape for 

environmental quality” (U.S. Department of Agriculture, 

Natural Resources Conservation Service, 2012a). Each of 

these various studies focuses on a particular location or 

aspect of measuring or preventing water-quality effects from 

agriculture and (or) informing the decisions of policy-makers 

and agricultural managers. This NAWQA study focused on the 

effect of water flowpaths that define the connection between 

agricultural activities and water quality.

Hydrology is Key.—An accounting of local hydrology—

the amount of water that enters the watershed, moves 

within the watershed, and leaves the watershed—forms the 

foundation for understanding the movement of agricultural 

chemicals in each of the study areas. In the following chapters 

of this report, the connections between agricultural activities 

and water quality are made by way of the movement of 

water through and between the various components of the 

hydrologic system (water flowpaths). 

Knowledge about the predominant water flowpaths 

and understanding their characteristic transport times can 

guide selection of the most effective management practices 

in improving water quality for particular agricultural 

settings. For example, buffer zones around streams can be 

effective in improving water quality where overland flow is 

important, but would be less effective where much water in 

a watershed travels to the stream through subsurface drains 
NAWQA Studies on Agriculture and Water Quality 27

Table 1.1. Locations and characteristics of study areas from the NAWQA studies on connections between agriculture and water 

quality.

Location Major crops and 

animal production 

Primary agricultural 

management 

practices 

Hydrogeologic 

setting Major water-quality issues 

Washington

 Yakima River

 Granger Drain

 DR2

Orchards, vineyards,

corn, hay, mixed row

crops, dairy, cattle

feedlots 

Sprinkler and

gravity irrigation;

conventional tillage

and no-till;

subsurface drains,

surface ditches

Arid; deep loess soils

underlain by basalt 

Pesticides and nutrients, water

temperature, sediment, and fecal

bacteria

California

 San Joaquin River 

 Lower Merced River

 Mustang Creek

Orchards, vineyards,

mixed row crops,

dairy 

Spray, furrow, flood,

and drip irrigation;

conventional tillage

and no-till 

Arid; permeable

sands with relatively

shallow water tables

and poorly drained

soils with deeper

water tables 

Nitrate in groundwater, insecticides

in streams with toxicity to aquatic

invertebrates

Nebraska

 Elkhorn River

 Maple Creek

 Unnamed tributary

Corn, soybean,

alfalfa, hay, and

wheat; beef cattle,

some dairy and hogs 

Mostly dryland;

central pivot

irrigation;

conventional/

conservation tillage,

no-till increasing

Semiarid; permeable

surface and

subsurface 

Herbicides and nutrients in streams and

groundwater, including drinking-water

wells; sediment in streams 

Indiana

 White River 

 Sugar Creek

 Leary-Weber Ditch

Corn and soybean No irrigation;

conventional and

reduced tillage; tile

subsurface drains,

surface ditches 

Humid; poorly

drained soils

Herbicides, nutrients, accelerated

transport of pesticides and nutrients to

streams by way of subsurface drains 

Maryland 

 Morgan Creek

Corn and soybean;

some dairy 

Some central

pivot irrigation;

conventional tillage

and no-till 

Humid; moderately

to well drained soils;

permeable aquifer

composed of sand

and gravel 

Herbicides and nutrients in streams and

groundwater, long-term storage of

nitrate in groundwater

Iowa 

 Iowa River

 South Fork at NP1

 South Fork at BB2

Corn and soybean;

extensive hog 

confined feeding

operations 

Conventional and

conservation tillage;

subsurface drains,

surface ditches 

Humid; poorly

to moderately

drained soils from

glacial till 

Herbicides, nutrients, accelerated

transport of pesticides and nutrients

to streams by way of subsurface drains 

Mississippi 

 Bogue Phalia

 Tommie Bayou

 Clear Creek

Cotton, rice,

soybean, corn;

catfish, few other

animals 

Flood, pivot, spray,

and furrow irrigation 

Subtropical; poorly

drained alluvial soils 

Residual organochlorine insecticides

and currently used pesticides, fine

sediment, and low dissolved oxygen in

streams

1

South Fork of the Iowa River near New Providence, Iowa.

2

South Fork of the Iowa River near Blarisburg, Iowa.

or by deep groundwater flowpaths. Knowledge concerning 

which water flowpaths predominate in particular settings 

also helps managers develop appropriate monitoring plans. 

In situations where most water in a watershed is transported 

along deep groundwater flowpaths, responses to management 

changes may not be apparent in stream-water quality for 

years. In contrast, responses in stream quality may be 

apparent almost immediately in areas with short flowpaths, 

such as watersheds with extensive overland flow and (or) 

subsurface-drainage systems.
28 Agriculture—A River Runs Through It—The Connections Between Agriculture and Water Quality

The French were the first Europeans to visit the present State of Iowa in the late 1600s. Zebulon Pike in 1805 

explored parts of Iowa to survey locations for forts and trading posts. These Iowa explorers, traveling by river, would 

have been surrounded by dense hardwood forests. Away from the river in drier upland areas, oak savannas bordered 

the tallgrass prairie that encompassed most of the landscape. In the dry autumn, the prairie ecosystem was prone to 

routine wild fires, which kept trees from invading the open land and recycled nutrients to the soil. The topography 

was flat or gently rolling hills with thousands of small depressions dotting the landscape. These topographic 

depressions, left behind during the last glacial period about 14,000 years before the present era (Steinwand and Fenton, 

1995), contained isolated wetlands. The soil beneath the prairie and wetlands was deep and rich in organic matter. 

The cast-iron plows the settlers had brought with them from the East were designed for the light, sandy New 

England soil. The rich, Midwest soil clung to the plow so that every few steps it was necessary to scrape off the soil, 

making plowing a slow and laborious task. Many pioneers were discouraged and considered moving on, or heading 

back East. By the mid-1800s, with the help of John Deere’s new steel plow (“self-polishers”) with its highly polished 

and properly shaped moldboard that scoured itself as it turned the furrow slice, the rich lands of Iowa were plowed 

and cultivated for agriculture (Drache, 2001). Farmers plowed the prairies and planted mostly corn, wheat, oats, and 

hay for livestock feed. By the 1900s, settlers had converted nearly the entire prairie to farmland. The wetlands that 

remained quickly disappeared because of the practice of drainage through ditch digging, trenching, and installation 

of subsurface drains. The economy of north-central Iowa was driven by agriculture and the other industries that 

supported agriculture. Webster City (fig. 1.3) was home to a factory that would produce much of the clay pipe used 

for subsurface drains. The net effect of European settlement in Iowa was, thus, a change from a prairie ecosystem 

that supported a diverse flora and fauna to uniform blocks of monoculture farmland. “Incredibly, this astounding 

transformation from a natural landscape of wild places teeming with wild creatures to a checkerboard of manicured 

fields, cities, and roads, took place in barely 60 to 70 years, less than a lifetime” (Flickinger, 2010). 

Recent trends in agriculture in north-central Iowa have included an increase in the number of large concentrated 

animal feeding operations (CAFOs) established for swine and poultry, a substantial increase in the amount of 

subsurface drainage, and an increase in the area planted in corn that is used for biofuel. CAFOs, each one 

potentially housing thousands of pigs, have transformed animal agriculture from relatively small numbers of animals 

housed at many farm sites to large numbers of animals housed at a much smaller number of locations. Recent studies 

by the USGS and USDA on the effects of agriculture on water quality in north-central Iowa have focused on nutrient 

transport in streams and subsurface drains (Tomer and others, 2010), sediment yield (Merten and others, 2016), 

occurrence of glyphosate (Chang and others, 2011; Coupe and others, 2011), trends in pesticide concentrations 

(Sullivan and others, 2009), trends in herbicide transformation products (Kalkhoff and others, 2012), and the 

occurrence of phytoestrogens and mycotoxins (Kolpin and others, 2010).

Agriculture in North-Central Iowa

A B C

The landscape of north-central Iowa. (A) An isolated wetland typical of the time before agriculture, (B) typical 

cropped fields, and (C) the view from the air (23 square kilometers). Photograph A by Lynn Betts, U.S. Department 

of Agriculture, Natural Resources Conservation Services photo gallery, NRCSIA99470, 1999; photograph B by Paul 

Capel, U.S. Geological Survey, 2008; photograph C from U.S. Department of Agriculture, Farm Service Agency, 

National Agriculture Imagery Program, 2010.
Overview of Agriculture and Water Quality 29

—Overview of Agriculture and Water Quality 2 Chapter

Why are there Water-Quality Effects 

from Agriculture? 

Agriculture in the United States supplies a large part of 

the Nation’s food, feed, and fiber needs. Hundreds of different 

crops and agricultural products are grown each year in the 

United States to meet the needs and expectations of society. 

In response to both societal demands and market forces, 

agriculture has expanded, diversified, modified the landscape, 

and intensified. Crop and animal yields have increased through 

advances in mechanization, genetics, breeding, biotechnology, 

and chemicals. For commercial success, each agricultural crop 

and animal requires its own set of agricultural activities—

procedures that fulfill the requirements of the production 

of crops or animals. Many of these activities include the 

application of chemicals to the land—fertilizers and soil 

amendments for crop growth and soil health, herbicides for 

weed control, and insecticides for insect control.

There is considerable public concern—as well as 

scientific evidence—that many agricultural activities adversely 

affect the quality of water, air, and soil, and the overall health 

of the environment. These effects and concerns are the result 

of complex interactions among agricultural activities, natural 

processes, societal needs, and market forces—interactions that 

must be understood on a fundamental scientific basis in order 

to design agricultural policies and management approaches 

that can successfully address the sustainability of both 

agriculture and the environment. Many of the environmental 

concerns originate from the off-site movement of water, 

agricultural chemicals, and eroded soil through water and air. 

The identification and characterization of transport pathways 

into, through, and away from agricultural areas are essential 

for understanding connections between agricultural activities 

and their effects on water flowpaths and water quality.

As the National Water Quality Assessment (NAWQA) 

Project began its second decade of assessment in 2002, one of 

the highest priorities identified by scientists and stakeholders 

was to improve our understanding of the sources and transport 

of agricultural chemicals in the hydrologic system. This 

priority was addressed by a set of coordinated and systematic 

studies in agricultural settings of the Nation that represent 

a wide range in the diversity of both agricultural products 

and activities, and in natural settings with their associated 

hydrologic systems. Findings from these studies, combined 

with selected findings from other NAWQA studies and 

extensive contributions from numerous other programs, 

agencies, and organizations, demonstrate and explain the 

connections between agriculture and water quality—and 

how essential it is to understand these connections for 

effective management.

Even though greater than 80 percent of the Nation's 

population lives in urban areas (Mackun and Wilson, 

2011), everyone has a connection to agriculture. Despite 

the ever-increasing expectations for high-quality, abundant, 

diverse, and inexpensive food, many people have little direct 

contact with (or understanding of) the agricultural activities 

that provide these products. For many Americans, their 

closest contact with agriculture consists of shopping at the 

supermarket for food and the department store for clothing. 

To meet the needs and expectations of society, American 

agriculture is highly diverse (fig. 2.1). Hundreds of different 

crops and agricultural products are grown. Each crop and 

animal requires its own unique set of agricultural activities 

and resources—including space, soil, water, nutrients (plants), 

food (animals), protection (from pests and weather), and 

disposal of the wastes—for commercial success. Agricultural 

activities are defined herein as all farming procedures that lead 

to the production of crops or animals (see “Specific Terms 

Used in this Report”). These activities include the growing 

and harvesting of crops and animals, modifications of the 

landscape, application of chemicals, and disposal of wastes. 

For annual crops, the soil is prepared, and crops are planted, 

maintained, and harvested. For perennial crops, such as 

orchards and vineyards, the plants are pruned and maintained 

according to their seasonal needs. Most crops are fertilized or 

protected with pesticides, and water is added to or removed 

from the landscape as needed. Animals are housed or fenced, 

fed, watered, and harvested; their manure is stored and used 

on the landscape. Agricultural activities generally fulfill one or 

more of the crop or animal requirements in an economically 

sustainable fashion (table 2.1), or they are implemented to 

help minimize the adverse effects of agricultural activities on 

water flowpaths, water quality, and (or) the environment.
30 Agriculture—A River Runs Through It—The Connections Between Agriculture and Water Quality

tac11-0566_fig02-01

20,000 hogs and pigs

10,000 cattle and calves

Livestock, one dot equals—

1,000,000 broilers

60,000 layers

100,000 turkeys

EXPLANATION

EXPLANATION

Corn

Soybean

Wheat

Other crops

Cotton

Sorghum

Crops and grassland

Vegetables and ground 

fruit

Rice

Orchards and grapes

Citrus

Grassland (includes 

pasture and hay) 

A

B

Note: Cell resolution is 56 meters

Figure 2.1. Diversity of (A) crops and (B) livestock in the conterminous United States (U.S. Department of Agriculture, 2009).
Overview of Agriculture and Water Quality 31

Table 2.1. Agricultural activities fulfilling crop and animal requirements and their environmental concerns. [Abbreviations: ET, evapotranspiration; BOD, biochemical oxygen demand; DDT, 1,1,1-trichloro-2,2-di(4-chlorophenyl)ethane; –, not applicable]

Requirement Agricultural activity for crops Potential movement Agricultural activity for animals Potential movement

Nutrients/food Fertilizer, manure, and lime

applications

Nitrate, nitrite, ammonia, nitrogenoxides, organic nitrogen,phosphorus

Feed: derived from plant primaryproduction

Manure, methane

Sufficient water Irrigation Increased water in streams,

increased infiltration, increased ET, mobility of dissolved ions(increased salinity), groundwaterdepletion, streamflow depletion

Farm ponds, building plumbing,free range to streams and ponds

Pathogens, BOD, phosphorus,sediment

Protection from weeds Herbicide application Metolachlor, glyphosate, atrazine,

many other specific compounds

– –

Protection from insects Insecticide application Carbaryl, permethrin, DDT, many

other specific compounds

Insecticides Lindane, DDT, many other

specific compounds

Protection from disease Fungicide application Benomyl, captan, thiabendazole,

many other specific compounds

Antibiotics Tetracycline, many other specific 

compounds

Protection from excess water Surface drainage, subsurface

drainage, vertical drains 

Increased discharge, nitrate, nitrite,ammonia, organic nitrogen,phosphorus, pathogens, organiccarbon (BOD), sediment,metolachlor, permethrin, DDT,many other specific compounds

– –

Waste disposal – – Lagoons, manure spreading Odors, nitrate, nitrite, ammonia,

nitrogen oxides, organic nitrogen,phosphorus, pathogens, organiccarbon (BOD)

Enhanced yield Prophylactic application of

fungicides 

Propiconazole, azoxystrobin,chlorothalonil

Hormones, prophylactic use ofantibiotics 

Tetracycline, many other specificcompounds

Protection from wind damageand erosion

Hedge rows, wind rows Decreased leakage of soil and

chemicals

– –

Protection of soil from watererosion

Terraces, contour farming, grassed waterways

Decreased leakage of soil andchemicals

– –

Protection of environmentfrom soil erosion

Buffer strips, sedimentationbasins, constructed wetlands

Decreased leakage of soil andchemicals

– –
32 Agriculture—A River Runs Through It—The Connections Between Agriculture and Water Quality

Specific Terms Used in This Report

Agricultural activities are all farming procedures that lead to the production of crops or animals. These 

activities include the growing and harvesting of crops and animals, modifications of the landscape, application of 

chemicals, and disposal of wastes. Every agricultural activity has the purpose of fulfilling one or more of the crop 

or animal requirements in an economically sustainable fashion. Agricultural activities also are implemented in 

many cases to minimize any effects of agricultural activities on the broader environment.

Agricultural chemicals are associated with both plant and animal agriculture. Many chemicals, such as 

fertilizers, lime, pesticides, hormones, and antimicrobials are purposely used because they directly benefit crops 

or animals. Both manufactured chemicals and manure can be used as fertilizers. Some chemicals are produced by 

the crops or animals as waste by-products. Other chemicals are natural occurring and mobilized by agricultural 

activities. Although eroded soil particles (sediment), microorganisms, and manure are not chemicals, they are 

discussed in this report because they share many of the same sources and environmental transport processes, and 

can cause adverse effects on water quality. 

Agricultural modifications are any agricultural activities that change the landscape, and, therefore, usually 

change the manner in which water moves across the landscape. The goals of agricultural modifications generally 

are to increase crop yield, minimize soil loss, maintain soil fertility, protect crops and animals, and (or) minimize 

adverse effects on the broader environment. Some modifications—such as irrigation, drainage, reservoirs, and 

constructed wetlands—have become a permanent part of the landscape and have substantially changed it. Other 

types of modifications, such as tillage, crop type, and choice of land cover, are frequently revised, and only 

temporarily change the movement of water on the landscape. 

Environmental concerns are the potential effects that agricultural activities can have on ecosystems and people. 

A few broad environmental categories are of general societal importance, but there can be many specific concerns 

within each of these broad categories. In specific areas, at specific times, some concerns are realized by actual 

adverse environmental effects.

Field describes any area where agricultural activities take place. This area could be a cropped field, orchard, 

vineyard, pasture, grazed land, or animal feeding operation.

Sustainability means meeting the needs of the present without compromising the ability of future generations to 

meet their own needs.

Each agriculture type has a unique set of agricultural 

activities. The production of corn is different from the 

production of grapes. The production of cotton is different 

from the production of asparagus. The production of sheep 

is different from the production of eggs. For example, cattle 

farming covers a broad range of agricultural settings (fig. 2.2). 

Near one end of this range are low-density operations like 

grazing and ranching. Small dairies, where cattle are raised 

in combination with row crops, have about the same animal 

density as grazing operations, but these two types of activities 

function and dispose of their wastes differently. At the other 

end of the range are concentrated (high density) animal 

operations—large dairies and beef feedlots—where feed, 

medications, diseases, and waste are managed on a much 

larger scale. Each agriculture type connects to and affects 

water quality differently. 

The early European settlers quickly developed 

commercial agriculture to supply urban areas and provide 

exports. Agriculture expanded to meet the needs and 

expectations of society through an expansion of agricultural 

lands, modification of the landscape, and technological 

advances in mechanization, chemical use, conventional 

breeding and, more recently, genetic engineering that resulted 

in increased production (fig. 2.3). Although each of the 

methods listed above has resulted in increased production 

(Chapter 3), the cumulative effect of this agricultural 

expansion and intensification has led to adverse effects, 

including changes in water quantity, water quality, and the 

health of ecosystems. Some of these effects, such as the 

sedimentation of rivers and other surface waters downstream 

of agricultural areas, are obvious and appear immediately after 

a heavy rain. Other adverse effects, such as the deterioration 
Overview of Agriculture and Water Quality 33

tac11-0566_fig02-02

Head per square

kilometer

0

30

60

200

50,000

200,000

Operation

Crop only

Beef – grazing 

Dairy – small

Dairy – large

Beef – confined feeding (outdoor)

Beef – confined feeding (indoor)

A B

C D

Figure 2.2. Each agriculture type has a unique set of agricultural 

activities. Cattle densities vary for different types of agricultural 

settings in the United States. (A) Crops only (grapes, California); 

(B) grazing (Colorado); (C) small dairy (Iowa); (D) confined 

animal feeding operation (Washington) (Capel and Hopple, 

2018). Photographs A, C, and D by Paul Capel, U.S. Geological 

Survey, 2010. Photograph B by Jeff Vanuga, U.S. Department 

of Agriculture, Natural Resources Conservation Service photo 

gallery, NRCSCO02015, 2002.

of coastal ecosystems caused by the discharge of fertilizerderived nutrients at rates exceeding the capacity of these 

ecosystems to absorb them, may go unnoticed for decades. 

The changes in the ecosystems of Chesapeake Bay and the 

Gulf of Mexico are examples of these latter types of effects.

As part of the management of agricultural systems, 

there are material inputs and outputs on a reoccurring basis 

(daily to annually). Material inputs include water, nutrients, 

feed, pesticides, and chemicals for enhanced crop and animal 

growth. Material outputs include harvested products—crops, 

meat, milk, and eggs. Water for crops comes either from 

precipitation or irrigation. Water is provided to animals 

either indoors (through plumbing) or outdoors (from farm 

ponds, water troughs, natural streams, and other sources). 

Nutrients are applied to crops through fertilizers or manure, 

supplementing the nutrients available in the soil. Food is 

provided to animals as feed (from crops). A by-product of 

life and growth is the generation of waste. Plant waste (crop 

residue) generally is returned to the soil after the plant’s death 

or senescence. Subsequent microbial decomposition returns 

the plant’s chemical building blocks (elements) to the soil. 

Waste from animal agriculture (manure) is often spread on 

the land as a soil amendment, a source of nutrients for crops, 

and (or) as a disposal method (MacDonald and others, 2009). 
34 Agriculture—A River Runs Through It—The Connections Between Agriculture and Water Quality

tac11-0566_fig02-03

U.S. population

Land in farms

Corn yield

0

0.2

0.4

0.6

0.8

1.0

1910 1920 1930 1940 1950 1960 1970 1980 1990 2000 2010

U.S. population, area of land in farms, and 

corn yield, normalized to highest value

Year

EXPLANATION

Figure 2.3. Changes in population, land in farms, and yield of corn 

(for grain) in the United States, 1910–2010. Data are presented 

as normalized values where the maximum value is equal to 1.0. 

Maximum U.S. population—309 million (2010). Maximum area of 

land in farms—488 million hectares (1952–54). Maximum corn 

yield—10,340 kilograms per hectare (2009) (Grey and others, 2012).

Additionally, many types of inputs are used to protect the 

product of interest. Herbicides protect crops against competing 

weeds, insecticides protect crops and animals against insects, 

fungicides protect crops and animals against fungi, and 

antimicrobials protect animals against bacterial diseases. Other 

chemicals, such as lime, are used to improve soil conditions. 

Finally, some chemical inputs are designed to enhance growth. 

For crops, nitrogen is sometimes applied in excess of the basic 

plant requirements to enhance resilience against poor weather 

conditions. For animals, hormones are used to yield faster 

growth and larger animals.

About 54 percent of the land in the conterminous United 

States is used for agriculture. About 13 percent is cropland 

and 41 percent is pasture and grasslands used for grazing 

(Baker and Capel, 2011). Land with sufficient soil and water 

is used for growing crops, whereas more arid grasslands and 

scrublands are more commonly used for grazing. Much of the 

animal agriculture, outside of the arid areas, is interspersed 

within the areas of crop agriculture (fig. 2.1). At each location 

where agriculture activities began, there was an initial one-time 

transition from a natural landscape to an agricultural landscape. 

Before agricultural use of the land began, the landscape 

had a natural vegetative cover and natural water flowpaths 

(movement of water). Water moved through natural landscapes 

in expected ways—referred to as the “water cycle”—

controlled primarily by the local vegetation, soils, topography, 

and the geologic framework. The most important input of water 

is precipitation. The major flowpaths for the removal of water 

include return to the atmosphere by way of evaporation and 

evapotranspiration (through plant respiration), runoff across 

the landscape to streams and other surface-water bodies, and 

recharge through the soil to groundwater and, eventually, 

discharge to streams, lakes, and coastal areas. The relative 

importance of water flowpaths is dependent on the specific 

location on the landscape and season of the year. 

Modifications of the landscape for agriculture often resulted 

in substantial changes in the water flowpaths, rates of water 

movement, and its abundance on the natural landscape. The goals 

of the agricultural landscape modifications are to increase crop 

yield, minimize soil loss, maintain soil fertility, protect crops 

and animals, and (or) minimize adverse effects to the broader 

environment. (The term “modification” is defined here as any 

agricultural activity that changes the landscape, and, therefore, 

usually changes the manner in which water moves across the 

landscape.) Some modifications—such as irrigation, drainage, 

and the construction of reservoirs and wetlands—have become a 

permanent part of the landscape and have substantially changed 

water movement and agricultural abundance. Other types of 

modifications or practices, such as tillage, crop type, and choice 

of land cover, are frequently revised, and only temporarily change 

water movement on the landscape.

Each orchard, field, pasture, range, and animal feeding 

operation has unique landscape characteristics and unique 

water flowpaths. Each water flowpath has a beginning and an 

end, persists for a specific period, with variable intensity (flow 

rate) over time. The resulting water flowpaths from any specific 

agricultural location are the sum of the multiple modifications that 

have been made to the landscape, superimposed over the features 

of the original hydrologic system. Modified flowpaths can contain 

water that has moved from fields, such water can also move 

sediment (eroded soil) and agricultural chemicals from fields and 

farms into the environment, where these materials can interact 

with people and ecosystems, and are a possible source of concern. 

(Wind also can move soil and agricultural chemicals into the 

broader environment through the air.)

The generalized connection among agricultural activities and 

water-quality effects can be thought of as a series of cause and 

effect events. Crops and animals have requirements for growth; 

therefore, agricultural activities are conducted to meet these 

requirements. The water, chemicals, and sediment that move from 

the agricultural areas can have adverse effects on water quality, 

water quantity, and other aspects of the environment. The specific 

environmental effects aggregate and result in broad societal 

concerns about the environmental effects of agriculture (table 2.2). 
Overview of Agriculture and Water Quality 35

The practice of irrigation allows agricultural land use in arid areas 

(Washington). Photograph by Paul Capel, U.S. Geological Survey, 

2010.

Runoff from an almond orchard after a heavy rain (California). 

Photograph by Joseph Domagalski, U.S. Geological Survey, 2004. 

Table 2.2. Environmental concerns from agriculture are grouped in a few broad 

categories of societal concerns. 

[Within each of these broad categories, there are many specific concerns. See “Human and 

Ecosystem Health Effects of Agricultural Chemicals,” for a detailed summary of one of these 

broad categories]

Broad concerns Specific examples

Economic sustainability

Increased treatment for drinking water required

Loss of infrastructure

Increased cost of navigation, dredging

Loss of water storage (reservoir infilling)

Agricultural sustainability

Loss of water for irrigation (because of poor quality)

Loss of soil

Loss of soil fertility

Loss of crop

Loss of livestock (sickness/death)

Loss of aquatic harvest

Weed resistance to herbicides

Aesthetics and recreation

Aesthetics

Fishing

Swimming

Odor

Health (human and ecosystem)

Loss of habitat, loss of species

Change in ecosystem

Human–sickness/death

Animals–sickness/death

Cancer

Developmental/reproduction problems

Antimicrobial resistance

Climate change
36 Agriculture—A River Runs Through It—The Connections Between Agriculture and Water Quality

Agricultural chemicals applied to the land can be carried through precipitation- or irrigation-derived 

flowpaths—either in dissolved form or bound to sediment particles—to the broader environment. The 

chemicals may be transformed into other compounds by natural chemical and (or) biological processes, or 

become concentrated in the water, sediment, and (or) tissues of organisms living within the water. Relatively 

persistent chemicals may be further concentrated in the bodies of predators that feed on contaminated 

organisms living in the water—a process known as biomagnification. In long-lived organisms, such as many 

mammals, fish, and birds, persistent chemicals pose a greater risk of negative health effects over a long life span 

as more of the chemical accumulates in the body tissues of the organism.

The range of deleterious health effects caused by agricultural chemicals is large, and varies with the 

species, sex, and developmental stage of the organism in question. Factors such as dose, length of exposure, 

type of substance, and the physiological response of the organism to the chemical are all important in 

determining the extent of damage. As a result, not all exposures to potentially hazardous chemicals will cause 

harm. Because of similarities in biological systems across species, chemicals that are harmful to one type of 

organism are often harmful to other closely related organisms. By contrast, a particular biological species’ 

unique (or nearly unique) sensitivity to a particular chemical is often the reason for its use as a pesticide. 

All living organisms have systems designed to eliminate wastes and foreign substances. Many 

chemicals that are harmful to organisms have properties that cause them to bypass an organism’s detection or 

detoxification processes. Some chemicals may act to block normal cell function, or spread across membranes 

into adjacent cells, organs, and the circulatory system. Exposure to high concentrations of hazardous chemicals 

may cause a disruption in oxygen or nutrient uptake, birth defects, learning disabilities, early onset of puberty, 

hormonal imbalances, reproductive failure, lowered immunity against infections and a variety of other 

problems (table 2.3). Furthermore, some chemicals can bind directly to DNA, causing mutations that may 

predispose an individual to cancer. Mutations in reproductive cells can be passed on to future generations where 

they may perpetuate negative health effects. One example of variable health effects ranging across different 

groups of organisms is the organochlorine insecticide DDT. Originally thought to be non-toxic to humans, 

DDT was used extensively as an insecticide after World War II (U.S. Environmental Protection Agency, 

2012d). By 1972, because of its biomagnification in predatory birds feeding on contaminated fish—and the 

health disruptions that resulted—DDT was banned in the United States. A notable effect of DDT was thinning 

eggshells and reduced numbers of offspring in bald eagle populations. DDT also causes reproductive failure 

in other species, such as fish and mammals. Additionally, impairment of the nervous, immune, and endocrine 

systems has been linked to DDT, as well as mutations that could lead to cancer (table 2.3).

Human and Ecosystem Health Effects of Agricultural Chemicals
Overview of Agriculture and Water Quality 37

Table 2.3. Selected agricultural chemicals and their health effects for a range of biological species. [From Capel and Hopple (2018). Health effects can be acute or chronic, damaging one or more systems within the organism, or the whole organism. Type of toxicity: A, acute toxicity (refers to the adverse effects of a substance that result either from a single exposure or from multiple exposures in a short space of time); C, chronic toxicity (refers to the adverse effects of a substance that result 

from long-term exposure). Shading: Green denotes a relatively low effect; blue denotes a relatively moderate effect; pink denotes a relatively high effect. Biological species: B, bird; F, fish; H, human; 

I, insect; R, rat. Immune system: A complex and integrated body system of organs, tissues, cells, and cell products such as antibodies that differentiate the body’s cells from foreign objects and protect 

the organism from potentially pathogenic organisms or substances. Endocrine system: Composed of a group of cells and glands that make, store, and secrete hormones into the body that regulate body functions. Developmental/teratogen: Teratogen is a chemical that causes defects in development between conception and birth. Carcinogen: A chemical that causes cancer. LD-50: The lethal 

dose of a material, given all at once, which is expected to cause the death of 50 percent of an experimental population when exposure is by swallowing, through skin contact, or by injection. This one 

measure of the short-term acute toxicity of a chemical. LC-50: The concentration of a chemical in air or water to which exposure for a specific length of time is expected to cause death in 50 percent of 

an experimental population when exposure is through drinking or inhalation. This is one measure of the short-term acute toxicity of a chemical. Phytotoxicity: Describes the degree of toxic effect by a 

compound on plant growth. Abbreviations: L, liter; mg/kg, milligrams per kilogram; mg/L, milligram per liter]

Example

chemical

Type of toxicity

Reproductive 

system

Immune

 system

Respiratory/ 

circulatory 

system

Nervous 

system

Endocrine 

system

Developmental/ 

teratogen

Mutagen

Carcinogen

Rat- oral LD-50

 (mg/kg)

Bluegill 96-hour 

LC-50 (mg/L)

Phytotoxicity

Bioaccumulation

H R F B I H R F B I H R F B I H R F B I H R F B I H R F B I H R F B I H R F B I

Permethrin A 2,280 0.0008

insecticide C

Metolachlor A 2,780 15 (7)

herbicide C

Nitrate A 20.0039 21.21 (7)

fertilizer C

DDT1 A 150 0.0086

insecticide C

Mercury A 326 30.2 (4)

insecticide C

Arsenic A 5763 50.02 (4)

insecticide C

Total ammonia A 60.26–2.97

fertilizer C

Sodium chloride A 3,000 9,627

salinity C

1DDT is 1,1,1-trichloro-2,2-di(4-chlorophenyl)ethane.2LD-50 value is average of 0.0012–0.0066 mg/kg in rats, LC-50 value is average of 0.42–2 mg/L in fish.3Mercuric chloride (inorganic) for rats; methylmercury (organic) for fish.4Some chemicals with mercury-carbon and arsenic-carbon bonds can bioaccumulate.5Arsenate/arsenite (inorganic) for rats; arsenobetaine/arsenocholine/dimethyl arsenic acid (organic) for fish.6pH 8.7Bioaccumulation occurs in vegetation.
38 Agriculture—A River Runs Through It—The Connections Between Agriculture and Water Quality

For some agricultural chemicals, the series of events 

that leads to these effects is straightforward. For example, 

ammonia is oftentimes applied to crops as a nitrogen source 

(fertilizer) and is detected at high concentrations (greater 

than background concentrations) in animal waste. Ammonia 

is acutely toxic to fish at low concentrations (at or near 

background concentrations) (see “Human and Ecosystem 

Health Effects of Agricultural Chemicals”), such that an 

ammonia or manure spill can cause local fish kills (Wilton, 

2002; U.S. Environmental Protection Agency, 2012f). The 

observation of a fish kill causes concerns about the health 

and stability of the aquatic ecosystem. For other chemicals, 

complex cause and effect relations, combined with other 

chemicals and environment processes, result in multiple 

environmental concerns. Nitrogen provides an important 

example of this complexity (see “Nitrogen”).

The magnitude of a water-quality effect is linked to 

the strength of the chemical source (concentration), the 

characteristics of the chemical (for example, toxicity and 

persistence), and the water flowpaths coming from that source. 

When these characteristics and factors are well understood, 

cause-effect relations for an adverse effect can be quantified 

for a specific location over a specific timeframe. Therefore, 

agricultural activities can be implemented to minimize or 

diminish the movement, and thus, the water-quality effect. 

An understanding of these cause-effect relations with the 

understanding of specific flowpaths can help set realistic 

expectations for the effectiveness of an agricultural activity to 

diminish adverse effects on water quality.

Everyone and everything has a connection to the 

environment. The environment is the source of air and water. 

tac110566photoE06

Application of nitrogen before planting (Iowa). Photograph by 

Paul Capel, U.S. Geological Survey, 2008.

It is where people live and recreate. A healthy, sustainable 

environment is critical to the well-being of society. 

Sustainability is defined as meeting the needs of the present 

without compromising the ability of future generations to 

meet their own needs (U.S. Environmental Protection Agency, 

2012i; U.S. Department of Agriculture, National Agricultural 

Library, 2015). Environmental concerns about agriculture arise 

when agricultural activities cause adverse effects on the health 

and (or) ecological sustainability of people and ecosystems. 

These effects are the result of complex interactions not just 

between agriculture and the natural environment (noted 

earlier), but also among agriculture, the environment, and 

choices of human society. 
Overview of Agriculture and Water Quality 39

Nitrogen is one of the elemental building blocks of living organisms and a critical component of amino acids 

and deoxyribonucleic acids (DNA). Nitrogen can be present in many chemical forms in the environment. Nitrogen 

changes back and forth among these different forms through natural biological and chemical reactions. For crop 

production, the natural supply of nitrogen is often supplemented with synthetic fertilizer and manure in the forms of 

nitrate, ammonia, and (or) organic nitrogen (fig. 2.4). 

Nitrogen

Aesthetics

Fishing

Swimming

Odor

Non-tangible needs

Climate change

Economic sustainability

Increased treatment for drinking water required

Loss of infrastructure

Increased cost of navigation, dredging

Loss of water storage (reservoir infilling)

Agricultural sustainability

Loss of water for irrigation (poor quality)

Loss of soil

Loss of soil fertility

Loss of crop

Loss of livestock (sickness/death)

Loss of aquatic harvest

Weed resistance to herbicides

Cancer

Health sustainability

Loss of habitat

Change in ecosystem

Sickness/death - human

Sickness/death - animals

Developmental/reproduction problems

Antimicrobial resistance

Fertilizer

Nitrogen dioxide

(NO2)

Nitrate (NO3

-

)

Nitrite (NO2

-

)

Organic nitrogen

(OrgN)

Nitric oxide (NO)

Nitrous oxide

(N2O)

Nitrogen (N2)

Ammonia (NH3)

Urea

Source Nitrogen form Environmental Perturbation Environmental concern

Greenhouse gas accumulation

Combustion

Harmful algal blooms

Decreased dissolved oxygen

Eutrophication

Acidic precipitation

Figure 2.4. The complex connections among nitrogen from fertilizer and various water-quality and other environmental 

concerns. The lines show the many potential pathways that connect the sources of nitrogen used in agriculture with the 

many different concerns. A realized concern has an adverse effect on economic or agricultural sustainability, aesthetics 

and recreation, human and ecosystem health, or climate change. The nitrogen that is applied as fertilizer, both chemical and 

manure, undergoes transformation through chemical and biological reactions to different forms of nitrogen. Each of these 

forms of nitrogen interacts with the environment in a different way and through different processes. Some forms of nitrogen 

can cause direct adverse effects on the environment (shown by direct lines (red) from a nitrogen form to an environmental 

concern). Some of the nitrogen forms can cause environmental perturbations (changes in the condition of the environment, 

blue lines), which then can produce adverse effects on the environment (Capel and Hopple, 2018).
40 Agriculture—A River Runs Through It—The Connections Between Agriculture and Water Quality

Some forms of nitrogen can cause health problems 

for humans or other organisms (fig. 2.4). Some of these 

problems directly affect the environment. Nitrate can be 

toxic to some livestock—because they are not able to 

metabolize it—as well as to infants younger than 6 months 

of age, in which it can cause methemoglobinemia, or 

“blue-baby” syndrome. Ammonia has a strong odor, 

can cause breathing problems in humans (Agency for 

Toxic Substances and Disease Registry, 2013), and is 

toxic to fish (Russo, 1985). Other concerns indirectly 

affect the environment. For example, nitric oxide can 

react with water in the atmosphere and contribute to 

acidic precipitation that can damage buildings and other 

infrastructure. Nitrous oxide is a potent greenhouse gas 

that traps heat 310 times more effectively than carbon 

dioxide and is one of the key compounds that leads to 

ozone depletion (Ravishankara and others, 2009). Nitrate, 

ammonia, and organic nitrogen, when present in surface 

water, act as fertilizers and increase the growth of aquatic 

plants. This process, known as eutrophication, creates a 

number of environmental concerns (fig. 2.4).

Eutrophication can occur naturally over centuries as 

environmental processes erode rock and organic matter 

and transport them to streams. However, over the past 

5–10 decades, agriculture and other human activities 

have caused eutrophication rates to increase dramatically 

(Dubrovsky and others, 2010). Runoff from fertilized 

fields and livestock areas can contain high concentrations 

of nitrogen, accelerating the eutrophication process. 

Excess nitrogen entering a stream, at rates exceeding 

the capacity of the stream to assimilate it, can disrupt 

normal equilibrium of the stream chemistry, which can 

in turn have many cascading negative effects, sometimes 

permanently disrupting and changing the ecosystem. Many 

streams and rivers flow into lakes, estuaries, and oceans; 

therefore, the excess nitrogen delivered to these water 

bodies may cause eutrophication far from the original 

nitrogen source.

The eutrophication process can cause both short-term 

and long-term effects. The primary effects of excessive 

plant growth can include a reduction in the aesthetic 

value of the water body, unpleasant odors, decreased 

recreational activities, and disruption to navigation. The 

secondary effects of eutrophication can have cascading 

adverse effects on water quality, the food web, and the 

aquatic organisms that it supports, including decreases 

in water clarity, dissolved oxygen concentrations, usable 

habitat, and species diversity. The cascading effects begin 

tac11-0566_photo-E-07

A eutrophicated pond with extensive aquatic plant and 

algal growth, adjacent to agricultural fields (Minnesota). 

Photograph by Paul Capel, U.S. Geological Survey, 2009.

with decreased water clarity, which decreases the depth 

of light penetration, and, consequently, decreases the 

ratio of oxygen to carbon dioxide in the water column. 

With less light penetration, photosynthesis is hindered 

or blocked, resulting in increases in carbon dioxide 

concentrations and decreases in available oxygenated 

habitat. Water bodies are particularly susceptible to this at 

night, when the algae and plants continue to respire, but 

are no longer producing oxygen through photosynthesis. 

As biomass decays in the water column, oxygen also is 

consumed and carbon dioxide is released. This process 

is especially acute for mussels, fish, insect larvae and 

other invertebrates, and other organisms that require 

well-oxygenated water in order to survive (Camargo and 

Alonso, 2006). Some aquatic organisms are able to tolerate 

low oxygen concentrations and reduced light conditions 

and can survive, or even thrive, in the eutrophic water. For 

example, blue-green algae, which typically are responsible 

for the formation of green “scum” on the surfaces of water 

bodies late in the summer, have specialized pigments 

that allow them to photosynthesize under low light 

conditions. Some blue-green algae can release toxins that 

are harmful to aquatic fauna, terrestrial animals, and even 

humans. Under the conditions caused by eutrophication, 

blue-green algae can perpetuate this negative cascade 

accumulation of events, helping change well-oxygenated, 

native aquatic ecosystems, which are more biodiverse but 

may be relatively unproductive, to a condition of higher 

productivity but relatively low species diversity.
Overview of Agriculture and Water Quality 41

Summary

Agriculture in the United States supplies a large part of 

the Nation’s food, feed, and fiber needs. Hundreds of different 

crops and agricultural products are grown each year in the 

United States to meet the needs and expectations of society. 

In response to both societal demands and market forces, 

agriculture has expanded, diversified, modified the landscape, 

and intensified. Crop and animal yields have increased through 

advances in mechanization, genetics, breeding, biotechnology, 

and chemicals. 

For commercial success, each agricultural crop and 

animal requires its own set of agricultural activities—

procedures that fulfill the requirements of the production of 

crops or animals—space, soil, water, nutrients (plants), food 

(animals), protection (from pests and weather), and disposal of 

wastes. Agricultural activities include growing and harvesting 

crops and animals, modifications of the landscape, application 

of chemicals, and disposal of wastes. Many of these activities 

include the application of chemicals to the land—fertilizers 

and soil amendments for crop growth and soil health, 

herbicides for weed control, fungicides for fungus control, and 

insecticides for insect control.

There is considerable public concern—as well as 

scientific evidence—that many agricultural activities adversely 

affect the quality of water, air, and soil, and the overall health 

of the environment. These effects are the result of complex 

interactions among agricultural activities, natural processes, 

societal needs, and market forces—interactions that must be 

understood on a fundamental scientific basis in order to design 

agricultural policies and management approaches that can 

successfully address the sustainability of agriculture and the 

environment. Many of the environmental concerns originate 

from the movement of water, agricultural chemicals, and 

sediment from agricultural areas through water and air. The 

identification and characterization of transport pathways into, 

through, and away from agricultural areas are essential for 

understanding connections and relations between agricultural 

activities and their environmental effects.
42 Agriculture—A River Runs Through It—The Connections Between Agriculture and Water Quality

Agriculture in the Panhandle of Texas

The panhandle of Texas (fig. 1.3), in the southern High Plains, has a semi-arid climate, receives an average of 

only 44 cm of precipitation annually, and has a growing season averaging 185 days each year. The flat landscape 

of the southern High Plains is dotted by thousands of closed-basin depressions. This landscape overlies the vast 

High Plains (Ogallala) aquifer. Prior to settlement, short-grass prairie covered the entire region, which was home to 

bison and other prairie wildlife that thrived on abundant grass resources. The region was (and is) critical habitat for 

migrating birds that travel the Central Flyway. Hunter-gatherer Native Americans (Apache and Comanche) lived 

in the area. Early European explorers referred to the southern High Plains as the Llano Estacado, or staked plain, 

because of the vast featureless vistas. 

With few exceptions, this region of Texas was largely isolated throughout early European settlement, and opened 

to westward expansion about 1875. In 1879, the State of Texas sold 12,000 km2 of land to finance the building of a new 

State Capitol. The new landowners created the XIT Ranch to run cattle until the land could be parceled off. Circa 1885, 

with the introduction of barbed wire, fences were erected to contain longhorn cattle being brought into the area. Within 

the next year, 2,000 km2

 of range were fenced and some 100,000 cattle had been purchased. By the late 1890s, because 

of difficulties posed by droughts, prairie fires, and declining markets, the XIT Ranch began selling off land.

With the attraction of seemingly plentiful groundwater extracted by windmills in the High Plains (Ogallala) 

aquifer, homesteaders began pouring into the region during the early 1900s. Wheat and sorghum were common crops, 

but, with increasing irrigation and improved farming technology, cotton and corn also were planted. By 1930, cotton 

was an important crop. Cattle ranching, while declining in economic importance, remained important in the area. 

With the advent of new drilling and pump technology in the 1930s–1940s, widespread drilling of irrigation wells 

aided agricultural expansion. The development of a large beef cattle feedlot industry in the 1960s reinforced the 

importance of cattle, with dairy cattle also becoming increasingly abundant.

Currently, beef and dairy cattle, as well as wheat, corn, cotton, and sorghum, are important in the panhandle 

region. Water from the Ogallala aquifer is extremely important to agriculture. In many parts of this region, the 

water table is declining and economic concerns for this dwindling resource have led to more research into water 

conservation practices, irrigation efficiencies, development and use of more drought-tolerant crop commodities, 

mechanisms of recharge, as well as contamination from agricultural chemicals (fertilizers and pesticides) (Gurdak and 

others, 2009; McGuire, 2009). Other recent USGS studies on the evaluation of effects of agricultural activities and 

natural processes on groundwater quality in the southern High Plains include Gurdak and Qi (2006), McMahon and 

others (2006), Stanton and Fahlquist (2006), and McMahon and others (2007).

A B C

The landscape of the Western Panhandle of Texas. (A) The short-grass prairie with a shallow, intermittent 

salt lake typical of the time before agriculture, (B) sorghum field, and (C) the view from the air (23 square 

kilometers). Photograph A by Wyman Meinzer, U.S. Fish and Wildlife Service, Muleshoe National Wildlife 

Refuge, 2008; photograph B from U.S. Department of Agriculture, Natural Resources Conservation Service, 

Texas, 2010; photograph C from U.S. Department of Agriculture, Farm Service Agency, National Agriculture 

Imagery Program, 2010.
Changes in the Nation’s Agriculture Over Time 43

3 Chapter —Changes in the Nation’s Agriculture Over Time 

How are Past and Future Changes in Agriculture 

Connected to Water Quality and Water Quantity? 

Native American Indians began farming the North 

American continent as early as 5,000 BCE. The native people 

planted seed from the most productive wild plants, and with 

many years of this selective practice created crop varieties that 

were adapted to diverse environments from the cool northern 

Great Plains to the hot, dry Southwest. Their farming system 

was based on corn, beans, and squash, although various other 

crops (tobacco, sunflower, and potatoes) also were grown 

(Hurt, 2002).

Agriculture for European settlers began in the 1600s 

(fig. 3.1). Although they adopted corn into their agriculture, 

the settlers largely retained their traditional European farming 

practices in raising grains, apples, tobacco, and livestock. 

Little attention was given to crop rotation, fertilization, or 

proper tillage practices. During the latter part of the 1700s, 

agriculture expanded across the Appalachians and along 

the Gulf Coast. Commercial production included cotton 

exports and an expanding food market, which thrived along 

rivers where farmers could easily ship produce to market. 

In the southwestern areas of the country, Spanish colonists 

established agriculture using irrigation for cultivation of 

grapes, fruits, vegetables, and grains, and also grazed cattle 

(Hurt, 2002).

The Louisiana Purchase and the “Indian Removal Act” 

(early 1800s) rapidly extended westward settlement, opening 

new opportunities for agricultural and economic development 

(fig. 3.2). Land with sufficient rainfall was used for crops; 

arid grasslands and scrublands were used for grazing. New 

technology, such as subsurface drains (tiles) and Government 

support for the conversion of wetlands to farmland, opened 

vast expanses of fertile land, particularly in the Midwest 

(fig. 3.3) (Dahl and Allord, 1996). The “Homestead Act” 

further encouraged westward development of agriculture by 

giving out free land on the (arid) Great Plains. By the mid1800s, successful agriculture developed in the Northwest, 

such as potatoes in Idaho and fruit and vegetables in Oregon. 

By 1910, irrigation allowed large-scale fruit and vegetable 

agriculture in the Central and Imperial Valleys of California, 

which became some of the most productive agricultural land 

in the world.

In the early 1900s, the Nation realized that if agricultural 

production was to be successful on the Great Plains and 

throughout the West, then some of the land would have to 

be irrigated. The National Reclamation Act of 1902 (Public 

Law 57-161; 32 Stat. 388) provided funding for irrigation 

projects designed to “reclaim” arid lands in 17 Western States. 

Initially, five projects were constructed in 1903: Milk River 

in Montana, Truckee River in Nevada, North Platte River 

in Nebraska and Wyoming, Salt River in Arizona, and the 

Gunnison River in Colorado. Later projects included Hoover 

Dam in Nevada and Grand Coulee Dam in Washington. More 

than 180 irrigation projects resulted from the Reclamation 

Act, which brought water to 40,000 km2

 of farmland and made 

large-scale agriculture possible in much of the West. Many of 

these projects have had adverse environmental impacts since 

their construction (Bureau of Reclamation, 2011).

The early 1920s brought a rapid expansion of 

agriculture largely due to advances in mechanization with 

new tractors and combines. Availability of trucks allowed 

for the harvest to be moved more easily. Overproduction 

led to agricultural surpluses and a collapse of agricultural 

prices, and the beginning of an agricultural depression that 

lasted almost two decades. The removal of native plant cover 

and overproduction, combined with severe droughts and 

extraordinary heat in the 1930s, eventually led to the “Dust 

Bowl” in widespread areas of the Great Plains. The topsoil on 

the barren fields literally blew away with strong prairie winds. 

Without topsoil, the land was useless for growing crops or 

grazing. The Dust Bowl was one of the defining moments in 

the Nation’s agriculture. In response, Congress passed laws to 

facilitate soil conservation and assist agricultural development. 

The Dust Bowl, together with the Great Depression, left the 

farm economy in turmoil and led to greater Government 

involvement. Since then, conservation has been intertwined 

with agricultural production. 

During the last half of the 20th century, the extent of 

agricultural land did not substantially change (compare 

fig. 3.2 for 1949 and 2002), although agricultural activity 

became more intensive on much of the existing farmland. 

Mechanization of farming helped reduce the agricultural labor 

force by 50 percent between 1950 and 1970 (Conkin, 2008). 

(See “Agriculture in the “Delta” region of Mississippi,” in 

Chapter 6, as an example of the changes in farm labor.) By 

the 1970s, the USDA pushed the idea of “get big or get out” 

and planting “fence row to fence row” to encourage full 

production of farms to feed the Nation and increase exports 

(Anderson and Jansen, 2010). Small farms were consolidated 

(fig. 3.4), and the major gains in productivity occurred on 

large farms. 
44 Agriculture—A River Runs Through It—The Connections Between Agriculture and Water Quality

tac11-0566_fig03-01

Year

Corn, cotton, potatoes and tobacco 

domesticated by Indians in South and North 

America

Early 1600s European colonists learn 

agricultural practices from Indians

National landscape undergoes large scale 

deforestation and drainage of wetlands (1700s 

to early 1900s)

First steam engine locomotive in the United 

States by Peter Cooper (New York)

Growing use of factory-made agricultural 

machinery increased farmers' need for cash 

and encouraged commercial farming (1840s)

United States Reclamation Service (precursor 

to the Bureau of Reclamation) was established 

and aided in the development of many large 

agricultural irrigation and drainage projects

Passage of the First National Pesticide Act 

regulated the composition of pesticides sold to 

consumers

Manufacturing of synthetic ammonia (HaberBosch Process) begins in the United States

Expansion of agriculture westward as Eastern 

farm soils become exhausted

Growing market for factories producing 

bone and lime for agricultural supplements

Formation of the U. S. Department of Agriculture 

and the passage of the Morrill Act, which 

established land for agricultural colleges

Completion of Transcontinental Railroad

Information on plant genetics, proposed by 

Gregor Mendel, is used to accelerate plant 

breeding

Beginning of the series of droughts coined the 

"Dust Bowl"

Agricultural Adjustment Act of 1933 established 

the Commodity Credit Corporation to stabilize, 

support and protect farm prices and supplies

Passage of the Soil Conservation and Domestic 

Allotment Act

Passage of the Rural Electrification 

Administration Act funds to aid establishment of 

reliable, affordable electricity in rural areas

Passage of Agricultural Act of 1954 authorized 

a Commodity Credit Corporation reserve for 

foreign and domestic relief

Passage of Agricultural Act of 1956 established 

the Soil Bank Program to reduce crop surpluses 

by taking cropland most subject to erosion out of 

production for up to 10 years

Publication of Rachel Carson's Silent Spring 

brings public attention to the environmental 

effects of pesticides 

Green Revolution, promoting high yield 

agriculture from improved crop hybrids and 

fertilizer supplements is led by Norman Borlaug

World War II helps shift farming from animal to 

mechanical power

Invention of center-pivot irrigation by Frank 

Zybach (Nebraska)

Formation of the Environmental Protection 

Agency, a consolidation of several federal 

agencies previously dealing with environmental 

issues 

Passage of the Clean Water Act (CWA) to 

restore and maintain the integrity of the Nation’s 

waterways through prevention of point and 

non-point pollution sources

Passage of Federal Environmental Pesticide 

Control Act classified pesticides as "general" or 

"restricted" use and required certification for 

restricted use

U. S. Supreme Court rules that geneticallymodified life forms can be patented

Glyphosate (herbicide) is registered for 

commercial use

Advances in genetics (recombinant gene 

technology) allows for mass production of 

synthetic hormones for additives in animal feed

97 percent of farms have electricity

Mechanization of farming helps reduce the 

agricultural labor force by one-half (1950-1970)

Improvements to no-till planting equipment 

increases the number of farmers converting to 

conservation tillage

Switch from open-pollinated towards hybrid 

corn

Average farm size increases and there are 

fewer farms (1950s-1970s)

Confined animal feeding operations (CAFOs) are 

used for livestock

USDA pushes the idea of "plant hedgerow to 

hedgerow" and "get big or get out" encouraging 

full production of farm acreage

Specialization and intensification of farming 

leads to increased numbers of confined animal 

feeding operations (CAFOs)

Rural Clean Water Program establishes 

federally sponsored program to address 

agricultural non-point source pollution

Formation of the Conservation Reserve Program 

(CRP) volunteer program providing cost sharing 

for removing environmentally sensitive land 

from agricultural production (1985 Farm Bill) 

EPA finds 74 different pesticides in the 

groundwater of 38 states

Formation of the Wetland Reserve Program 

(WRP) establishes long-term (30-year to 

permanent) contracts with landowners 

conserving wetlands (1990 Farm Bill)

Food and Drug Administration declares that 

genetically-modified foods are "not 

inherently dangerous" and do not require 

special regulation

Passage of Food Quality Protection Act (FQPA) 

fundamentally changes the way EPA regulates 

pesticides, required a complete re-assessment 

of pesticide tolerances

Formation of the Environmental Quality 

Incentives Program (EQIP) provides technical, 

financial, and educational assistance for a 

wide range of agri-environmental activities

Widespread use of genetically engineered 

glyphosate-tolerant crops contribute to the 

rise in "no-till" tillage practices

Increases in numbers of organic farms 

(1990s)

Passage of the Harmful Algal Bloom and 

Hypoxia Research and Control Act requires a 

plan for controlling hypoxia in the northern 

Gulf of Mexico

Development of the Unified National Strategy 

for Animal Feeding Operations outlines plan 

using existing legal regulatory authority to 

reduce water quality and public health impacts 

from improperly managed animal wastes

Bacillus thuringiensis (Bt) plant-incorporated 

biopesticides (insecticide) are registered for 

use by EPA

Implementation of organic labeling system by 

the USDA 

Passage of the Farm Security and Rural 

Investment Act of 2002 creates a National 

Dairy Program, also significantly increases 

spending for conservation programs, creating 

the Conservation Security Program and the 

Grasslands Reserve Program

Passage of the Energy Policy Act of 2005 

supports increased use of biofuels

Crop agriculture is the largest producer of 

nitrous oxide (greenhouse gas), 70% of 

annual emissions (2008)

Fifteen weeds have developed resistance to 

glyphosate (herbicide) in the United States 

(1996-2010)

Wind farms generate electricity in 31 states

EXPLANATION

Note:

USDA: U. S. Department of Agriculture

EPA: U. S. Environmental Protection Agency

FDA: U. S. Food and Drug Administration

Crop (landscape) and animal changes 

Biological and chemical changes 

Mechanical changes

Regulatory and societal changes 

1500

1600

1700

1830

1840

1850

1860

1862

1869

1903

1900

1910

1921

1930

1932

1933

1935

1939

1941

1947

1950

1954

1956

1960

1960

1961

1962

1968

1970

1970

1971

1972

1975

1979

1979

1980

1980

1989

1982

1985

1990

1990

1992

1996

1996

1996

1998

1999

2001

2002

2002

2005

2008

2008

2010

Figure 3.1. Important events in the changing agriculture of the Nation that could be related to water quality and quantity (Grey and 

others, 2012). For further information on the history of agriculture in the United States, see U.S. Department of Agriculture, Agricultural 

Research Service (2008) and Spielmaker (2012). 
Changes in the Nation’s Agriculture Over Time 45

EXPLANATION

A. 1790

Proportion of total county area in improved farmland, in percent

>1 to 5

>5 to10

Population density, in people per square kilometer

1 

Not enumerated or zero >10 to 20

>20 to 30

>30

EXPLANATION

>10 to 20

1 to 10 >20 to 40

Not enumerated or zero >40 to 60

>60 to 80

>80

C. 1850

E. 1949

B. 1830 

D. 1900

F. 2002

tac11-0566fig03-02

Figure 3.2. Expansion of agricultural lands in the United States over time. Population density is used as a surrogate for agricultural 

lands in (A) 1790 and (B) 1830. Data for actual agricultural lands are not available before 1850. Agricultural lands as a percentage of 

county area in (C) 1850, (D) 1900, (E) 1949, and (F) 2002 (Baker and Capel, 2011).
46 Agriculture—A River Runs Through It—The Connections Between Agriculture and Water Quality

tac11-0566_fig03-03

A B

C D

Figure 3.3. Agricultural land was expanded through drainage and irrigation. (A) Hand dug subsurface drains in Wisconsin, 1916; (B) 

steam engine digging surface drains in southern Minnesota, 1922 (see “Big Ditcher Quickly Turns Swamps Into Farms”); (C) construction 

of irrigation canals in Washington circa 1880; (D) construction of Grand Coulee Dam, Washington, 1941 (for hydroelectric power and 

irrigation). Photograph A courtesy of the University of Wisconsin Archives; photograph B by Paul E. Walline, courtesy of Michael A. 

Johnson; photograph C courtesy of the Yakima County Museum, Washington; and photograph D from Bureau of Land Management.

Land in farms

Number of farms

Average size of farm

1910 1920 1930 1940 1950 1960 1970 1980 1990 2000 2010

Year

0.2

0.4

0.6

0.8

1.0

Number of farms, average size of farms, and area of 

land in farms, normalized to highest value

EXPLANATION

Figure 3.4. Changes in number of farms, average size of 

farms, and area of land in farms in the United States from 1910 

to 2010. Data are presented as normalized values, where the 

maximum value is equal to 1. Maximum land area in farms 

equals 488 million hectares (1952–54). Maximum number 

of farms equals 6 million (1910–50). Maximum average size 

of farm equals 188 hectares (1991–92). Intensification of 

agriculture after the 1950s led to fewer and larger farms. 

Land area in farms has slightly decreased, partly due to 

improvements in agricultural technologies, which has led to 

higher yields (Grey and others, 2012).
Changes in the Nation’s Agriculture Over Time 47

“Big Ditcher Quickly Turns 

Swamps Into Farms”

From Popular Mechanics, March 1922 (Windsor, 1922): 

“The immense scale on which reclamation 

of swamp lands in Freeborn and Mower 

counties, in southern Minnesota, is carried on, 

means that, within a short time, great tracts 

will be added to the productive lands of the 

state. Work is being concentrated on a tract of 

15,000 acres, which has been subdivided into 

eight units of from 1,000 to 3,000 acres each. 

Each unit is in turn subdivided into farms, most 

of which contain either 80 or 120 acres.

“The chief agent in this reclamation work 

is a wheel excavator of unusual design and 

capacity. This 76-ton ditching machine crawls 

steadily ahead like some great land monster, 

and as it advances it digs a ditch that is 12 ft. 

wide on top and 7 ft. deep. A huge revolving 

wheel scoops out the dirt and a conveyor 

belt is kept busy carrying it out 20 ft. to the 

side where it builds up a spoil bank. Working 

steadily with a crew of five men, the machine, 

which is 65 ft. long overall and has a 110 hp 

engine can drive a ditch ahead at the rate of 

about a quarter mile in a working day. 

“After the tile drains are laid, the sides of the 

ditches are thrown in to cover the tile. This 

work is done with a grading machine drawn by 

a tractor. Submains and laterals are added after 

the main drains have been laid. Then follows 

the building of bridges, roads, and groups of 

farm buildings. Work is done on a big scale, 

dozens of teams, workmen, and tractors being 

employed. Artesian wells are driven, fences 

built, and in short time the marshlands have 

been changed into highly productive farms.” 

The increased efficiency of mechanized farms, 

combined with the use of fertilizers and advances in 

crop genetics, allowed farmers to produce large excesses 

of food, which required new markets (cattle feeding 

operations and international exports). Some agricultural 

land was removed from production for conservation 

purposes, to reduce excess production, and allow for 

expansion of urban areas, resulting in nearly 15 percent 

decrease in domestic cropland from 1949 to 2007 

(1.93–1.65 million km2

) (Nickerson and others, 2011).

Crop and Animal Changes.—Crops primarily 

provide food for people, feed for animals, fibers for 

clothing, and biomass for fuel. Early subsistence farming 

gave way to commercial farms that produced for distant 

markets. Although agriculture in the United States had 

always been diverse, throughout much of American 

history the major crops have been corn (predominantly 

for animal feed), wheat (for human food), oats (for horse 

feed), cotton (for fiber), and more recently soybean (for 

feed and food) (fig. 3.5). The soybean was first adopted 

as a commercial crop in the early 1930s and have become 

the second largest crop in the Nation. Before World War 

II, a variety of crops, including legumes like clover, were 

planted in rotation to replenish the soil nitrogen and avoid 

nutrient deficiency of the soil.

0

10

20

30

40

50

Millions of hectares planted

Corn

Soybean

Wheat

Cotton Oats

1910 1920 1930 1940 1950 1960 1970 1980 1990 2000 2010

Year

Figure 3.5. Changes in corn, soybean, wheat, oats, and 

cotton planted in the United States from 1910 to 2010 

(Grey and others, 2012).
48 Agriculture—A River Runs Through It—The Connections Between Agriculture and Water Quality

Technological advances in the 20th century changed the 

face of agriculture, leading to dramatic increases in crop yields 

starting in the 1930s (fig. 3.6, table 3.1). Hybrid varieties 

were first developed for corn and widely adopted after the 

Great Depression. Building upon the success of corn, hybrids 

for other crops were developed for the United States and 

the world during the 1950s and 1960s; these advances were 

often referred to as the “Green Revolution” (Borlaug, 1972). 

Compared to crops produced through open pollination, the 

hybrids gave increased yield and had attractive characteristics 

such as enhanced disease resistance or drought tolerance. 

Beginning in the 1940s, the availability of synthetic nitrogen 

fertilizers and herbicides allowed for the intensification of crop 

agriculture. Table 3.1 shows corn as an example. Altogether, 

these advances dramatically increased crop yields (fig. 3.6) so 

that, starting in the mid-1950s, grain surpluses were common. 

Beginning in the mid-1990s, genetically modified crops 

that were embedded with herbicide resistance, insecticidal 

properties, or other desirable characteristics were introduced. 

The majority of the corn, soybean, cotton, and sugar beet 

crops that are planted today are genetically modified varieties 

(see “Genetically Modified Crops”).

0

1,000

2,000

3,000

4,000

5,000

0

10,000

20,000

30,000

40,000

50,000

1910 1920 1930 1940 1950 1960 1970 1980 1990 2000 2010

Yields of soybean, wheat, oats, and cotton,

 in kilograms per hectare 

Yields of corn for grain and corn for silage, 

in kilograms per hectare

Year

Corn for grain

Corn for silage

Soybean

Wheat

Oats

Cotton

EXPLANATION

Figure 3.6. Average yields of corn for grain and corn for silage, soybean, wheat, 

oats, and cotton in the United States from 1910 to 2010 (Grey and others, 2012).
Changes in the Nation’s Agriculture Over Time 49

Table 3.1. Changes in corn production in the United States from 1800s to 2009. [Data source: Grey and others (2012). Abbreviations: $, dollars; kg, kilogram; ft, foot; in., inch; %, percent; –, not applicable]

Circa 1800 Circa 1850 Circa 1900 1950 2000 2009

Planted area: all corn 

(hectares)

– – 40,300,000 33,600,000 32,200,000 35,900,000

Harvested area: corn 

for grain (hectares)

– 12,100,000 38,400,000 29,300,000 29,300,000 32,800,000

Harvested area: 

corn for silage (hectares)

– – 1,440,000 1,990,000 2,460,000 2,270,000

Production: corn for 

grain (kg)

– 18,600,000,000 67,600,000,000 70,200,000,000 251,000,000,000 334,000,000,000

Production: corn for 

grain ($)

– – – 4,222,366,000 18,499,002,000 48,588,665,000

Production: corn for 

silage (metric tons)

– – 24,400,000 37,200,000 92,700,000 98,200,000

Yield: corn for grain 

(kg/hectare)

– 1,525 1,764 2,398 8,592 10,199

Yield: silage (metric 

ton/hectare)

– – 17 19 38 43

Seed Open pollination Open pollination Open pollination

(hybrids emerging)

Hybrid Genetically modified 

(29%), herbicide (glyphosate) tolerant (5%), insect (Bt) resistant (18%)

Genetically modified (91%), herbicide (glyphosate) tolerant (82%), insect (Bt) resistant (69%)

Plant spacing Mounds 3–6 ft (spacing 

in rows or scattered) 

40–44 in. rows (horse 

width)

20–40 in. (experimental 

new widths)

36–42 in. (adjustable to 

tractor width)

30 in. or less 20 in. 

Rotation Corn, beans, squash Corn, wheat/rye/oats/

barley/turnips, clover, fallow (4–5 year rotation)

Corn, legume (pea, clover, 

alfalfa) (variable year rotation)

Corn, oats, soybean, 

alfalfa (4–5 year rotation)

Corn, soybean/wheat 

(2–3 year rotation)

Corn, soybean/wheat (2–3 year rotation)

Soil tillage Moldboard plow 

(horse power)

Steel plow 

(horse power)

Steel plow (tractor power) Conservation tillage 

with herbicides

Conservation tillage with 

herbicides

Conservation tillage with 

herbicides

Nitrogen fertilizer: 

source

Manure, fish remains Guano, manure Chemical (from atmospheric 

nitrogen through HaberBosch process, manure)

Chemical, manure Chemical, manure Chemical, manure

Nitrogen average

U.S. application rate–selected states (kg/hectare)

– – – 65 152 140

Phosphorus fertilizer: 

source

Mineralized bone 

material, manure

Phosphate rock, coprolites 

(fossilized animal fecal matter), manure

Phosphate rock, manure Phosphate rock, 

manure

Phosphate rock, manure Phosphate rock, manure

Phosphorus average 

U.S. application rate–selected States (kg/hectare)

– – – 46 64 60
50 Agriculture—A River Runs Through It—The Connections Between Agriculture and Water Quality

Genetically Modified Crops

Conventional breeding methods involve the hybridization of crops or animals to enhance or combine 

desirable traits. Such techniques have been used for thousands of years to improve crop yield and resilience. In 

recent decades, molecular genetics technology—involving such methods as gene cloning, protein engineering, 

and DNA-strand insertion—has been used to create genetically modified (GM) crops in which a broad range of 

desirable traits have been enhanced or introduced. By far, the most widely used applications of GM technology, 

however, have involved the engineering of crops to be resistant to specific herbicides, to impart insecticidal 

properties to the plant itself, or both, called “stacked traits” (National Research Council, 2010). Other aspects 

of genetic engineering research focus on methods to increase crop yields, biomass production, or growth rates; 

increase tolerance to stress (from drought, cold, or salinity); delay ripening; increase oil content; or produce 

chemical substances for use as pharmaceuticals.

Although GM crops have been grown in a wide variety of locations around the world since 1996, the United 

States has been the world leader in implementing these technologies (Committee on the Impact of Biotechnology 

on Farm-Level Economics and Sustainability, National Research Council, 2010). GM tomatoes were introduced in 

1994 as the first commercial GM crop. At present, GM cotton, soybean, and corn—first introduced in 1995, 1996, 

and 1997, respectively—are the most widely planted GM crops in the United States. GM versions of these crops 

have largely replaced most of the conventional varieties throughout the country (fig. 3.7). GM canola, papaya, 

sugar beets, and alfalfa (commercialized in 1995, 1999, 2005, and 2011, respectively) also have been introduced to 

American agriculture.

0

20

40

60

80

100

1996 1998 2000 2002 2004 2006 2008 2010

Year

Percentage of total corn, cotton or soybean

planted with a genetic modification

Corn

Cotton

Soybean

A B

EXPLANATION

Corn

Cotton

Soybean

1 to 25

> 25 to 75

> 75 to 90

> 90 to 98

> 98

EXPLANATION

Percentage of total crop (2009) 

that is genetically modified to 

be glyphosate tolerant mapped 

by crop reporting district

Figure 3.7. (A) Genetically modified corn, cotton, and soybean harvested in United States from 1996 to 2010. 

(B) Percentage of total corn, cotton, and soybean (2009) that is genetically modified to be glyphosate tolerant mapped 

by crop reporting district (Grey and others, 2012).
Changes in the Nation’s Agriculture Over Time 51

Herbicide-tolerant GM crops are designed to survive treatment with a single, broad-spectrum herbicide such 

as glyphosate, glufosinate, or imazethapyr. As of 2009, glyphosate-resistant GM crops comprised about 85, 82, 

and 89 percent of all GM cotton, corn, and soybean, respectively, in the United States (Grey and others, 2012). 

These and other herbicide-tolerant GM crops were produced to save the grower time and money, narrow the 

variety of conventional herbicides used, and reduce soil erosion by diminishing or eliminating the need to till the 

soil. Glyphosate was first registered for use in the United States in 1974, and the first glyphosate-resistant crop 

was introduced in 1996 (Monsanto Company, 2013). Glyphosate is less persistent in the environment than many 

conventional herbicides. 

As a result of the widespread use of glyphosate on both GM and non-GM crops, several weed species have 

developed resistance to the herbicide in the United States and around the world. In 1998, the first glyphosateresistant weed was reported in the United States. As of 2015, 35 weed species are known to be resistant to the 

herbicide (Heap, 2016). In response to this important challenge, GM crops resistant to herbicides other than 

glyphosate have been introduced. Concerns have arisen about the ability of glyphosate to chelate with some 

metals (manganese, zinc), which can result in micronutrient deficiencies in glyphosate-resistant crops. The 

cultivation of GM crops have been associated with reduced abundance and biodiversity of several invertebrate 

species (Bohan and others, 2005). 

Insect-resistant GM crops have a gene from the naturally occurring soil bacterium Bacillus thuringiensis

(Bt

) inserted into their DNA. The Bt bacterium produces a protein that is activated in the gut of susceptible 

insects to produce a toxin specific to those insects. In Bt crops, the ability to make this toxic protein has been 

transferred to the entire plant. Because the protein must be ingested to be activated, and is specific to the target 

insect, the protein is relatively non-toxic to mammals and other non-target organisms. The cultivation of insectresistant GM crops offers the potential to save the grower time and money, and has led to reductions in the 

amount and variety of conventional insecticides used (see section, “Changes in Conventional Pesticide Use Due 

to Development of Genetically Modified Crops,” Chapter 8). However, because of the continuous exposure of 

insects to the Bt toxin, the U.S. Environmental Protection Agency (EPA) mandated that non-Bt crops be planted 

among or adjacent to areas planted with Bt crops, in order to avoid (or slow) the development of Bt resistance 

among these insect species (National Research Council, 2010). It has been observed that non-Bt crops planted 

near high-density Bt cropped areas receive the same benefits (higher yields) as do the Bt crops, owing to regionwide reductions in the populations of the target insects (Hutchison and others, 2010). 

Resistance of the cotton bollworm to Bt cotton between 2003 and 2006 was documented in Mississippi and 

Arkansas (Tabashnik and others, 2008), but current numbers of Bt-resistant insects are not well documented. 

Some environmental concerns have become evident from the cultivation of Bt crops, including harm to beneficial 

insects, gene transfer to native plant species, and allergenic effects of the toxin. Bt crop residue has been reported 

in streams near agriculture fields, but the environmental effects of this phenomenon remain unclear 

(Rosi-Marshall and others, 2007).
52 Agriculture—A River Runs Through It—The Connections Between Agriculture and Water Quality

Animals have always been an important part of the 

Nation’s agriculture. Through much of American history, most 

farms raised animals for family food, for market, and to help 

with farm work. Horses were used to pull the machinery for 

tilling, planting, and harvesting. By the 1950s, horses had 

largely been replaced by self-propelled tractors (Gardner, 

2002). This led to a decrease in the numbers of work horses, 

and thus a major decrease in the production of oats (fig. 3.5). 

Livestock breeding developed more productive animals. 

Average milk production, per cow, increased 4.4 times 

between 1944 and 2007 (Capper and others, 2009). 

The surpluses of grain gave rise to the development 

of cattle feeding operations in the 1960s, enabling the 

increased production of relatively inexpensive meat (Ebeling, 

1979). Electricity allowed for mechanization of beef and 

dairy operations as well as the heating and cooling of 

enclosed animal facilities, which changed the production of 

hogs, poultry, and dairy cattle (Conkin, 2008). The use of 

antimicrobials allowed the expansion of feeding operations 

to hogs, chickens, and turkeys by controlling the spread 

of disease under the concentrated conditions in which the 

animals were kept (Khachatourians, 1998; U.S. Environmental 

Protection Agency, 2012c). As a result, animal agriculture 

moved from being highly dispersed across the landscape to 

more concentrated areas (fig. 2.1B).

Mechanical Changes.—The Nation’s agriculture started 

largely as subsistence farming with horse-drawn, single-row 

plows. The mechanical inventions of the past two centuries 

have transformed agriculture into large-scale, commercial 

ventures with self-propelled, air-conditioned, satellitepositioned tractors that can plant up to 24 rows of crops at one 

time. In the early to mid-1800s, inventions like the iron plow, 

interchangeable parts, mechanical harvesters, and the cotton 

gin transformed agriculture. Railroads brought agriculture 

products to market. Near the end of the 19th century, the 

invention of steam-powered machines opened the way for 

drainage and irrigation projects throughout the Midwest and 

West. The subsequent development of internal combustion 

engine technology led to the invention self-propelled tractors 

and harvesters (White, 2008).

The period between 1930 and 1970 saw dramatic 

growth in the Nation’s agriculture through a combination of 

advances in mechanization, electrification, chemistry, and 

genetics (Conkin, 2008). Horses were largely replaced by 

tractors (White, 2008). Combines were introduced in 1953, 

and by 1970, almost all corn in the Midwest was harvested 

and processed in the field. Electrification of rural areas was 

largely completed (97 percent of all farms) by the mid-1950s. 

Electrical power, together with the invention of center-pivot 

irrigation, facilitated the rapid expansion of irrigation. 

In the past few decades, changes in farm machinery have 

largely been directed toward making existing approaches more 

efficient and precise. New machines were developed to allow 

efficient no-till planting. Computer and satellite technology 

has been incorporated into farm tractors for precision 

agriculture so that crop yields can be measured continuously 

across the field. 

Biological and Chemical Changes.—Nutrients are 

the elemental building blocks upon which crops depend 

for successful growth. Once the nutrients were depleted 

from the soil, crop yields decreased. For centuries, nutrient 

requirements were fulfilled through crop rotations that 

included fallow years and, to a lesser extent, through the 

application of manure. In the 19th century, the major crop 

nutrients were identified as phosphorus, potassium, and 

nitrogen. Phosphorus was available as a natural mineral, 

whereas potassium was available from processed wood ashes 

or from other natural minerals. By the 1880s, both were 

marketed to farmers to increase crop yields. Nitrogen was 

the nutrient that was not readily available until 1913, when 

ammonia was first synthesized from atmospheric nitrogen 

and methane using the Haber-Bosch process (Galloway and 

Cowling, 2002). It was not until the 1940s, however, that 

nitrogen fertilizer (in the form of ammonia nitrate) became 

widely available for agriculture. Use of the chemical nitrogen 

fertilizer was quickly accepted, and steadily increased for 

decades (fig. 3.8). This led to increased crop yields (fig. 3.6, 

table 3.1). The ability to add all three major nutrients to soil 

on an annual basis allowed for the intensification of crop 

agriculture during the following decades.

In addition to fertilization, crops are protected from 

weeds, insects, fungi, and other unwanted organisms. 

Traditionally, crop rotations and cultivation were used to 

control weeds in agricultural fields. This changed with the 

development of synthetic chemical herbicides. In the 1940s 

2,4-D and 2,4,5-T were introduced as the first herbicides. 

Early success of these herbicides led to the development 

of other chemical classes of herbicides, and an increasing 

dependence on herbicides over the following decades 

(fig. 3.8). As a result, 242,000 metric tons of herbicides (and 

plant growth regulators) were used in crop agriculture in 2009; 

61 different herbicides were used in quantities of greater than 

100 megagrams per year (Mg/yr; Grey and others, 2012). 

The widespread use of herbicides eliminated the need for 

routine cultivation of many crops. However, unwanted plants 

can develop resistance to herbicides, a phenomenon that has 

required the ongoing development and use of new herbicides 

to control the growth of weeds that develop such tolerance. 
Changes in the Nation’s Agriculture Over Time 53

tac11-0566_fig03-08

0

200

100

300

400

500

600

0

2,000

4,000

6,000

8,000

10,000

12,000

1940 1950 1960 1970 1980 1990 2000 2010

Total pesticides applied, in thousands of megagrams

Nitrogen (N) and phosphorus (P) applied, in thousands of megagrams

EXPLANATION

 USGS

 USDA

Nitrogen from fertilizer, as N

Nitrogen from manure, as N, USDA

USGS

USDA

Phosphorus from fertilizer, as P

Phosphorus from manure, as P, USDA

USGS

EPA

Total pesticides (active ingredient)

Figure 3.8. National applications of nitrogen, phosphorus, and total pesticides in crop agriculture in the United States from 

1940 to 2010 (Grey and others, 2012). EPA, U.S. Environmental Protection Agency; USDA, U.S. Department of Agriculture; 

USGS, U.S. Geological Survey.

In the 1970s, glyphosate was introduced as a broad-spectrum 

herbicide that could control almost all green plants. Through 

the use of genetic engineering, tolerance to glyphosate was 

incorporated into the biochemistry of crops (see “Genetically 

Modified Crops”). Glyphosate is the most extensively used 

pesticide in the United States (Grube and others, 2011). 

In recent years, many crop varieties have been genetically 

modified to incorporate multiple traits, including tolerance to 

more than one herbicide. 

With the rise of intensified agriculture, much of farming 

relies on chemicals for controlling crop damage from 

unwanted insects. In the 19th and early 20th centuries, toxic 

metals (especially lead, arsenic, and mercury) were used to a 

limited extent for this purpose. After World War II, synthetic 

insecticides, such as the organochlorines (for example, 

DDT) and organophosphates (for example, parathion), were 

introduced and widely accepted. In the subsequent decades, 

a variety of other insecticides were developed and used in 

both crop and animal agriculture (fig. 3.8). By the early 

1960s, it was discovered that DDT accumulated in animal 

tissues, causing a variety of health problems, including the 

thinning of eggshells in bald eagles and other predatory 

birds (Carson, 1962). This finding eventually led to a ban on 

DDT use in 1972, along with increased public awareness of 

the environmental impacts of synthetic insecticides and the 

need to develop safer, less persistent ones. In 2009, 17,400 

metric tons of insecticides were being used in the Nation’s 

crop agriculture; 26 of these compounds were used in 

quantities of greater 100 metric tons per year (Grey and others, 

2012). Recombinant DNA technology allowed agricultural 

crops to be genetically engineered to produce a naturally 

occurring insecticidal toxin from the soil bacterium Bacillus 

thuringiensis (Bt

), making the entire plant toxic to corn 

borers, budworms, bollworms, and other target insects (see 

“Genetically Modified Crops”). 
54 Agriculture—A River Runs Through It—The Connections Between Agriculture and Water Quality

Many of the diseases that damage crops are caused by 

fungi and yeasts (for example, mold, rust, scale, blight, scab, 

and smut). These diseases have been controlled through 

the development and use of fungicides and, more recently, 

some genetically modified crops (for example, papaya). 

Sulfur has been used as a fungicide for many decades. In the 

past few decades, many new synthetic chemical fungicides 

have been developed and are now used widely. In general, 

these fungicides are disease-specific and expensive, so they 

generally are used in response to specific outbreaks of disease, 

rather than on the routine basis that is commonly used for 

herbicides and nutrients. In 2009, 47,500 metric tons of 

fungicides were used; 26 were used in quantities of greater 

than 100 metric tons per year (Grey and others, 2012).

Animal agriculture also has seen an increase in chemical 

use over the past several decades, although to a much lesser 

extent than for crops. Insecticides are used to control ticks 

and flies on cows and sheep. After World War II, penicillin 

and other antimicrobial drugs were used to reduce chronic 

illnesses in livestock—a concern that became increasingly 

problematic with the development of concentrated animal 

feeding operations (CAFO; U.S. Environmental Protection 

Agency, 2012c). Low (“sub-therapeutic”) levels of 

antimicrobials and antiparasitics were used to improve 

growth and feed-use efficiency. As a result, since the 1980s, 

antimicrobials have commonly been added on a regular basis 

to feed for poultry and hogs and, to lesser extent, to feed for 

beef cattle (Khachatourians, 1998; Love and others, 2011). 

In 2009, there were 13,000 metric tons of antimicrobials 

used in animal agriculture, almost 4 times the amount used 

for people (U.S. Food and Drug Administration, 2010; Chai, 

2010). As with weeds, the organisms that antimicrobials were 

designed to control can develop resistance to the compounds 

over time, reducing or eventually eliminating the effectiveness 

of the compounds in controlling the target species. As a result, 

the widespread use of antibiotics has caused concerns about 

the possibility that this practice may be reducing the overall 

effectiveness of antimicrobials in controlling disease in humans, 

as well as in livestock (Khachatourians, 1998; American 

Academy of Microbiology, 2009; Landers and others, 2012). In 

addition to antimicrobials, hormones also have been found to 

promote animal growth and increase meat and milk production. 

First introduced in 1951, bovine somatotropin (bST), also 

called bovine growth hormone (rBGH), is now widely 

administered to dairy cattle (Conkin, 2008; U.S. Food and Drug 

Administration, 2011). Hormones also were found to increase 

meat and milk production in cows. Other natural and synthetic 

growth hormones, such as testosterone, trenbolone acetate, and 

melengestrol acetate, are used to promote the rate of weight 

gain and (or) improve feed efficiency in beef cattle (U.S. Food 

and Drug Administration, 2002).

Changes Off the Farm.—Society and government 

have been strong influences on the direction and changes 

in agriculture throughout American history. The Federal 

Government has promoted agriculture. It provided public 

lands at little to no cost to settlers during various periods. 

By 1862, the USDA was created and land grant colleges 

were established for agricultural research. In 1902, the U.S. 

Reclamation Service (precursor to the Bureau of Reclamation) 

was established and aided in the development of many large 

agricultural irrigation and drainage projects. Establishment of 

transportation infrastructure (roads and railroads) as well as 

monetary assistance (credit and subsidies) also contributed to 

agricultural advancement.

Since the 1930s, the Government has used farm 

subsidies as incentives for soil and water conservation and 

environmental protection. The Great Depression and the Dust 

Bowl, which left the farm economy in turmoil, led to greater 

Government involvement. The Agricultural Adjustment Act 

of 1933 (Public Law 73-10; 48 Stat. 31) provided Federal 

Government subsidies to remove agricultural land from 

production to stabilize crop prices and reduce soil erosion (see 

“U.S. Department of Agriculture Conservation Programs”). 

This was followed by the Soil Conservation and Domestic 

Allotment Act (1935; Public Law 74-461; 49 Stat. 1148), 

which established the Soil Conservation Service to develop 

and teach erosion-control techniques. Setting aside land 

for erosion control spiked again after the mid-1950s with 

incentives to convert land back toward a more natural state 

as a result of enactment of the Agricultural Act of 1956 

(Public Law 84-540; 70 Stat. 188) (establishing the Soil 

Bank Program) and the Food and Agricultural Act of 1962 

(Public Law 87-703) (establishing the Resource Conservation 

and Development Program). The Farm Security and Rural 

Investment Act of 1985 sought to protect vulnerable 

lands such as highly erodible areas and wetlands through 

the Conservation Reserve Program. The 2002 Farm Bill 

substantially increased spending for conservation programs, 

creating the new Conservation Security Program and the 

Grasslands Reserve Program.

The Federal Government also has been active in 

protecting human health. The Pure Food and Drugs Act of 

1906 (Public Law 59-384; 34 Stat. 768) required Federal 

inspection of meat products and led to the creation of the Food 

and Drug Administration (FDA). The first national pesticide 

act (Federal Insecticide Act of 1910; Public Law 61-152; 

36 Stat. 331), originally promulgated to ensure the quality 

of commercial insecticides, was modified in the Federal 

Insecticide, Fungicide, and Rodenticide Act of 1947 (FIFRA) 

to protect human and environmental health. With the 1962 

publication of Rachel Carson’s “Silent Spring,” environmental 

concerns on the use of agricultural chemicals were heightened 

when DDT was linked to the declining bald eagle populations 
Changes in the Nation’s Agriculture Over Time 55

(Carson, 1962). As part of the Government response, the U.S. 

Environmental Protection Agency (EPA) was formed in 1970 

and given the responsibility for FIFRA. With the increase in 

animal feeding operations, legislation including the Poultry 

Products Inspection Act (1957; Public Law 85-172), Food 

Additives Amendment (1958), and the Wholesome Meat Act 

(1967; Public Law 90-201) were enacted to protect public 

health in terms of meat and meat processing. More recently, 

the Food Quality Protection Act of 1996 (Public Law 104‐170; 

110 Stat. 1489) further mandated health-based standards 

for pesticides in food and provided incentives to create 

safer pesticides.

Changes in agriculture led to societal changes. During 

the 1950s to 1970s, the mechanization and consolidation of 

farms displaced many workers from farming to urban areas. 

By 2007, less than 2 percent of the American population 

worked on farms (Dimitri and others, 2005). Prior to this 

urban migration, much of the Nation’s population were 

farmers or closely connected to agricultural communities, and 

held a diverse knowledge of agricultural life. By 2010, about 

83 percent of the Nation’s population lives in urban areas 

(Mackun and Wilson, 2011). Many urbanites have little to no 

contact with (or understand) agriculture. Often, Americans 

become aware of agricultural activities through the media, 

which in many cases focuses on the human and environmental 

health concerns. News about water-quality concerns from 

agricultural practices (such as those in Chesapeake Bay and 

the Gulf of Mexico), as well as the expansion of biofuels, 

continue to draw attention. Human-health concerns related to 

pesticides in foods, antimicrobials in meat and dairy products, 

and genetically modified crops in the food supply are perhaps 

even more widespread. These and other concerns have put 

public pressure on agriculture and have led to an increased 

demand for products from organic farms (see “Organic 

Agriculture”).

Current and Future Challenges and 

Opportunities

Since World War II, there have been tremendous changes 

in almost every facet of agriculture, including the widespread 

use of self-propelled machinery, advances in plant and animal 

breeding, the development of new chemicals (fertilizers, 

pesticides, antimicrobials, and hormones) and genetically 

modified crops, and farm intensification. Advances in 

computer technology, such as the use of global positioning 

system (GPS)-guided precision agriculture, have improved 

planting accuracy and customized agrichemical applications. 

Public responses to potential adverse impacts on human and 

ecosystem health also have changed agriculture through 

regulations, conservation programs, and other government 

initiatives. Many of the current challenges and opportunities 

for agriculture are likely to continue into the foreseeable 

future—increased demand for its products; shortages of 

agricultural land and water, environmental impacts, and 

reliance on incremental solutions from new technologies, 

and sustainability. 

Increased Demands for Agricultural Products.—A 

growing domestic and global population will drive increasing 

demands for agricultural products (food, feed, fiber, and fuel) 

and will present an enormous opportunity for agriculture. 

In 2012, the population of the United States was about 

313 million and is projected to increase to 400 million by 

2050 (Mackun and Wilson, 2011; U.S. Census Bureau, 2013). 

The world population is projected to increase from 6 billion 

to 9.1 billion people by 2050 (United Nations, 2009). This 

substantial increase in the American and global population 

will drive food demand and consumption. As a result, it 

has been estimated that global food production will need to 

increase by between 52 and 109 percent by 2050 (Tweeten 

and Thompson, 2008). Projected increases in population 

are anticipated to be most dramatic in developing nations, 

especially China and India, whose economies are experiencing 

some of the most rapid growth on the planet. The United 

States is the world’s major provider of food aid to developing 

countries (60 percent) (Hanrahan and Canada, 2011). The 

increased demand for food, feed, fiber, and fuel will continue 

to challenge the Nation’s agriculture, which operates on a 

continually diminishing area of cropland (fig. 3.4). The energy 

needs of human societies are vast and increasing. Because the 

use of traditional forms of energy (that is, oil, coal, natural gas, 

and hydroelectric power) involves a wide range of adverse 

environmental impacts from their development, distribution, 

and use—as well as the reliability of their supply—there has 

been considerable interest in the development of other, more 

environmentally benign and (or) reliable sources of energy 

since the early 1970s. Among these other sources of energy, 

several are intimately linked with agriculture. Biofuel, the 

product of the conversion of biomass to fuel sources, is one 

type of energy in which the United States has heavily invested 

(see “Biofuels”). In 2013, 1.4 percent of the Nation’s energy 

consumption was met with biofuels (U.S. Energy Information 

Administration, 2014), but their importance may increase in 

future years. The trade-offs between producing biofuels, rather 

than food, on the finite amount of arable land and fresh water 

available are topics of societal and ethical concerns. In 2015, 

44 percent of the Nation’s corn harvest was used to produce 

biofuels (U.S. Department of Agriculture, Economic Research 

Service, 2016c). 

The numbers and sizes of wind turbine “farms” are 

expanding in many agricultural areas, and competing for 

cropland. In the future, the same may be true for solar power 

facilities, should they be built on a large scale. Biogas 

facilities, which capture and burn reduced gases emanating 

from animal manures and other agricultural wastes, also may 

become more widespread in the future.
56 Agriculture—A River Runs Through It—The Connections Between Agriculture and Water Quality

U.S. Department of Agriculture Conservation Programs

The Dust Bowl had a devastating effect on American agriculture in the 1930s, leading to the loss of topsoil from 

about 40 million hectares of land. This devastating event prompted the establishment of the first formal efforts by the 

Federal Government to promote soil conservation by the Nation’s farmers—policies that continue to the present day. 

These voluntary programs have changed over the decades (fig. 3.9); they were designed to help farmers to conserve 

their land and to alleviate erosion, thus helping minimize or avoid the increased sediment loads and impaired water 

quality that erosion causes. The popularity of these programs increased and decreased over time, partially due to 

national and international demands on agriculture in the United States. Little or no land was included in conservation 

programs during and after World War II and during global wheat shortages in the 1980s. Brief descriptions of the 

principal programs in current use are presented below.

Conservation Adjustment Program

Agricultural Conservation Program

Soil Bank

Cropland Adjustment Program

Acreage Reduction Program

Conservation Reserve Program

Wildlife Habitat Incentives Program

Wetlands Reserve Program

Environmental Quality Incentives 

Program

EXPLANATION

0

5

10

15

20

25

30

35

1930

Land in program, in millions of hectares

1940 1950 1960 1970

Year

1980 1990 2000 2010

Figure 3.9. A number of U.S. Department of Agriculture programs have set aside land for conservation purposes over 

the past decades. Due to changes in the conservation programs, the total area has been highly variable (Grey and others, 

2012; modified from Crosswhite and Sandretto, 1991).

The Conservation Reserve Program (CRP) was established in the 1985 Farm Bill as one of several 

approaches for reducing soil erosion (U.S. Department of Agriculture, Natural Resources Conservation Service, 

2012b). The CRP provided incentives to remove highly erodible land from production. In this program, farmers 

signed a contract with the USDA agreeing to take the land out of production for 10 years. The original target was 

for about 18 million hectares. In 1993, 14.2 million hectares were enrolled, but decreased to 10.3 million hectares 

by 2014. Under the CRP, land is not simply taken out of production, but is stabilized and replenished by growing 

a vegetative cover such as grass, shrubs, or trees. The Federal Government provides incentives to farmers to enroll 

in this voluntary program by paying them for the loss of crop revenue while the land is out of production, and by 

sharing the costs associated with re-establishing native vegetation on the land. After the initial contracts expire, 

some are renewed and others are not. In order to continue to receive government support, farmers who return their 

land to agricultural production are required to meet rigorous standards.
Changes in the Nation’s Agriculture Over Time 57

The Conservation Reserve Enhancement Program (CREP) is a cooperative program in which the Federal 

Government partners with States, Indian Tribes, and local governments to provide long-term environmental 

protection for cropland, animal habitat, and pastureland (U.S. Department of Agriculture, Farm Service Agency, 

2012). The area enrolled in this program is not included in figure 3.9. CREP, related to the CRP, is designed to 

protect and improve water quality by restoring and enhancing riparian habitat, estuaries, drainage ditches, and 

wetlands. The USDA and cooperating agencies provide technical assistance to help landowners plan and implement 

CREP practices, such as filter strips and forested buffers. The program provides incentives for farmers to enroll for 

periods of 10, 15, or 30 years. The landowners voluntarily plant or preserve their more environmentally sensitive 

lands—especially those close to water bodies—in perennial vegetation for the enrollment period and receive annual 

payments plus reimbursements for the cost of implementing these practices. 

The Wildlife Habitat Incentive Program (WHIP) provides cost-share assistance to private landowners 

to help them implement improvements to wildlife habitat on agricultural lands. The Federal Government pays 

the farmer up to 75 percent of the implementation costs (U.S. Department of Agriculture, Natural Resources 

Conservation Service, 2009). WHIP was initially part of the 1996 Farm Bill, reauthorized in 2002, 2007, and 

2008, but ended in 2014. WHIP agreements between the USDA and the landowner generally last for between 

5 and 10 years. This program has proven to be a highly effective and widely accepted program across the 

country. By helping establish wildlife-habitat-improvement projects on any type of land, WHIP provides 

assistance to conservation-minded landowners whose land cannot meet the specific requirements of other USDA

conservation programs.

The Wetlands Reserve Program (WRP) was established to protect, restore, and enhance the Nation’s 

wetlands (U.S. Department of Agriculture, Natural Resources Conservation Service, 2012c). Conversion of 

wetlands to cropland was a common practice during the initial modification of the American landscape for 

agriculture. Since that time, however, wetlands have been found to represent some of the most biologically 

productive and useful ecosystems on Earth (U.S. Environmental Protection Agency, 2012b). Improved 

understanding of the critical importance of wetlands for protecting and enhancing water quality, for flood control, 

and as wildlife habitat, has motivated changes in wetland policy and management over the past several decades. As 

a reflection of this, the WRP was initiated by the 1985 Farm Bill and reauthorized or amended in every subsequent 

Farm Bill since then. In 1996, the maximum area of land that was authorized for enrollment in this program was 

64,000 hectares. This was expanded in 2002. As of 2014, 1.1 million hectares of wetlands and related uplands were 

enrolled in the WRP across the Nation. The WRP was replaced by the Agricultural Conservation Easement Program 

in 2014.

The Environmental Quality Incentives Program (EQIP) is a voluntary program for farmers and ranchers 

who are willing to promote agricultural production and environmental quality (U.S. Department of Agriculture, 

2012). It offers financial and technical assistance to participants who install structures or implement management 

practices on agricultural land (for either crops or livestock) that are environmentally sustainable. EQIP was 

approved in 1996 as an amendment to the 1985 Farm Bill and reauthorized in 2002 and 2008. As of 2014, about 

7.9 million hectares of land receive conservation practices from the program.
58 Agriculture—A River Runs Through It—The Connections Between Agriculture and Water Quality

Organic Agriculture

Organic foods are currently the fastest growing sector of the Nation’s agricultural industry, with sales having 

increased from $1 billion in 1990 to $47 billion in 2016 (U.S. Department of Agriculture, National Institute of Food 

and Agriculture, 2018; U.S. Department of Agriculture, Economic Research Service, 2016a). Some of this growth 

in demand is likely derived from a growing public awareness of the human and environmental health concerns 

from agricultural chemicals. Growth in consumer demand for organic agricultural products (U.S. Department 

of Agriculture, Economic Research Service, 2010a) eventually led to (1) the enactment of the Organic Foods 

Production Act of 1990 (Public Law 101‐624; 104 Stat. 3935) and its subsequent amendments, (2) the establishment 

of the National Organic Program by the USDA, and (3) the development of national standards for the production of 

foods certified as “organic” (U.S. Department of Agriculture, Economic Research Service, 2010a).

Organic agriculture is being practiced throughout the Nation on small areas of farmland (fig. 3.10). In 2011, 

2.18 million hectares of agricultural land were certified as organic. This is 0.83 percent of total cropland and 

0.49 percent of total pasture and grassland. Rates of adoption of organic methods vary considerably among different 

sectors of the agricultural industry, with relatively small areal percentages among the top field crops (0.26 percent 

for corn, 0.17 percent for soybean, and 0.63 percent for wheat), moderate rates among livestock (2.0 percent 

for layer hens and 2.8 percent for dairy cows), and the highest rates among fruits and vegetables (for example, 

4.9 percent for apples, 12 percent for lettuce, and 14 percent for carrots) as of 2011 (U.S. Department of Agriculture, 

Economic Research Service, 2016b).

One dot equals 100 

hectares 

EXPLANATION

Organic crops

Figure 3.10. Organic agriculture in the United States, 2007. The cropland for organic agriculture is about 1 

percent of all cropland (U.S. Department of Agriculture, 2016).
Changes in the Nation’s Agriculture Over Time 59

The USDA defines “organic production” as “a system that is managed ... to respond to site-specific 

conditions by integrating cultural, biological, and mechanical practices that foster cycling of resources, promote 

ecological balance, and conserve biodiversity” (U.S. Department of Agriculture, Agricultural Marketing 

Service, 2012a). Agricultural products that are certified as organic must be grown in soil to which no “prohibited 

substances” have been applied within the past 3 years and produced in a manner that does not include the use of 

genetic engineering, ionizing radiation, or sewage sludge. Crops are fertilized primarily through the application 

of animal manure and crop waste, the planting of leguminous cover crops, and the mechanical incorporation 

(tillage) of these materials into the soil on a regular basis. Many organic farms use an agricultural system, 

commonly referred to as “permaculture,” that involves a much greater diversity of crops and livestock than most 

conventional farms. As suggested by the USDA definition given above, such systems place considerable emphasis 

on encompassing the full cycle of nutrient flow within the overall operation, using animal waste to fertilize crops 

that, in turn, are grown to produce feed for the livestock, in addition to other agricultural products. 

Organic methods of pest control include a variety of physical, mechanical, and biological approaches—and, 

when necessary, the application of substances approved for use in organic agriculture. Although organic farming 

eschews the use of most synthetic chemicals used in conventional agriculture, the application of a considerable 

number of substances to the land—most of them naturally occurring—is permitted (U.S. Department of 

Agriculture, Economic Research Service, 2010a; U.S. Department of Agriculture, Agricultural Marketing Service, 

2012b). Additional control of insects also is provided by the balance between insect predators and prey on organic 

farms (Crowder and others, 2010). Although conventional agriculture depends primarily on pesticide applications 

to control weed growth, organic agriculture typically relies on more complex crop rotations and tillage for this 

purpose (for example, Pimentel and others, 2005). The use of crop rotations also helps control insect damage 

and maintain soil fertility. In addition, the USDA has established standards to address the treatment of seeds, 

planting stock and livestock, as well as procedures for the handling of all foods certified as organic up to their 

time of sale. For example, the use of growth hormones or antimicrobials on livestock for any reason is prohibited 

(U.S. Department of Agriculture, Economic Research Service, 2010a; U.S. Department of Agriculture, National 

Institute of Food and Agriculture, 2011; U.S. Department of Agriculture, Agricultural Marketing Service, 2012a). 

Soils that have been cultivated using organic methods are commonly found to have significantly higher 

amounts of organic carbon and nitrogen (Drinkwater and others, 1998; Tilman, 1998; Pimentel and Patzek, 2005), 

exhibit greater aggregate stability (Mäder and others, 2002), and show lower rates of nitrate leaching (Drinkwater 

and others, 1998; Pimentel and Patzek, 2005) than soils under conventional agriculture. This observation 

suggests that the more extensive use of cover crops and manure applications by organic agriculture—relative 

to conventional methods—can help compensate for any potentially adverse effects of tillage on soil quality 

(Drinkwater and others, 1998). More research is needed on the various ways that organic agriculture may affect 

the health of the ecosystems in which it is practiced, compared to conventional agriculture.
60 Agriculture—A River Runs Through It—The Connections Between Agriculture and Water Quality

Biofuels

Biofuels are fuels produced directly or indirectly from organic matter (biomass), such as plants and animal 

waste. Corn-based ethanol is the most common type of biofuel produced in the United States, with approximately 

34 billion liters produced in 2008 (Grey and others, 2012), whereas in Brazil, ethanol is produced from sugarcane. 

Biodiesel made from soybean also is important in the United States. In the future, other sources of biomass have the 

potential to be commercialized for biofuels. Switchgrass, a fast-growing variety of perennial prairie grass that can 

be grown in many areas of the country, has been explored as a future source of biofuels (Mitchell and others, 2014). 

Several other sources of biofuels have been suggested including algae, wood chips, animal manure, and crop residue.

The recent emphasis on the production of biofuels is at least in part due to national security issues related 

to creating a long-term and stable supply of fuel for the country. The Biofuels Initiative was implemented by the 

U.S. Department of Energy Biomass Program in late 2006 to help meet the goals of the Energy Independence and 

Security Act (Public Law 110-140; 121 Stat. 1492). The goal of this Act is to increase the production of renewable 

and alternative fuels in the United States and to reduce dependence on foreign oil. Two primary goals for the 

Biofuels Initiative and Biomass Program are to reduce the cost of ethanol so that it is competitive with gasoline and 

to produce 136 billion liters of renewable fuel (biofuels) by 2022 (as a partial replacement of gasoline). Because 

only 42 percent of total biofuels is allowed to be derived from corn grain, the Act also encourages the development 

of diverse crops as the basis of biofuel production. The production of ethanol from biofuels increased by about 

30 percent between 2007 and 2008 and again by 13 percent between 2008 and 2009 (fig. 3.11). The Biofuels 

Initiative created an important market, increased demand, and encouraged more area to be planted in corn. In 2009, 

almost 35 percent of the corn production in the Nation was used in the production of ethanol. Some of the corn used 

for the production of ethanol has come from increases in corn production and some has come from an increase in 

yield (fig. 3.11), but the rest has come from a change in use of the existing production capabilities.

The use of corn for ethanol production poses several concerns. In the production of ethanol by current methods 

(2012), the ratio of the energy gained from ethanol to the energy needed to grow and process corn for ethanol is 

not large. This new area planted in corn comes at the expense of areas formerly used for other crops, the removal 

of land from conservation reserve programs, and 

(or) the addition of marginal lands that generally 

are not well suited for row-crop agriculture. The 

environmental impacts of the additional water, 

fertilizers, and herbicides that are needed to grow 

the increased corn are additional concerns. For 

example, in 2007 in northwestern Mississippi, 

about 184,000 hectares of cotton were replaced 

with corn (Welch and others, 2010). In Mississippi,

corn requires more irrigation water and more 

fertilizer than cotton, so the switch has created 

two issues: it has exacerbated an already declining 

water level of the alluvial aquifer, and it has 

increased the export of nitrogen from the landscape 

to the Mississippi River, and, eventually, to the 

Gulf of Mexico (Coupe and others, 2012). The 

increase in corn production across the Mississippi 

River Basin may therefore be working at crosspurposes against the numerous management 

practices that are being implemented to decrease the 

load of nitrogen discharging to the Gulf of Mexico.

0

10

20

30

40

50

1990 1992 1994 1996 1998 2000 2002 2004 2006 2008 2010

Millions of hectares, megagrams per hectare,

percent, or billions of liters

Year

Area of corn planted, millions of hectares

Annual corn yield, megagrams per hectare

Percentage of corn used for fuel

Annual ethanol production, billions of liters

EXPLANATION

Figure 3.11. Annual area of corn planted in the United States, 

annual yield of corn for grain, percentage of corn for grain used 

for fuel, and annual ethanol production (Capel and Hopple, 2018).
Changes in the Nation’s Agriculture Over Time 61

Increased Shortages.—The area of land used for 

agriculture has decreased over the past few decades. In 

some areas, agricultural land has become more valuable for 

conservation, recreation, or urban development purposes. 

From 1949 to 2007, there was a 15-percent decrease in 

cropland area (from 1.94 to 1.65 million km2) (Nickerson 

and others, 2011). It is estimated there will be an additional 

13-percent decrease in cropland area by 2050 (down to 

1.42 million km2), whereas the area of urban lands is projected 

to increase from 3.1 percent in 2000 to 8.1 percent in 2050 

(Nowak and Walton, 2005). Additional decreases in cropland 

may be due to climate changes, soil salinity, and lack of 

irrigation water. With this projected decrease in crop area over 

the next few decades, production from existing cropland must 

be increased to meet domestic and global needs.

Water is the primary limiting factor for agricultural 

production in many areas; this limitation is expected to 

increase as available freshwater resources are used at rates 

faster than they can be replenished (Schaible and Aillery, 

2012). Rain-fed agriculture is dependent on the amount 

and timing of precipitation, whereas irrigated agriculture 

(supplied by either groundwater or surface-water withdrawals) 

has a more reliable source of water, resulting in increased 

agricultural yields. Water used for irrigation accounts for 

61 percent of all non-consumptive withdrawals from surface 

and groundwater supplies in the United States (Barber, 2009). 

Although it is considered a renewable resource, groundwater 

is often available in relatively limited supply, and is truly 

renewable only when the rate of withdrawal is less than or 

equal to the rate of groundwater recharge. In some areas, 

recharge from rainfall is less than the rate of withdrawal, 

rendering the groundwater a non-renewable resource. 

Phosphorus is an important nutrient for vigorous growth 

of plants. The United States is the world’s largest producer of 

phosphorus fertilizer. The majority of the Nation’s supply of 

phosphorus is mined from shallow geologic deposits. It has 

been estimated that the known global reserves of phosphate 

rock will last from 50 to 100 years and the domestic, easily 

mined reserves will last about 25 years given the current rate 

of consumption (Cordell and others, 2009; Vaccari, 2009). 

Because no other element can substitute for the biological role 

of phosphate, options for preserving and recycling phosphate 

sources would help to extend the lifetime of the current 

supply. These options might include reduction in use while 

sustaining crop yield, minimization of the phosphorus lost in 

field runoff, reclamation from livestock waste, reclamation of 

urban effluent, and improved technologies for mining currently 

inaccessible deposits (Herring and Fantel, 1993). 

Changing Environmental Stresses.—It is expected 

that global climate change will increase average temperatures 

and change rainfall patterns for many arable areas in the 

United States, which could force a spectrum of change from 

locally grown hybrids to large changes in cropping patterns. 

tac12-0566_photo-E-09

Urbanized areas continue to expand and compete for farmland (South 

Dakota). Photograph by Paul Capel, U.S. Geological Survey, 2010.

Perhaps even more important is the potential for more 

variable weather and more extreme events—such as droughts, 

floods, hurricanes, tornadoes, and wildfires—in the coming 

decades. More frequent extreme events could be disruptive 

locally and (or) regionally and will be difficult to plan for 

and mitigate. Crop agriculture may be looked to for methods 

to help sequester excess carbon in the crops and in the soil. 

The beginnings of market-driven incentives to store soil 

carbon have been initiated (Kollmuss and others, 2008). 

The storage of carbon from crop residue is supported by the 

goals and methods used for soil conservation, such as the 

use of continuous no-till tillage, but may oppose the goal 

of producing biofuel from crop residue (Cruse and Herndl, 

2009). Row-crop agriculture also is likely to come under 

greater scrutiny for its part in the production of nitrous oxide, 

a greenhouse gas that traps heat approximately 310 times as 

effectively as carbon dioxide. In 2010, row-crop agriculture 

was the Nation’s largest producer of nitrous oxide, having 

released almost 70 percent of total estimated emissions (U.S. 

Environmental Protection Agency, 2012a).

Reactive nitrogen chemical forms (forms other than 

atmospheric nitrogen gas, N2

) are being produced through 

human activities at more than twice their natural rate. This is 

occurring in part because of fossil fuel combustion and the 

cultivation of legumes, but primarily from the synthesis of 

fertilizer from atmospheric nitrogen gas, discussed earlier 

(Galloway and Cowling, 2002). This extra reactive nitrogen, in 

addition to increasing crop yields, contributes to the formation 

of atmospheric smog and haze; adversely impacts forests; 

causes acidification of soils, lakes, and streams; contributes 

to global warming and to the destruction of the ozone layer; 

and contributes to the eutrophication of our streams, lakes, 

reservoirs, and oceans, odors from livestock operations, and 

toxicity to fish (see “Nitrogen” in Chapter 2).
62 Agriculture—A River Runs Through It—The Connections Between Agriculture and Water Quality

Many of the lands that were enrolled in some Federal 

land conservation programs in the 1990s are nearing the end 

of their contract (typically 10–15 years) (see “U.S. Department 

of Agriculture Conservation Programs”). The area of cropland 

in Federal conservation programs decreased by 3 percent 

from 1997 to 2002. The return of retired, environmentally 

vulnerable cropland to production is a cause of environmental 

concern, including loss of wildlife habitat and loss of 

sediment, nutrients, and pesticides to streams (Zelt and Munn, 

2009). Choices are continually made to put these vulnerable 

lands back into some form of production or keep them out of 

active crop production to protect the environment. 

The use of corn and soybean for biofuel production has 

increased over the past decade. The expansion of the area 

needed to grow crops for biofuels has reduced the amount of 

land available to grow crops for food, feed, and (or) fiber or 

returned previously retired, environmentally vulnerable lands 

to production and, in some areas, increased the use of water 

and chemicals on crops for biofuels (Welch and others, 2010). 

Incremental Solutions from New Technologies.—The 

current genetically modified crops (corn, cotton, and soybean) 

have been a commercial success that has led to continued 

expansion to new crops (U.S. Department of Agriculture, 

Economic Research Service, 2017) and new varieties with 

more than one genetically modified trait. It is probable that 

the area of land on which genetically modified crops are 

grown in the Nation will continue to expand. The widespread 

adoption and expansion of genetically modified crops carries 

with it potential unknowns. There are scientific and societal 

issues associated with the production and expansion of 

genetically modified crops including increases in resistance 

by the pests, food labeling (for consumers who are concerned 

about potential adverse effects on humans or other nontarget organisms), long-term impacts on wild organisms 

through inadvertent hybridization, potential losses of genetic 

diversity, and over reliance on a limited number of chemicals 

(Snow and others, 2005). Genetic modifications also are 

being explored in animals. Future efforts within this realm 

may include the introduction of livestock with such traits as 

increased efficiencies of nutrient uptake from food, decreases 

in the amounts of waste excreted, improved adaptability to 

heat stress, improved parasite and disease resistance, and 

increased growth rates (U.S. Food and Drug Administration, 

2012). Other advances in biotechnology will continue to affect 

change in both crop and animal agriculture (Senthil-Kumar 

and Mysore, 2010; Rodrigues and others, 2012; Eldakak and 

others, 2013; Sherman and others, 2015).

The use of computer technology in agriculture continues 

to advance. The Nation had a goal of connecting all of 

rural America with broadband Internet (U.S. Department 

of Agriculture, 2011), analogous to the goal of bringing 

electricity to all of rural America during the Rural 

Electrification programs of the 1950s. The easy availability of 

satellite mapping equipment (global positioning systems, or 

GPS), together with user-friendly computer interfaces, have led 

to substantial improvements in the accuracy of land leveling, 

planting, and applications of chemicals—a suite of advances 

that is often referred to as “precision agriculture.” The use of 

GPS-guided planters helps avoid losses of crop acreage owing 

to uneven rows, and utilizes information on soil, tillage, and 

previous yields to allow for customized, spatially varying 

agrichemical applications to help maximize crop yields. Precision 

agriculture holds the future promises of decreasing the amounts of 

chemicals that are applied to the landscape, and modifying tillage 

and planting to minimize harm to environmentally sensitive 

areas within a field. A vast array of computer-controlled sensors 

is used to monitor physical parameters, such as temperature, 

soil moisture, and humidity, in real time in orchards, making it 

possible to apply irrigation water and agricultural chemicals at 

specific times and rates to individual trees, according to their

particular physiological needs at the time.

Nanotechnology has found widespread application in 

consumer products, but its incorporation into agriculture is 

not widespread (Parisi and others, 2015). The application of 

chemicals as nanoparticles could increase the efficiency of their 

use, and provide better control over their rates of release to 

the crop(s) of interest. The development of GPS-linked nanoenvironmental sensors could allow monitoring of soil moisture 

and crop yield throughout agricultural landscape (Scott and 

Chen, 2012). In many ways, nanoparticles are currently at the 

same stage of development as insecticides were in the 1960s and 

genetically modified crops were in the 1990s. Nano-materials 

offer many potential benefits for agriculture, but also present 

many unknowns regarding human and environmental safety. 

Although nanoparticles are widely used in food packaging, the 

presence of nanoparticles in the food itself is likely to cause 

concerns among some consumers.

Sustainability.—Incremental solutions aim to meet the 

future challenges of agriculture by staying within the present 

system. In addition to these solutions, the science is beginning to 

point to solutions that will have a “transformative” shift from the 

present agricultural system to a new and more comprehensive 

approach that will take into account the complex interactions 

between humans and the environment (Foley and others, 

2011). These solutions are based on changes made not only on 

the farm, but also at the consumer level—changes that could 

ultimately alter the way food is produced and what is eaten. 

On-the-farm solutions include organic farming and increased 

diversity of crops and livestock. Consumer solutions include 

changing dietary habits and educating consumers about their 

role in sustaining agricultural production for future generations. 

One example of an area in which both consumers and farmers 

can play a part is the reduction of food waste. Globally, food 

waste, generated both during harvest and after consumer 
Changes in the Nation’s Agriculture Over Time 63

purchase, accounts for nearly 40 percent of all food produced 

(Foley and others, 2011). Any reduction in such waste made at 

either the consumer or farm level will in effect increase overall 

yields without having to increase production. 

Most would agree that there exists a primary mandate 

for this Nation’s agriculture to feed the population in an 

environmentally and economically sustainable manner. 

Agriculture needs to be able to produce its bounty and 

help provide a healthy environment for this generation and 

the scores of generations to come. Environmental impacts 

of agriculture have been a continuing concern since its 

beginnings, but were first brought to the Nation’s attention 

following the period of the Great Dust Bowl. Tillage practices, 

modifications of the natural hydrology, manure disposal, 

chemical applications, groundwater withdrawals, and other 

agricultural activities have affected the quality of the Nation’s 

air, streams, lakes, groundwater, estuaries, and coastal areas. 

Nutrient enrichment issues are of particular concern in the 

Gulf of Mexico, Chesapeake Bay, and other coastal areas. 

Loss, degradation, and salinization of soil will continue to be 

concerns, as well as diminishing water resources for irrigation. 

Until now, the Federal Government has dealt with these 

concerns through subsidies and incentive programs to change 

agricultural activities, regulation, and building of infrastructure 

(drainage, irrigation, and channel modifications). Some have 

suggested that society, through the Federal Government, 

has developed an unwritten “contract” with agriculture 

within which society itself pays the cost of environmental 

impacts (non-point pollution and other impacts) rather than 

agriculture paying these costs. It has been suggested that 

agriculture should be held responsible for the complete costs 

of production including the costs of environmental impacts 

and stresses (Kling, 2010). All these demands on agriculture 

probably call for different paradigms of production in the 

future—paradigms that place primary value on both yield and 

sustainability. This future can be led by the collective vision 

of individual producers, by agribusiness, by consumer choice 

(market forces), by advocacy groups, and (or) by government, 

taking ownership of the future demand for environmentally 

and economically sustainable agriculture to work toward the 

common goal.

Summary

Agriculture has changed dramatically since the days of 

the early settlers and subsistence farming. It has responded 

to the needs of society for food, fiber, feed, and fuel through 

expansion, intensification, and adoption of new crops and 

new technologies. Agricultural expansion in the United 

States fundamentally changed the American landscape. The 

lands most suitable for agriculture, forests and grasslands, 

were cleared first. Arid lands and wetlands were made arable 

by the introduction of irrigation and drainage technologies, 

respectively. Landscapes that were not conducive for crop 

agriculture were used for grazing.

Advances in mechanization, electrification, chemistry, 

and genetics combined to create the unprecedented growth 

in yields of both crop and animal agriculture from the 1950s 

through the 1970s. More recently, advances in biotechnology 

have further increased yields. This growth and intensification 

of agriculture has had adverse effects on agricultural lands 

and the broader environment. Many early farmers used their 

land in ways that degraded its ability to continue supporting 

agriculture in the future. As a result, soil was depleted of its 

nutrients and (or) lost altogether through wind and water 

erosion. Increasing demand for agricultural products has led to 

increases in the use of agricultural chemicals, the widespread 

introduction of genetically modified crops, and extensive 

modifications of natural water movement, resulting in a 

variety of important, if inadvertent, environmental concerns. 

Because demand for food, feed, and fiber will continue 

to grow, agriculture will continue to face these and other 

challenges into the future. Extensive improvements in our 

understanding of the connections between agricultural 

activities and water quality will be needed to meet 

these challenge.
64 Agriculture—A River Runs Through It—The Connections Between Agriculture and Water Quality

Agriculture In Eastern Nebraska

Prior to the arrival of European settlers, the area that today is Colfax County in eastern Nebraska was 

primarily a vast expanse of rolling hills covered with tallgrass prairie (fig. 1.3). A network of streambeds cut 

through the prairie landscape, some fed by seasonal rainfall and others by springs. The prairie soil was deep and 

rich, supporting grazing buffalo herds, providing dens for coyotes and wolves, and supplying abundant food 

for prairie chickens, quail, grouse, and wild duck and geese. Along the rivers and protected from prairie fires, 

hardwood forests provided habitat for deer, elk, antelope, raccoon, otter, beaver, muskrat, and mink.

In the mid-1800s, American settlers came to the area seeking land for pasture to graze cattle and sheep, and 

to grow corn. In 1856, the first town was settled, in the bottomlands where water and fuel were easily obtained. 

Prior to the building of the Union Pacific Railroad, only about a dozen families could have been considered 

actual settlers in the area. Having already built the eastern part of the transcontinental railroad, the Union Pacific 

extended the line west from Omaha and completed its line across southern Colfax County by 1866. As settlement 

pressure increased, cattlemen were pushed westward and pasture land was converted mostly to the production of 

cultivated crops, such as corn, alfalfa, wheat, and oats. By the 1900s, corn and wheat were the main cash crops 

being grown. By the 1930s, however, cattle and hog production had increased substantially, and the crops shifted 

toward livestock feed, such as corn and oats. As cultivation of the lands progressed, increasing water demands led 

to the use of water from streams and the installation of wells to provide more irrigation water for use on the fields. 

In the late 1940s, a center-pivot irrigation system that could accommodate hilly terrain was invented. During the 

droughts of the 1950s, many farmers were forced to invest in irrigation systems to remain in business, leading 

to the widespread use of center-pivot irrigation systems. Ponds also were constructed for watering livestock. 

Most lakes and ponds in that area were not naturally occurring but were constructed as impoundments for this 

purpose. A major trend from 1940 to 1990—in this and many other agricultural areas across the Nation—saw the 

consolidation of farmland into fewer, larger farms.

Corn and soybean are the most important crops. Over the past decade there has been an appreciable increase 

in the area of corn planted. Concentrated animal feeding operations for cattle also have become common. Since 

2007, there has been an appreciable loss of farmland enrolled in the Conservation Reserve Program (CRP; U.S. 

Department of Agriculture, Farm Service Agency, 2013; see “U.S. Department of Agriculture Conservation 

Programs”). Recent studies by the USGS on the effects of agriculture on water quality and quantity in eastern 

Nebraska have focused on groundwater age and quality (McGuire and others, 2012), groundwater levels and 

storage (McGuire, 2011), effects of irrigation on baseflow of streams (Stanton and others, 2010), and influence of 

nutrients and habitat on stream ecosystems (Frankforter and others, 2009).

A B C

The landscape of eastern Nebraska. (A) The tallgrass prairie typical of the time before agriculture, (B) typical cropped fields, 

and (C) the current view from the air (23 square kilometers). Photograph A from U.S. Fish and Wildlife Service (Boyer Chute 

National Wildlife Refuge), 2011; photograph B by Jason Vogel, Oklahoma State University, 2011; photograph C from U.S. 

Department of Agriculture, Farm Service Agency, National Agriculture Imagery Program, 2011.
Terrain, Climate, Soil, and Water 65

4 Chapter —Terrain, Climate, Soil, and Water 

Why is Agriculture Located Where It Is?

The location of agriculture is determined by natural, 

economic, and societal factors. The natural environment 

offers a wide range in conditions of crop requirements: 

space, sunlight, warmth, water, gentle slopes, proper soil, and 

drainage. On some lands, these conditions are ideal, and crops 

and commercial agriculture thrive. On other lands, however, 

rugged slopes, poor (infertile) soil, lack of or excess water, 

and (or) inhospitable climate make commercial agriculture 

unprofitable or even impossible. Most agricultural lands are 

somewhere between the ends of the spectrum. In general, the 

natural factors that govern the extent of crop agriculture are 

terrain, climate, soil, and soil water. It is the combination of 

these four natural factors that allow specific crops to be grown 

in certain areas. 

Agriculture has evolved over time and changed the 

natural vegetation on the landscape to agricultural vegetation 

(see “Initial Modification of the Landscape for Agriculture”). 

Early European settlers in America practiced agriculture 

primarily for subsistence, and farmers grew a wide range of 

crops and livestock in order to survive. Their choice of crops 

and animals was likely similar to those in the areas from 

which they had emigrated, and may not have been particularly 

suited to the natural conditions of their new homes. Today 

in commercial agriculture, in which crops and livestock are 

produced for widespread distribution and consumption by 

others, farmers tend to grow crops that are well suited to 

the particular conditions of the area in order to maximize 

production with minimal inputs.

Cropland (row crops, grains, fruits, nuts, and vegetables, 

but not including hay) occupies about 13 percent of the total 

land area of the United States (fig. 2.1A; Baker and Capel, 

2011). Agricultural land also is used for pastures (lands that 

have been seeded and primarily are used for the production of 

domesticated forage plants), hay (grasses and legumes, such as 

timothy and alfalfa, respectively, that are typically cut, dried, 

and stored for livestock fodder), and rangelands (lands on 

which the native vegetation, such as grass, grass-like plants, 

and shrubs, is grown for animal grazing). Grasslands (pasture 

and hay) and rangelands occupy another 41 percent of land 

area in the United States.

Terrain 

Agricultural land used for commercial production is 

constrained by both elevation and slope. Only 1 percent of 

land used for commercial agriculture within the conterminous 

United States is above 2,000 meters (m) elevation (Baker 

and Capel, 2011). These high elevation areas generally have 

low temperatures, high wind velocities, high precipitation, 

appreciable snow accumulation, and, largely as a result of 

these factors, poor soil quality. 

The slope of the landscape also is an important 

determinant of its suitability for agriculture, because slope 

affects soil formation, climate, water drainage, soil water 

availability, and the operation of machinery. Areas that are 

nearly level (3-percent slope or less) generally are suitable 

for row-crop agriculture. Flat areas, however—such as the 

floodplains of rivers and streams, coastal areas, and glaciated 

landscapes—commonly contain wetlands where soils are wet 

through much or all of the year. American agriculture has 

traditionally excavated ditches and (or) installed subsurface 

drains in these flat areas to lower water levels in the soil either 

permanently or seasonally to enable crop production. Such 

drainage modifications have reduced the area of wetlands 

by more than one-half (about 56 percent, Dahl and Allord, 

1996; Dahl, 2006). These flat or nearly level slopes pose no 

constraint for farm machinery. Gently rolling areas (from 3 to 

6 percent slopes) also are not serious obstacles to cultivation, 

but intense or sustained rainfall can cause soil erosion, so 

terracing is sometimes implemented to reduce soil erosion. 

Steep slopes are not readily accessible by farm machinery and 

also are subject to erosion and soil loss. Slopes that are too 

steep for row crops may still be suitable tree fruit orchards, 

grapes, vineyards, vegetables, other grains, or animal grazing.

The distribution of major crops among areas of 

different slopes across the conterminous United States is 

shown in figure 4.1. Nearly 82 percent of the cropland in the 

conterminous United States occupies land with slopes of 3 

percent or less, including nearly all land planted in cotton, 

rice, and citrus. Only about 8 percent of crops are grown on 

land with 4-percent slopes. About 20 percent of grassland is on 

slopes steeper than 5 percent. 
66 Agriculture—A River Runs Through It—The Connections Between Agriculture and Water Quality

Initial Modification of the Landscape for Agriculture

The expansion of agriculture in the United States resulted in widespread change to the natural landscape as 

farmers converted forest, wetlands, prairie grasslands, and scrublands to agricultural lands (table 4.1). The resulting 

modifications of the landscape changed the water budgets and water flowpaths of many areas, and substantially 

altered each type of ecosystem present.

Table 4.1. Natural vegetation in the United States that was converted to agricultural land. 

[From Baker and Capel, 2011. Total cropland area in class: Does not include grasslands and hay. km2, square kilometer] 

Natural

vegetation class

Total land area

in class 

(1,000s km2

)

Total cropland 

area in class 

(1,000s km2)

Total grassland 

and hay area 

in class

(1,000s km2

)

Land in class 

converted to 

cropland

(percent)

Land in class 

converted to 

grassland 

and hay

(percent)

Total 

agricultural 

land 

(percent)

Broad-leaf forest 2,092 190 429 9.1 20.5 21.0

Needle-leaf forest 1,163 10 117 0.9 10.1 4.3

Grassland 2,208 520 1,074 23.6 48.6 54.0

Wetland 1,014 250 177 24.7 17.5 14.5

Shrubland 1,271 30 156 2.4 12.3 6.3

All land 7,748 1,000 1,953

From Forest to Agriculture.—Deciduous and coniferous forest covered about 40 percent of the conterminous 

United States prior to the arrival of European settlers. The forests were cut for timber and space, and the cut areas 

were burned to remove stumps and the understory plants. These practices interrupted the natural nutrient cycles 

that had previously depended on the decay of leaves, fallen trees, and other organic materials to replenish the soil. 

The removal of the native land cover affected the amount of water and soil retained in the landscape. Prior to their 

removal, the tree roots and other vegetation had helped to stabilize the soil, whereas the forest canopy captured 

rainfall and returned it slowly to the atmosphere through evapotranspiration. Rainfall that reached the forest floor 

was retained in the leaf litter, sometimes collecting in depressions to form temporary pools. Much of the water 

that infiltrated the land surface was retained for long periods (weeks to months) within the organic-rich soils, and 

released slowly throughout the year to feed perennial streams. With the loss of the forest canopy, the rain fell 

directly upon the soil and caused increased surface runoff and increased erosion. 
Terrain, Climate, Soil, and Water 67

From Grassland to Agriculture.—Grasslands covered about 30 percent of the United States prior to the 

introduction of agriculture (fig. 5.1A). These grassland areas generally received less precipitation than do the forested 

areas, but the deep-rooted grasses helped to capture and store rainfall in the soil for slow release to the atmosphere 

and nearby surface waters. Seasonal fires halted the encroachment of tree seedlings and other invasive vegetation onto 

the grassland areas, and helped maintain the fertility of the soil. With the invention of the moldboard plow, the thick 

sod could be easily turned, exposing organic-rich soil that was ideal for cultivating crops. At the same time, however, 

the conversion to agricultural land destroyed the native habitat, reduced the frequency of seasonal fires, decreased 

the amount of evapotranspiration, and increased surface runoff and erosion. Because agricultural crops covered the 

landscape only seasonally rather than throughout the year (as the original grasses had done), the soil became much 

more vulnerable to erosion by water and wind.

From Scrubland to Agriculture.—Arid scrublands covered about 17 percent of the United States prior to 

agriculture (fig. 5.1A). These areas do not receive enough natural rainfall to produce the dense natural vegetation 

found in forests, wetlands, and grasslands. In their natural state, scrublands return most of the rain that they receive 

back to the atmosphere through evapotranspiration, and many streams and rivers are dry except during flooding 

events. The advent of irrigation allowed these dry lands to become extremely productive for agriculture. The irrigation 

water was obtained from aquifers and mountain snowmelt. In some areas, large irrigation projects included the 

construction of dams and reservoirs to capture the snowmelt, canals to distribute the water, and ditches to capture the 

excess water. The addition of the irrigation water transformed the water budgets and water flowpaths in these areas. 

In many of the areas where available surface-water irrigation was scarce, the local water table declined as water 

from the underlying aquifers was withdrawn faster than it was being replenished by recharge. In these areas, the 

salinity in surface waters and soils increased because irrigation water was applied too sparingly to enable flushing of 

accumulated salts from the soil.

From Wetland to Agriculture.—Wetlands, areas where saturated soils support a rich array of water-tolerant 

vegetation, covered about 13 percent of the United States prior to agriculture (fig.5.1A). Most of these wetlands 

were small and widely dispersed. Some wetlands were seasonal, whereas others were permanent. Wetlands retain 

excess rainfall because of their low-lying topography and sponge-like soils. As agriculture expanded into these areas, 

surface ditches were dug to divert water into adjacent streams. In some areas, subsurface drains were installed to 

bring the water level farther below the land surface. Because of their rich vegetation and the slow rates at which 

water flows through them, wetlands are characterized by highly active ecosystems that efficiently capture and store 

large quantities of nutrients. The highly fertile soils created by these ecosystems were excellent for cultivating crops, 

resulting in some of the most productive lands in the country. Drainage of these areas, however, also destroyed the 

wetland ecosystems, and increased the volume and velocity of water that flowed off the land surface into nearby 

streams and rivers.
68 Agriculture—A River Runs Through It—The Connections Between Agriculture and Water Quality

tac11-0566_fig4-01

0

20

40

60

80

100

All crops

Corn

Soybean

Wheat

Other crops

Cotton

Sorghum

Vegetables and ground fruit

Rice

Orchards and grapes

Citrus

 Cropland area by slope range,

in percent 

A

B

C

EXPLANATION

>2 to 3

>3 to 4

>1 to 2

0 to 1

>4

Slope range, in percent

EXPLANATION

EXPLANATION

>2 to 3

>3 to 4

>1 to 2

0 to 1

>4

Slope range, in percent

Crops on land with 3 percent

or greater slope

Other cropland

Figure 4.1. (A) Slope (or gradient) of land in the United States; (B) areas of steep slope (greater than 3 percent) 

used for cropland; and (C) percentage of cropland area used for selected crops for each slope class in the United 

States (Baker and Capel, 2011). 
Terrain, Climate, Soil, and Water 69

Local Climate 

Climate is a primary determinant in the location of crops 

and accounts for much of the regional difference in the types 

of crops grown across the Nation. The climate of a location 

(average or prevailing weather conditions) is defined by the 

precipitation and sunlight (solar radiation that determines 

light intensity and temperature) it receives. Climate is largely 

determined by latitude, terrain, elevation, and proximity to ice, 

snow cover, and water bodies. 

The light and heat provided by the sun are essential 

for the formation of chlorophyll and the operation of 

photosynthesis in plants. Different plants have different 

requirements for the amounts of light and heat that they 

need to reach maturity. For many plants, the growth rate 

from emergence to maturity is directly related to the total 

amount of thermal energy absorbed by the organism over 

its lifetime. Each plant has its own minimum temperature 

threshold for development. Cumulative growing degree-days 

is a metric used to quantify the solar radiation requirements 

for crops and to represent the accumulated product of time 

and temperature above the minimum temperature threshold 

for a given crop for each day (fig. 4.2; Ahren, 2011). One 

degree-day for a specific crop represents one 24-hour period 

with an air temperature 1 degree Fahrenheit (°F) above the 

minimum temperature threshold for that crop. For example, 

the minimum temperature threshold for corn is 50 °F. Thus, if 

the air temperature remains at 53 °F (3 °F above the threshold) 

for 24 hours, three degree-days are accumulated for corn. The 

distribution of growing degree-days across the United States 

for some major crops is shown in figure 4.2. The number of 

accumulated growing degree-days, however, does not capture 

0

20

40

60

80

100

All crops

Corn

Soybean

Wheat

Other crops

Cotton

Sorghum

Vegetables and ground fruit

Rice

Orchards and grapes

Citrus

 Cropland area by growing degreeday category, in percent 

A

B

EXPLANATION

>3,000 to 4,000

>4,000 to 5,000

>2,000 to 3,000

0 to 2,000

>5,000

Growing degree-day, base 50°F

Cropland

Figure 4.2. (A) Location of cropland, and the 

growing degree-days for base 50 degrees Fahrenheit 

(°F) (1971–2000); (B) percentage of cropland area 

used for selected crops for each growing degree-day 

category in the United States (Baker and Capel, 2011).
70 Agriculture—A River Runs Through It—The Connections Between Agriculture and Water Quality

all of the effects of temperature on crop growth. Many crops 

stop growing when the air temperature exceeds a certain 

maximum value. In addition, some crops, such as apples, 

grapes, and winter wheat, require a period of cold dormancy 

in order for seeds or fruit to develop. Spring-seeded annual 

crops, such as corn, soybean, rice, and cotton do not require 

this period of cold dormancy in order to mature. 

Plants take up water from the soil through their roots. In 

natural landscapes, soil water consists largely, if not entirely, 

of precipitation that has infiltrated into the soil. The quantity of 

precipitation that falls at any given location has a strong effect 

on the plants that thrive there. The distribution of precipitation 

across the United States and the percentage of cropland within 

each precipitation category for some major crops are shown 

in figure 4.3. Different plants have different requirements 

for the amount of water that they need in order to develop to 

maturity. Plants give off water as they grow (a process called 

transpiration). A plant’s transpiration varies as a function 

of air temperature, humidity, and soil water availability. It 

changes during the course of the day and the course of the 

growing season. If water is not available to the plant (because 

soil moisture has dropped below its wilting point, or for some 

other reason), transpiration cannot occur.

In many parts of the Nation, the amounts and timing of 

precipitation are sufficient to meet crop water needs. In some 

areas, however, although the overall annual precipitation is 

sufficient to meet crop growth requirements, rain does not 

fall during the season when the water is required. In such 

cases, crop water needs can be met by using groundwater 

or water drawn from storage in reservoirs (fig. 4.4). In other 

areas where annual precipitation does not meet crop water 

requirements, precipitation is supplemented by irrigation water 

drawn from another part of the same watershed (perhaps the 

water retained as snow at high elevations), from a different 

watershed, or from “mined” groundwater. Mined groundwater 

is water that is extracted at a rate exceeding recharge, resulting 

in declines in the water table (Galloway and others, 2000; 

Bartolino and Cunningham, 2003). In some cases, the mined 

groundwater has been in the ground for centuries or millennia, 

and may have entered the subsurface at a location far from its 

point of extraction. 

Examples of the three precipitation/irrigation scenarios 

described above can be found in Mississippi, California, 

Texas, and Washington. The Delta region of Mississippi 

(Chapter 6) receives ample precipitation for the common 

crops, but at the wrong times of the year for efficient crop 

growth. Some of the precipitation that falls on the Delta, 

however, recharges the groundwater system. As a result, 

irrigation, derived from the local groundwater, is used 

extensively to supplement rainfall. In recent years, intensive 

agriculture within the area has required more water than 

is recharged every year, resulting in a substantial lowering 

of the water table. California’s Central Valley (Chapter 8) 

has temperatures and soil conditions that are ideal for a 

wide variety of crops, but receives an insufficient amount 

of precipitation over the course of the year to support those 

crops. To the east, however, the Sierra Nevada Mountains 

receive abundant precipitation in the form of snow. As the 

snow melts in the spring, it provides water that flows down 

through the riverine system, and is used extensively for 

irrigation within the Central Valley. In the Western Panhandle 

of Texas (Chapter 3) and other areas in the Great Plains, 

extensive irrigation is required in order to grow the types and 

quantities of crops that are planted. Much of the water used for 

irrigation within this area is obtained by mining groundwater 

from the High Plains (Ogallala) aquifer system, water that 

was recharged thousands of years ago. Groundwater mining 

has been common in many areas of the Great Plains, causing 

water-table elevations to decline sharply beneath much of 

this region. In central Washington (Chapter 7), abundant 

water from snowmelt in the Cascades is used extensively 

for irrigation. Canals and irrigation systems have been built 

to change the direction of water flow along the land surface, 

moving the water across natural watershed divides—a 

procedure known as interbasin water transfer (see “Hydrologic 

Consequences of Agriculture in a Watershed Where Natural 

Conditions Were Too Dry for Agriculture,” in Chapter 7).
Terrain, Climate, Soil, and Water 71

tac11-0566_fig4-03

0

20

40

60

80

100

All crops

Corn

Soybean

Wheat

Other crops

Cotton

Sorghum

Vegetables and ground fruit

Rice

Orchards and grapes

Citrus

 Cropland area by precipitation

category, in percent 

A

B

EXPLANATION

>913 to 1,271

>1,271 to 2,032

>538 to 913

0 to 538

>2,032

Annual precipitation, in millimeters

per year

Cropland

Figure 4.3. (A) Annual precipitation (1971–2000), 

and the area used for cropland (2009); (B) percentage 

of cropland area used for selected crops for each 

precipitation category in the United States (Baker 

and Capel, 2011).

The combined effects of precipitation and temperature in 

determining the most appropriate places to grow various major 

crops across the conterminous United States are illustrated in 

figure 4.5. Wheat can be grown in a variety of climates, largely 

because its heat and moisture requirement thresholds are 

lower than those for other major crops. Sorghum can tolerate 

drier climates, but is not as tolerant of cold temperatures 

as are many varieties of wheat. Corn and soybean are well 

adapted to the wet springs, humid summers, and dry autumns 

of the Midwest. In contrast, rice and cotton can be grown 

only in limited areas of the country. They can be grown in 

areas with similar temperature ranges, but rice is constrained 

by high water requirements. Crops that are most likely to be 

irrigated are rice, nuts, citrus and other tree fruits, and grapes 

(fig. 4.5C). A wide range of cultivars has been developed for 

each crop, and many of them have been bred specifically to 

produce acceptable yields under less favorable conditions, 

such as a short growing season or low precipitation. 

Livestock are usually not as constrained by climate as 

crops can be, because most such constraints can be overcome 

with additional inputs of fuel and other resources, such as 

shelter, water, and feed. As a result, animals are often grazed 

on rangelands with a growing season that is too short or too 

dry to support cultivated crops on an economically viable 

basis. These marginal lands may also be used to grow grasses 

used for hay, in order to provide food for livestock during 

the winter.
72 Agriculture—A River Runs Through It—The Connections Between Agriculture and Water Quality

tac11-0566_fig4-05

0

20

40

60

80

100

All crops

Corn

Soybean

Wheat

Other crops

Cotton

Sorghum

Vegetables and ground fruit

Rice

Orchards, grapes and citrus

 Cropland area, in percent 

A

B

C

>5,000 to 10,000

>10,000 

>1,000 to 5,000

1 to 1,000

Surface-water withdrawals, in hectare-meters

EXPLANATION

Cropland area, in percent

Irrigated

Nonirrigated

EXPLANATION

>5,000 to 10,000

>10,000 

>1,000 to 5,000

1 to 1,000

Groundwater withdrawals, in hectare-meters

EXPLANATION

Figure 4.4. Extent of irrigated cropland from (A) surface 

water and (B) groundwater sources; (C) percentage of 

harvested cropland that is irrigated for selected crops 

(2007) (Baker and Capel, 2011).
Terrain, Climate, Soil, and Water 73

A. Corn

EXPLANATION

C. Wheat

E. Sorghum

B. Soybean

D. Cotton

F. Rice

tac11-0566_fig4-05

Optimal

Optimal; additonal extent

for winter wheat

Minimum threshold for water and heat (growing degree-days) requirements

Sub-optimal; additional

extent for winter wheat

Sub-optimal Climate is not favorable for 

 selected crop

One dot equals 10,000

acres of corn 

One dot equals 10,000

acres of soybean 

One dot equals 10,000 acres

of wheat (includes both

winter and spring wheat) 

One dot equals 2,000 acres

of sorghum 

One dot equals 3,000 acres

of rice 

One dot equals 5,000 acres

of cotton 

Figure 4.5. Location of favorable climate for growing selected crops (2007) based on average annual growing-degree-days (GDD) and 

evapotranspiration (ET) (Baker and Capel, 2011).
74 Agriculture—A River Runs Through It—The Connections Between Agriculture and Water Quality

Soil

Soils provide physical support for plant roots, as well 

as most of the water, nutrients, and symbiotic organisms that 

plants require to survive. Soils are formed by the simultaneous 

actions of rock weathering and the decomposition of 

vegetation and other organic materials (principal source of 

organic matter). The degree to which vegetation can thrive in a 

given soil is determined by a number of soil properties. These 

include soil depth, texture (grain-size distribution), organicmatter content, nutrient concentrations, mineralogy, and the 

degree of weathering. Some areas—such as those containing 

dunes, shifting sands, salt flats, rock debris, desert detritus, 

glaciers, and snow fields—are not amenable to growing crops, 

but most areas in the Nation have soils that are or can be made 

suitable for commercial agriculture. Many soils, however, require 

amendments to provide optimal growing conditions for plants.

Soil depth is not a limitation for crops in most locations. 

Most crops need at least 100 cm of soil to grow, although 

some crops can be grown in shallower soils (Fischer and 

others, 2002). Shallow soils, which are common on steep 

slopes underlain by bedrock or hardpan, are highly erodible.

The texture of a soil is defined by the relative proportions 

of sand, silt, and clay that it contains (Folk, 1980). Texture, 

together with organic matter content, influences a wide 

variety of soil properties, including water-holding capacity, 

erodibility, and permeability to both water and air. Sandy soils 

allow water to move more freely and provide greater root 

aeration than do clay soils. However, clay soils have a greater 

water holding capacity than sandy soils. Soils with a high 

percentage of silt and clay are more easily eroded than sandy 

soils under the same conditions. Differences in texture also 

affect the soil’s content of organic matter, which decomposes 

more rapidly in sandy soils than in fine-textured soils. 

Although it is not practical to change soil texture, agricultural 

modifications can be implemented to make soils more suitable 

for growing crops. Organic matter content can be increased by 

adding manure or by leaving more crop residue on the surface. 

Soils with high water-holding capacities (rich in clay and (or) 

organic matter) can be artificially drained to allow for proper 

root development. 

The fertility of a soil refers to its nutrient content, and, 

is one measure of its capacity to support plant growth. Some 

essential nutrients—especially nitrogen, phosphorus, and 

potassium—are needed in large quantities by plants. Other 

elements, such as selenium and boron, are needed in small 

quantities, and, thus, are often referred to as micronutrients. 

More than 65 percent of all cropland in the United States 

(including that used to grow hay) is supplemented with 

commercial fertilizer, lime (powdered limestone), and (or) other 

soil conditioners to improve soil fertility, water retention, pH, 

and ultimately, crop yield (U.S. Department of Agriculture, 

Economic Research Service, 2010b).

The mineralogy of the soil particles, together with 

organic matter content, determines the chemical characteristics 

of a soil, such as pH, salinity, and cation exchange capacity. 

Soil pH strongly affects the degree to which phosphorus is 

available for plant uptake and, thus, affects soil fertility. The 

pH of acidic soils is commonly adjusted by applying lime 

(powdered limestone). Some minerals produce soils that are 

highly saline and have high concentrations of sodium, which 

can either slow or prohibit plant growth. Clay minerals in soils 

strongly adsorb cations, such as calcium, magnesium, and 

sodium, exerting substantial effects over soil cohesion, crust 

formation at the soil surface, and water infiltration rates.

In addition to its texture-related classification system, the 

USDA also classifies soils according to eight capability classes 

indicating their capacity to support crop growth without any 

deterioration in this capacity over time. The eight classes are 

determined by position in the landscape, slope, soil depth, 

soil texture, soil moisture (wetness), soil chemical properties, 

erosion potential, and climate (Helms, 1992). The spatial 

distributions of these capability classes (fig. 4.6) indicate 

that most soils in the East and Great Plains are suitable for 

cropland (classes I–IV), whereas most of the soils in the 

interior West are more suited to grazing (classes V–VIII). 

Only about 6 percent of all crops are grown on soils with 

slight limitations on their use for agriculture (class I). Fortyfour percent of crops are grown on soils that have moderate 

limitations (class II) and may require soil conservation 

measures to facilitate agriculture. Twenty-eight percent of 

crops are grown on soils that have severe limitations (class 

III) and require special conservation practices. Eleven percent 

of crops are grown on soils with very severe limitations 

(class IV) that restrict the choice of plants and require careful 

management. An additional 11 percent of crops are grown on 

soils considered unsuitable for cultivation (classes V–VIII). 

More than 50 percent of all tree nuts and fruits, and all grapes, 

are grown on soils categorized as class IV or higher. Because 

these crops do not require continued cultivation, they are more 

likely to thrive in these soils than row crops. Irrigation reduces 

many of the limitations of the class V–VIII soils. Wheat, 

cotton, and rice are more likely to be grown on poorer soils 

than are corn and soybean (fig. 4.6C). Still, many soils in the 

arid West are largely unsuitable for crop growth because of 

characteristics that are difficult to correct, such as shallowness, 

abundance of stones, low moisture-holding capacity, low 

fertility, and salinity. 
Terrain, Climate, Soil, and Water 75

tac11-0566_fig4-06

Undefined

 Cropland area by capability class,

 in percent 

Land capability class

Cropland on soils that have severe or very 

severe limitations that require special 

conservation or management practices 

(Class III–VIII soils)

Cropland on soils that have slight or 

moderate limitations that require 

moderate conservation practices (Class 

I and II soils)

Citrus

0

20

40

60

80

100

All crops

Corn

Soybean

Wheat

Other crops

Cotton

Sorghum

Vegetables and ground fruit

Rice

Orchards and grapes

A

B

C

EXPLANATION

Class I soils have slight limitations that 

restrict their use.

EXPLANATION

Class II soils have moderate limitations that 

reduce the choice of plants or require 

moderate conservation practices.

Class III soils have severe limitations that 

reduce the choice of plants or require 

special conservation practices, or both.

Class IV soils have very severe limitations 

that restrict the choice of plants or require 

very careful management or both.

Class V, VI, VII, and VIII soils have very severe 

limitations that make them unsuited to 

cultivation.

Undefined

Land capability class

EXPLANATION

Class I soils have slight limitations that 

restrict their use.

Class II soils have moderate limitations that 

reduce the choice of plants or require 

moderate conservation practices.

Class III soils have severe limitations that 

reduce the choice of plants or require 

special conservation practices, or both.

Class IV soils have very severe limitations 

that restrict the choice of plants or require 

very careful management or both.

Class V, VI, VII, and VIII soils have very severe 

limitations that make them unsuited to 

cultivation.

Figure 4.6. (A) Land capability class, (B) location of cropland, and (C) percentage of cropland area used for selected crops for each 

land capability class in the United States (Baker and Capel, 2011).
76 Agriculture—A River Runs Through It—The Connections Between Agriculture and Water Quality

Water Needs

The amount of water in soil that is available to plants is 

controlled by terrain, climate, and soil characteristics. This 

amount is usually the limiting factor for the location of crop 

agriculture in many areas. The water in soil comes largely 

from precipitation, although in some locations soil water also 

can reach plants by way of groundwater or surface-water 

flowpaths. In most places, however, agriculture is highly 

dependent on the amount and seasonal patterns of available 

soil water from precipitation and irrigation where utilized. 

(Animal agriculture is not directly dependent on soil water, but 

it is affected by the quantity and quality of the groundwater 

and (or) surface water that is available for watering livestock 

and for growing their food.) 

In addition to the soil texture and capability classes 

mentioned earlier, the USDA uses another classification 

system to reflect the dominant hazards that affect the use 

of different soils for agriculture (Helms, 1992). These 

hazards are associated with the limitations on plant growth 

posed by climate, soil, erosion, or excess water (fig. 4.7). 

Susceptibility to soil erosion from overland water flow or 

wind represents the predominant hazard affecting the use 

of most soils in the Nation. Fifty-four percent of crops are 

grown in “erosion class” areas, where erosion susceptibility 

and past erosion damage are the major factors affecting these 

soils. Twenty‐three percent of crops are grown in “water 

class” areas, where poor drainage, excessive wetness, high 

water table, and (or) overland flow represent the predominant 

hazards. Most of the rice and citrus produced in the United 

States are grown on “water class soils.” Similarly, many 

areas along the Gulf and East Coasts, and along river valleys 

across the Nation, have soils whose use for agriculture is 

limited by their excess water content. These soils can be 

(and often have been) artificially drained to overcome such 

limitations (fig. 4.8). 

Excessive soil water is especially detrimental to crops 

during their growth and harvest. Moisture stress during 

flowering, pollination, and grain-filling is harmful to most 

crops—especially corn, soybean, and wheat. Excessive soil 

water interferes with root development and plant nutrient 

uptake, and can increase the risk of plant disease, and delay 

planting or harvesting (Rosenzweig and others, 2002). 

However, some of the most fertile cropland in the Nation is 

in areas with an overabundance of water in the soil. Most of 

the Midwest and Eastern Piedmont were originally wetlands 

and, as a result, are still underlain by poorly drained soils. 

The landscape has low slopes and annual precipitation rates 

that typically exceed evapotranspiration. Most of the presentday cropland in this area was made suitable for agriculture 

by artificial drainage, which made the fields accessible for 

planting and harvesting, and helped to minimize root damage 

caused by excess water. Crops that are most likely to be 

grown in areas that have been artificially drained are corn, 

soybean, and citrus (see “Availability of Data on Locations of 

Subsurface Drains”).
Terrain, Climate, Soil, and Water 77

 Cropland area by hazard class,

 in percent 

Citrus

0

20

40

60

80

100

All crops

Corn

Soybean

Wheat

Other crops

Cotton

Sorghum

Vegetables and

ground fruit Rice

Orchards and

grapes

A

EXPLANATION

B

tac11-0566_fig4-07

Soil Hazard Class

Climate class is made up of soils for which the climate—the temperature 

or lack of moisture—is the major hazard or limitation.

Erosion class is made up of soils for which the susceptibility to erosion is 

the dominant problem or hazard affecting their use. Erosion susceptibility 

and past erosion damage are the major soil factors that affect soils in 

this subclass. 

Soils class is made up of soils that have soil limitations within the 

rooting zone, such as shallowness of the rooting zone, stones, low 

moisture-holding capacity, low fertility that is difficult to correct, and 

salinity or sodium content.

Water class is made up of soils for which excess water is the dominant 

hazard or limitation affecting their use. Poor soil drainage, wetness, a 

high water table, and overflow are factors that affect these soils. Undefined

EXPLANATION

Climate

Erosion

Soil

Water

Cropland on soils in selected hazard class Other cropland

C

Figure 4.7. (A) Soil hazard class, (B) percentage of cropland area used for selected crops for each soil hazard class, and (C) location 

of cropland on each soil hazard class (climate, erosion, soil, and water) in the United States (Baker and Capel, 2011).
78 Agriculture—A River Runs Through It—The Connections Between Agriculture and Water Quality

Citrus

0

20

40

60

80

100

All crops

Corn

Soybean

Wheat

Other crops

Cotton

Sorghum

Vegetables and

ground fruit Rice

Orchards and grapes

A

B

C

tac11-0566_fig4-08

EXPLANATION

Cropland on soils requiring artificial drainage

Other cropland

 Cropland area, in percent

Figure 4.8. Extent of land in the United States underlain by soils for which artificial drainage (A) at the surface and (B) at the 

subsurface is likely required to remove excess water in order to cultivate cropland; (C) percentage of cropland cultivated for selected 

crops on the areas requiring artificial drainage (Baker and Capel, 2011).
Terrain, Climate, Soil, and Water 79

Availability of Data on Locations of Subsurface Drains

Modern agriculture in much of the United States would not be possible without extensive drainage (see “Initial 

Modification of the Landscape for Agriculture”). Large areas of Ohio, Indiana, Illinois, and Iowa, along with many 

of the fertile lowlands along the Mississippi River Valley, Piedmont Plains, and southern Florida, would be too wet 

to farm. In addition, the development of irrigation in the West would have failed because of waterlogged soils and 

accumulated salt content.

Prior to European settlement, wetlands covered about 13 percent (more than 1 million km2) of the land area 

of the conterminous United States (Dahl, 1990). About one-half of these wetlands have now been drained. Illinois, 

Indiana, Iowa, Michigan, Minnesota, Ohio, and Wisconsin account for about one-third of the wetland area that 

was drained and converted to cropland (Pavelis, 1987). By 1987, more than 14 percent of the cropland in the 

conterminous United States had been made suitable for agriculture through artificial drainage (table 4.1). 

The removal of excess water has resulted in reliable crop production in wetland areas. However, contaminants 

transported in runoff and subsurface drainage from cropland are often the major contributors to water-quality 

problems in our Nation’s streams. For example, much of the nitrate that enters the Mississippi River and, 

subsequently, contributes to hypoxia in the northern Gulf of Mexico comes from water that has been artificially 

drained from agricultural fields in the Midwest (Petrolia and Gowda, 2006). The use of artificial drainage in irrigated 

areas with highly saline soils has led to elevated salinity (above background levels) in some western streams. In 

some cases, highly saline or nutrient-rich drainage waters have damaged aquatic ecosystems (Madramootoo and 

others, 1997).

Information on the location and areal extent of artificial drainage networks is crucial to understanding and 

quantifying their potential effects on water quality. The locations of surface drainage ditches are well known, 

because they are easily observable on the landscape. The extent of subsurface drainage systems, however, is poorly 

known in most areas because of their distributed nature, the extended period of installation, incomplete location 

maps, and a general lack of recent, systematic surveys of their spatial distributions. Surveys of on-farm drainage 

systems were included in the Censuses of Agriculture for 1920, 1930, 1969, and 1974. Censuses of drainage projects 

also were taken every 10 years from 1920 to 1960. In 1978, a Census of Drainage was conducted by county by the 

Soil Conservation Service (now known as the Natural Resources Conservation Service, NRCS) (U.S. Department 

of Commerce, 1981). In 1982 and 1992, statistically based surveys of both subsurface and surface drainage systems 

were carried out by the NRCS for the National Resources Inventory. Land capability classes (fig. 4.6) and crop 

information were used in these surveys to estimate the extent of artificially drained cropland between 1900 and 1985 

(Pavelis, 1987). In addition to the lack of drainage information in recent decades, the lack of a consistent datacollection method has resulted in great uncertainty as to the locations of subsurface drains throughout the country.

Networks of subsurface drainage systems have been installed beneath agricultural fields in the last few decades. 

In many cases, these systems have been installed as patterned drainage (fig. 4.9) to improve control over soil water 

and thus increase crop yield. Landowners, however, are not required to report the installation of subsurface drainage 

systems or keep track of their locations. As a result, the locations of these networks are largely unknown.

Various approaches have been used to quantify the areal extent of subsurface drainage systems beneath 

agricultural lands. Statistically based farm survey methods generally provide county-level estimates of the area 

of drained land but do not provide the spatial resolution critical to many watershed studies. Statistically based 

surveys plus mapping of poorly drained soils overlain by cropland provides a spatial reference for areas that are 

likely drained. However, artificial drainage is not always installed in areas that could potentially benefit from it, and 

conversely, it is not always limited to poorly drained soils. Mapping of individual subsurface drains gives the most 

spatially detailed results for limited areas, but this mapping is labor intensive, expensive or, in many locations, not 

possible. Sometimes this mapping can be done by combining infrared aerial photography with soils information, but 

results are not always reliable.
80 Agriculture—A River Runs Through It—The Connections Between Agriculture and Water Quality

Leary Weber Ditch Basin

Tile drain

Tile-drain outlet

Overland-flow outlet

EXPLANATION

0 0.5 1 Kilometer 

0 0.5 1 Mile

35°51'

85°51' 85°50'

35°52'

Base From 2003 National Agricultural

Imagery Program, County Tiles for Indiana, 

Hancock County. Universal Transverse 

Mercator projection, Zone 16, Nad83

Tile Drain

Site

Leary Weber

Ditch Gage

and

Sampling

Site

Overland Flow

Site

X

X

X

North Site

South Site

Mohawk

Leary Weber Ditch

Sugar Creek

Indiana Hancock County

Sugar Creek

Leary Weber

Ditch Basin

Mohawk

Figure 4.9. Approximate locations of the subsurface drains in Leary-Weber Ditch watershed, Indiana (Baker, 

Stone, and others, 2006).

The 2012 Census of Agriculture included two questions asking landowners about the number of acres of 

their land that are either (1) underlain by functioning tile drains, or (2) artificially drained by ditches 

(U.S. Department of Agriculture, 2013). The answers to these questions will likely be aggregated by county 

for public release. Once this information is made available, such information may be used to make State- and 

National-scale assessments of the extent and locations of artificially drained agricultural lands by combining 

it with the most up-to-date maps of cropland and poorly drained soils across the Nation. This level of 

information will still be insufficient, however, to fully address water-quality concerns in many locations. 

Because water flowpaths are local, each field has unique flowpaths to the stream. In order to make the best 

decisions for each field, the locations and extent of its subsurface drainage must be known. Knowledge of 

the specific location of subsurface drainage under individual fields conflicts with the tradition of privacy 

for landowners, but this knowledge would aid the science-based implementation of practices to improve 

water quality.
Terrain, Climate, Soil, and Water 81

Summary

The location of agriculture in the United States is 

determined by natural, economic, and societal factors. The 

natural factors for the locations and extent of crop agriculture 

are terrain, climate, soil, and soil water. It is the manner in 

which these four natural factors are combined that allow 

specific crops to be grown in certain areas. Some lands have 

conditions that are nearly ideal for commercial agriculture. 

Other areas have rugged slopes, poor soil, either lack or have 

an excess of soil water and (or) have an inhospitable climate 

that make commercial agriculture either unprofitable or 

infeasible. Most agricultural lands, however, are between the 

two ends of this range. Today, most of the cultivated cropland, 

and grassland used for livestock is devoted to commercial 

agriculture, growing crops and livestock that are adapted to 

the particular conditions of the area to maximize production. 

In the United States, cropland (row crops, grains, fruits, nuts, 

and vegetables, excluding hay) occupies about 13 percent of 

the total land area. Grassland and rangeland occupy another 

41 percent of the land area of the Nation. 

Most crops are grown on gentle slopes (less than 

3 percent) in areas where the temperature, precipitation, 

and soils are favorable for their growth. In many areas that 

are naturally too steep, too wet, or too dry for crops, the 

landscape has been modified to allow them to be grown and 

(or) thrive. Some of the natural limitations on crop growth 

can be overcome through agricultural modifications, but other 

limitations (such as climate) cannot be overcome. Agricultural 

modifications commonly affect water availability through 

irrigation and (or) drainage, and soil fertility (including soil 

organic matter) through the addition of manure, nutrients, 

and lime. In general, however, it is not feasible to modify the 

other natural factors—soil texture, soil depth, soil mineralogy, 

temperature, and terrain—at large spatial scales. 
82 Agriculture—A River Runs Through It—The Connections Between Agriculture and Water Quality

Agriculture in Maryland’s Eastern Shore

In the 1640s, when the European settlers first arrived in what is now the Delmarva Peninsula, Maryland 

(fig. 1.3), they encountered a relatively flat coastal upland that was heavily forested with a mixture of evergreen 

and hardwood trees. Beneath the lush forests was a diverse understory of plants and wildlife cleft by steep ravines 

that carried streamflow to the Chesapeake Bay. Wetlands and riparian forests covered the lower elevations and were 

likely frequented by large flocks of migrating waterfowl. 

With settlement came the clearing of the forests for timber, fuel, and agriculture. From 1658 to 1663, land patents 

were sold to wealthy immigrants who began raising livestock, cultivating corn, and supplementing their income by 

growing tobacco. Landowners acquired indentured servants to clear additional acreage in order to increase tobacco 

production. After 1720, the need for cropland expanded rapidly, driven largely by the increasing demand for wheat. 

Through the 1800s, the row-crop and animal agriculture underwent multiple cycles of growth and decline as farmers 

faced a variety of challenges, including reductions in soil quality. By 1900, the westward movement of farming had 

reduced the importance of Maryland’s agriculture to the Nation, leading to a substantial decrease in farmland in the 

State. Given the high productivity of the farmland and the proximity to eastern cities, fruit and vegetable production 

became more prevalent during the early part of the 20th century. As the cost of fuel and transportation decreased, 

however, fruit and vegetable farmers on Maryland’s Eastern Shore were unable to compete with producers in 

California, resulting in a shift to other products. In the 1950s, crop production on Maryland’s Eastern Shore began to 

shift toward production of corn, soybean, and small grains to support the burgeoning poultry industry. Much of this 

change was driven by the availability of low-cost inorganic fertilizer that became available after World War II, when 

plants producing ammonium for munitions were converted to production of fertilizer.

Today, the region remains mostly rural, with continued cultivation of row crops, mostly corn, soybean, 

and small grains, although there is some pressure to convert farmland to housing and other urban uses. Some 

agricultural land is used for pasture and hay to support dairy production and, in recent years, specialty vegetable 

producers, greenhouse nurseries, and organic industries have increased and are filling a growing niche. Over the 

past few decades, impairments of water quality in Chesapeake Bay have been partly attributed to nonpoint-source 

pollution from agriculture, especially on Maryland’s Eastern Shore, where agriculture remains the predominant 

land use. This has been an important factor for changing agricultural practices to better protect water quality in the 

area (Chesapeake Bay Program, 2013). Recent studies by the USGS in the Delmarva Peninsula and wider Northern 

Atlantic Coastal Plain have focused on nutrients and pesticides, including the hydrologic and geochemical controls 

on their distribution in groundwater (Ator and others, 2005; Ator, 2008); temporal trends (Debrewer and others, 

2007; Denver and others, 2010); and their contribution from groundwater to surface water (Ator and Denver, 2012).

A B C

The landscape of Maryland’s Eastern Shore. (A) The forests typical of the time before agriculture, (B) typical 

cropped fields, and (C) the view from the air (23 square kilometers). Photograph A by Judith Denver, U.S. 

Geological Survey, 2015; photograph B by Paul Capel, U.S. Geological Survey, 2010; photograph C from U.S. 

Department of Agriculture, Farm Service Agency, National Agriculture Imagery Program, 2010.
Water on the Pre-Agricultural Landscape 83

5 Chapter —Water on the Pre-Agricultural Landscape 

How Does Water Move Through Natural 

Watersheds?

Prior to human settlement, the North American landscape 

consisted of forest, grasslands, scrublands, wetlands, and 

barren areas (fig. 5.1). Adequate precipitation to support tree 

growth generally defined the forest-grassland boundary. Areas 

with insufficient precipitation to support grass gave way to 

scrubland. Deciduous broadleaf forests once covered most of 

the East, the Ohio and lower Mississippi River Valleys, and the 

middle Great Lakes region. Needle leaf forests covered much 

of the central and northern Pacific Coast, the higher elevations 

of the West, portions of the interior North, and a narrow belt 

in the Deep South. Grasslands covered much of the sub-humid 

interior lowlands of the Great Plains from Texas and New 

Mexico to the Canadian border. Scrublands were concentrated 

in the arid lowlands of the interior West, where vegetation 

varied from the dense, brushy chaparral of southern California 

to the cacti of the Southwest and the mesquite of Texas. The 

presence of wetlands was partly defined by topography, and 

wetlands were widely scattered across the humid portions of 

the landscape in low-lying areas. High densities of wetlands 

were found in the recently glaciated areas of the Midwest and 

in low-lying areas near coasts and large rivers, such as the 

Gulf Coast and the lower Mississippi River Valley.

Many of the characteristics shown on the maps in 

figure 5.1 are driven by geography. The broad humid area of 

the Eastern United States results from movement of moist air 

northward from the Gulf of Mexico. In contrast, the land mass 

of Mexico contributes appreciably to the aridity of the western 

landscape. Superimposed on this eastern-to-western moisture 

gradient is a general increase in evapotranspiration rates from 

north to south. At the local scale, the larger effects of regional 

climate on the water cycle are modified by topography, 

vegetation, and soil type (see “Soil Properties, Soil Water, and 

Soil Erosion”). These combined factors determine the amount 

of precipitation that falls on the landscape, how the water 

moves through the landscape, and how much of that water is 

retained and used by the vegetation within the landscape. 
84 Agriculture—A River Runs Through It—The Connections Between Agriculture and Water Quality

tac11-0566_fig05-01

A

B

C

EXPLANATION

Needle-leaf forest

Broad-leaf forest

Grassland

Scrubland

Wetland

Potential natural vegetation

EXPLANATION

914 to 1,271

1,272 to 2,032

539 to 913

0 to 538

2,033 to 7,280

Precipitation, in millimeters

EXPLANATION

0 to 200

>200 to 400

>400 to 600

>600 to 800

>800 to 1,188

Evapotranspiration, in millimeters

Figure 5.1. Spatial distributions of natural vegetation, average annual precipitation, and average annual evapotranspiration throughout 

the pre-agricultural conterminous United States (Baker and Capel, 2011).
Water on the Pre-Agricultural Landscape 85

Soil Properties, Soil Water, and Soil Erosion

Soil is the foundation of all terrestrial life. Soil is composed of solid earth materials, air, and water, with the 

solid phase containing both mineral and organic matter (fig. 5.2). The mineral component of soil forms when the 

parent rock material is broken down (weathered) by physical, chemical, and biological processes. Soil organic 

matter is formed from the decay of plant and other biological residues. Because of the varying effects of parent 

material, climate, topography, hydrology, and biological activity. Soil properties may vary dramatically from one 

location or depth to another.

Soils in many locations, however, tend to contain a characteristic series of layers with roughly similar 

features, reflecting the results of similar combinations of processes that lead to soil formation. As illustrated 

by the examples shown in figure 5.2, these layers, known as horizons, have different colors, texture (size 

distributions of their mineral particles), and (or) structure (degree of aggregation of individual particles).

SL SL

L

L

SCL

SCL

SiL

SiL

Sand

Silt

Clay

Organic material

Water

Air

Percentage of soil volume 

distribution

EXPLANATION

0

10

20

30

40

50

60

70

80

90

100

Depth below land surface, in centimeters

Figure 5.2. Compositions of four different soils are compared—sandy loam (SL), loam (L), silty clay loam (SCL), and 

silty loam (SiL). The pie diagrams show the volume distribution of the mineral solids (sand, silt, and clay), organic-matter 

solids, water, and air at field capacity. The soil columns show the variation in the color and degree of cracking over the top 

1 meter. The darker colors indicate layers that are richer in organic matter (Capel and Hopple, 2018). Photograph by Scott 

Kronholm, University of Minnesota, 2010.
86 Agriculture—A River Runs Through It—The Connections Between Agriculture and Water Quality

The uppermost (shallowest) soil layer 

consists largely of plant matter at various stages of 

decomposition. Below this uppermost layer lies the 

topsoil, which tends to contain a large amount of 

stable, decayed organic matter (humus), and mineral 

components derived from the weathering of the parent 

rock material in place. This layer also is the source of 

most of the water and nutrients for the plants, and home 

to most soil organisms (bacteria, fungi, arthropods, mites, 

earthworms, nematodes, insects, and others). The intense 

biological activity in this horizon is supported largely by 

energy obtained from the decomposition of plant residues 

during humus formation. Below this area lies the subsoil, 

which contains large amounts of iron and aluminum 

minerals leached from the overlying layers by infiltrating 

water and accumulated at this depth through chemical 

precipitation. The subsoil also contains weathered parent 

material. These uppermost layers contain most of the 

plant root systems, although some plants have deep 

roots that reach downward to greater depths. Deeper in 

the soil are zones that contain fragments of relatively 

unweathered parent rock material.

Weathering processes create mineral particles of 

different sizes (clay: less than 0.002 millimeter (mm), 

silt: 0.05–0.002 mm, sand: 2.0–0.05 mm; Soil Science 

Society of America, 2012). The distribution of the 

particle sizes determines the texture category of the soil. 

A soil with equal amounts of sand, silt, and clay (by 

weight) is referred to as a loam. In many soils, mineral 

particles become bound together by organic matter to 

form soil aggregates (fig. 5.3). The stability of aggregates 

is dependent upon the characteristics of the natural 

minerals they contain and the organic matter that holds 

these minerals together.

Soil aggregates, in turn, bind together to form clods. 

Between clods are pores, channels, and other conduits 

of different sizes and shapes. The small spaces within an 

aggregate are referred to as micropores, with diameters 

of 0.005–0.03 mm. The larger spaces between soil 

aggregates can act like small, interconnected pipes. The 

largest of these spaces, often referred to as macropores, 

are greater than 0.075 mm, and are formed by insects, 

earthworms, plant roots (after they have decomposed 

in place), and desiccation. The latter process creates 

channels in the soil from repeated cycles of expansion 

and contraction of clay particles in the soil as it absorbs 

water and subsequently dries out (fig. 5.4).

tac11-0566_fig05-03

Quartz

particle

Soil pore

Figure 5.3. Soil aggregate with a plant root. Some 

individual mineral particles, such as the sand-sized particle 

of quartz, can be seen. The organic matter gives the 

aggregate the brownish color. Macropores can be seen 

throughout the surface of the aggregate. Photograph by 

Gustavo Merten, University of Minnesota-Duluth, 2011.

Figure 5.4. Macropores at the surface of a bare soil 

caused by desiccation. Photograph by Paul Capel, U.S. 

Geological Survey, 2009.
Water on the Pre-Agricultural Landscape 87

Water largely flows through macropores by gravity, 

which can be a fairly rapid process after precipitation 

events. In micropores, water flows at a slower rate 

through the soil matrix by way of capillary action, 

which is governed by the interfacial tension between 

water, air, and soil particles (capillary forces). Because 

of these capillary forces, soil water in small pores 

tends to be tightly held, but water in macropores tends 

to move freely. The extent to which water is stored in 

the soil matrix (table 5.1)—as well as the rate at which 

it moves through soil—is largely dependent on soil 

texture. In general, fine-grained (clayey) soils can hold 

more water than coarse-grained (sandy) soils. Three 

reference points are commonly used to characterize the 

water-holding capacity of soils. Saturation is the water 

content at which all pore spaces are filled with water (all 

air in the pores thus having been displaced by water). 

Field capacity is the maximum amount of water that a soil 

can hold for extended periods of time (days). Permanent 

wilting point is the lowest soil-water content at which 

plant roots are able to withdraw water from the soil. 

Soil erosion is the process by which the forces 

generated by the movement of water and wind across 

the land surface cause the detachment of individual 

soil particles and their transport to other locations. 

During a rainstorm, this process can be initiated by the 

splash of a rain drop, which, if falling on a bare soil, 

can disrupt the soil structure (fig. 5.5), resulting in the 

transport of the dislodged soil particles downhill with 

Figure 5.5. A rain drop splashing on a soil surface and 

dislodging soil particles. Photograph from U.S. Department 

of Agriculture, Natural Resources Conservation Service 

photo gallery, NRCSIA99138, 1999. 

Table 5.1. Water contents of three soil textures.

[From Neitsch and others, 2002]

Soil 

texture

Percent 

clay

Water content

(fraction of total soil volume)

At 

saturation

At field 

capacity

At permanent 

wilting point

Sand 3 0.4 0.06 0.02

Loam 22 0.5 0.29 0.05

Clay 47 0.6 0.41 0.2

the runoff. The runoff, in turn, accumulates in tiny 

flowpaths that can dislodge more soil particles. These 

tiny flowpaths can merge to form small channels (rills) 

in which the water may flow with greater energy and, 

thus, has a greater capacity to dislodge soil particles. 

These small flowpaths can, in turn, merge to form larger 

flowpaths (gullies) through which the water flows with 

even greater energy and greater capacity to erode soil 

(fig. 5.6). Rills and gullies are observed in both natural 

and human-altered landscapes, particularly in areas with 

substantial topographic relief.

Figure 5.6. A gully formed from erosion (Kansas). 

Photograph by Jeff Vanuga, U.S. Department of 

Agriculture, Natural Resources Conservation 

Service photo gallery, NRCSKS02008, 2002.
88 Agriculture—A River Runs Through It—The Connections Between Agriculture and Water Quality

The loss of the soil has always been a major concern for crop agriculture. The areal extent of highly erodible 

cropland in the United States is shown in figure 5.7. In the 1930s, following the Dust Bowl, there was a substantial 

focus on soil conservation (Chapter 3), which was the impetus for many agricultural landscape modifications and 

many Federal subsidy programs (see ”U.S. Department of Agriculture Conservation Programs” in Chapter 3). Soil 

erosion can be reduced with soil conservation practices, such as minimum tillage and no-tillage (see “Effects of 

Tillage on Runoff and Recharge” in Chapter 6).

EXPLANATION

Cropland soils classified 

highly erodible (about 

 50 percent of all cropland)

Soils classified 

highly erodible 

Figure 5.7. Cropland soils vulnerable 

to water and wind erosion (Baker and 

Capel, 2011).

Through years of research, the major factors 

in the soil erosion process have been identified 

and quantified in the Universal Soil Loss Equation 

and subsequent equations (U.S. Department of 

Agriculture, Agricultural Research Service, 2010). 

The erosion of soil from an agricultural field is 

understood as the product of the following factors 

in the equation:

 A = R × K × S × L × C × P (5.1)

where

A is the average annual mass of soil lost;

R is characteristic of the rainfall and climate;

K is characteristic of the soil;

S is characteristic of the topography;

L is the length of the field (uninterrupted field

 surface);

C is the choice of crop and land-cover management; 

 and

P is the field-based agricultural modifications,

 such as installation of terraces, grassy

  waterways, and buffer strips.

Sedimentation, the settling of particles from 

water because of gravity and other forces, results 

when the energy of the water can no longer hold 

the soil (sediment) in suspension. In the landscape, 

sedimentation frequently takes place in topographical 

depressions, in the bottom of valleys, in buffer strips, and 

in grassy waterways where the rate of water movement 

(and its energy) decreases. Good land-management 

practices, such as the construction and (or) maintenance 

of buffer strips, are extremely important in reducing the 

transfer of eroded soil from the upland to the channel. 

Sediment in stream channels and floodplains is largely 

from eroded soils and eroded streambanks. It is one 

of the most frequent causes of impaired streams in the 

United States (U.S. Environmental Protection Agency, 

2012h; Merten and others, 2016). Too much sediment in 

stream water decreases light penetration, which affects 

the productivity of the stream and disturbs the food 

chain; this in turn may lead to a change in the aquatic 

community of the stream by eliminating the base of the 

food chain. This elimination usually means that the more 

sensitive species that cannot adapt to a changing diet 

can no longer be present in that environment, and only 

the more hardy species can survive. Too much sediment 

also destroys habitat and buries bottom-dwelling aquatic 

organisms, such as mussels.
Water on the Pre-Agricultural Landscape 89

Watersheds and Water Budgets

Watersheds are defined as areas in which water collects and discharges through the mouth of a stream or river—often 

flowing into a larger body of water. Watershed boundaries are determined by the topography of the landscape, and the term 

watershed can be used to describe a vast range of sizes from the regional scale (for example, the Chesapeake Bay watershed) to 

areas less than 0.1 km2 in size (fig. 5.8). 

Washington D.C.

A

B

C

D

Evapotranspiration

Precipitation

Figure 5.8. Relevant scales for water flowpaths and movement. (A) Regional scale, represented by the Chesapeake Bay 

watershed. (B) Watershed scale, represented by the Morgan Creek, Maryland watershed. (C) Catchment scale, shown in 

three dimensions to illustrate the connection between the surface and subsurface. (D) Field scale, showing the soil (brown) 

and groundwater (blue) zones. The arrows represent the water flowpaths for the components in the soil-water budget: 

precipitation, evapotranspiration, runoff, and recharge to groundwater to baseflow. Recharge from the topographically higher 

part of the watershed may flow into deep groundwater and leave the catchment as regional groundwater flow.
90 Agriculture—A River Runs Through It—The Connections Between Agriculture and Water Quality

The small drainage networks where streams first appear 

in the uppermost portions of the landscape (catchments) can 

aggregate to form watersheds, which in turn can aggregate 

into regional-scale watersheds often referred to as basins. 

The amount of time required for water movement through 

watersheds, and the processes that control that movement, are 

different over this range of scales (table 5.2). At all scales, 

however, it generally is possible to estimate the timescales 

and flowpaths by which water moves into and out of these 

watersheds on the basis of climate, terrain, geology, land 

cover, and soil characteristics. 

A common approach to understanding the important 

inputs and outputs of water through watersheds is to develop 

an annual water budget (Healy and others, 2007). 

Precipitation (P) is the primary source of water entering 

the landscape for most watersheds. Precipitation can result 

as rain, snow, or other forms of frozen water. Average annual 

precipitation across the United States is shown in figure 5.1B. 

Water from precipitation exits the watershed by three primary 

flowpaths: water can return to the atmosphere by way of 

evapotranspiration, recharge to groundwater and eventually 

discharge to streams, lakes and wetlands, or flow directly over 

the land surface to water bodies.

Evapotranspiration (ET) is the sum of two processes—

evaporation and transpiration. Evaporation (a physical/

chemical process) is largely controlled by energy imparted 

by the sun and the capacity of the atmosphere to contain 

additional water vapor. Transpiration (a biological process) 

is the loss of water to the atmosphere from plants, which 

actively move water upward from the soil through the roots, 

to the stems, and eventually to the leaves, where it evaporates 

to the atmosphere. Estimates of average annual ET for the 

United States are shown in figure 5.1C. The measurement of 

ET is inherently difficult, so it typically is estimated by using 

mathematical relations that consider vegetation, precipitation, 

air temperature, wind speed, and solar radiation.

Water not evaporated or taken up by plant roots can 

move downward into the soil and eventually be incorporated 

into the groundwater system. For most natural watersheds, 

including those considered here, groundwater ultimately 

discharges into the streams that drain the watershed. This 

groundwater flow to streams (often called baseflow) is critical 

to the health of streams because it sustains streamflow during 

low-precipitation periods (Winter and others, 1998). As this 

description suggests, it is useful and important to recognize 

that the same water that moves down through the soil as 

recharge ultimately becomes baseflow, and, as a result, the 

movement of recharge to groundwater to baseflow (RGB)

should be considered a single pathway that operates on 

timescales from days to centuries (table 5.2). 

Runoff (RO) is water that flows over the landscape 

and directly into the surface waters that drain the watershed 

(for example, streams). The importance of runoff as a water 

flowpath is affected by precipitation, vegetation, topography, 

and soil characteristics. Precipitation in excess of what the 

landscape can assimilate at a given time produces runoff. 

Table 5.2. Potential flowpaths and their associated transit times in a natural landscape. 

[Blue cells indicate the typical time ranges for each flowpath under natural landscapes. Blank cells indicate that these flowpaths or time scales are not important 

for this type of catchment]

Flowpath

Transit time

Minutes Hours Days Months Years Decades Centuries

Flowpaths to the atmosphere

Evaporation

Plant transpiration

Flowpaths to shallow groundwater

Macropore flow through soil

Matrix flow through soil

Seepage from wetlands

Flowpaths to streams

Local groundwater flow

Runoff

Macropore flow through soil to stream

Flowpaths to rivers and oceans

Regional groundwater flow
Water on the Pre-Agricultural Landscape 91

Thus, the propensity for runoff tends to increase with the 

volume and intensity of precipitation. In cold climates where 

snow accumulates on the landscape during winter, runoff 

events also result during spring snow melt and commonly 

cause flooding in some areas. As the timeframes in table 5.2

indicate, runoff typically occurs over minutes to hours 

following a precipitation event, but does not sustain surface 

waters during dry periods. Although streamflow increases 

caused by large precipitation events (for example, many 

centimeters of rainfall within a 24-hour period) are sporadic 

and short-lived, these large events primarily are responsible 

for shaping the stream channel and transporting sediment from 

erosion (see “Soil Properties, Soil Water, and Soil Erosion”).

Runoff flowpaths are generally fast (fastflow) and, 

sometimes, high energy (fig. 5.9). In contrast, flowpaths that 

result in infiltration that recharges groundwater are generally 

much slower (slowflow) (table 5.2). Fastflow comes in contact 

only with the surface and (or) upper layers of the soil for short 

periods (hours to days). Slowflow (RGB), which eventually 

discharges to a stream, will come in direct contact with the 

soil and (or) aquifer materials for long periods (months to 

decades). There are other slow flowpaths, such as wetlands, 

which have varying degrees of contact with the soil. 

Water in a stream is a mixture of the various sources of 

water that has moved off the landscape. The same flowpaths 

that move water out of the landscape deliver it to the stream. 

The mixture of slowflow and fastflow contributions to 

the stream forms a continuum that has been quantified by 

baseflow separation methods based on analysis of the stream 

hydrograph (Wahl and Wahl, 1995; Sloto and Crouse, 1996). 

Using these separation methods, the sources of water to a 

stream can be described as ranging from an end-member 

of 100 percent slowflow to an end-member of 100 percent 

fastflow. All natural streams can be located on this continuum 

(fig. 5.9). Streams in small watersheds can be categorized by 

their characteristic flowpath(s). 

In practice, distinguishing between waters arriving at a 

stream by way of slowflow and fastflow is usually difficult. 

For example, water entering the shallow subsurface may find 

preferential pathways that bring it to discharge points near 

the stream in a matter of hours. Alternatively, in some cases 

recharge to groundwater by infiltration of precipitation causes 

the water table to rise above land surface, resulting in seeps 

or other flow pathways that rapidly deliver groundwater to 

the stream. These pathways are difficult to categorize because 

they have characteristics of both runoff and groundwater flow. 

100 percent

slowflow

sourced

100 percent

fastflow

sourced

A

B

Groundwater aquifer

Stream

Slowflow

Fastflow

Figure 5.9. (A) Simplified diagram of the water flowpaths from the landscape to the stream. 

(B) Continuum of the water flowpaths from the landscape to the stream with water sourced from 

100 percent slowflow to 100 percent fastflow.
92 Agriculture—A River Runs Through It—The Connections Between Agriculture and Water Quality

From the perspective of streamflow, these pathways deliver 

water rapidly and contribute to the increased flows commonly 

associated with runoff. As a result, these pathways have 

traditionally been included in the RO component of the water 

budget. However, much of the water entering the stream by 

these pathways is not from the current precipitation event, and 

can be from months- or decades-old groundwater. As a result, 

the chemical characteristics of this water can be much closer 

to that of groundwater than to runoff from recent precipitation 

(Shanley and others, 2002).

Storage (S) of water in watersheds can occur in 

surface-water bodies (for example, lakes and wetlands), in 

subsurface environments (groundwater and soil moisture), and 

in snowpack. The storage of water in surface-water bodies 

can dampen high streamflow after precipitation events, and 

can help sustain streamflow during dry periods. For natural 

landscapes, the amount of water stored within the watershed 

can change on a year-to-year basis (ΔS); however, over multiyear periods, ΔS generally is a small component of the water 

budget for natural watersheds.

Wetlands are a broad term for any body of water formed 

when the groundwater elevation rises to land surface Wetlands 

can be connected to isolated from other water bodies and can 

range in size from the small trickle of a surface spring to vast 

expanses of flooded river plains to regional areas like the 

Everglades in south Florida. Wetlands fluctuate in size with 

precipitation events as well as with overall seasonal changes 

in water availability, and can be permanent or ephemeral 

(see “Agriculture in the “Delta” region of Mississippi” in 

Chapter 6). Wetlands can receive their water inputs from the 

atmosphere, surface runoff, flooding, and (or) groundwater. 

Seasonally, the flow of water can become almost stagnant, 

which allows sediment to settle to the bottom.

A simple annual water budget, on the basis of the five 

components discussed above, quantifies the amount of water 

(in units of water volume per land surface area) that moves 

through a watershed:

P = ET + RGB + RO + ΔS (5.2)

where

P is precipitation,

ET is evapotranspiration (sum of evaporation and 

plant transpiration),

RGB is recharge to groundwater to baseflow,

RO is surface runoff, and

ΔS is change in water storage within the 

watershed.

At the watershed scale, streams/rivers are integrators 

of all water flowpaths across and through the landscape 

(runoff, recharge to groundwater, surface-water flow). For 

many watersheds, annual streamflow (SF) is the sum of 

annual groundwater discharge to the stream plus annual 

runoff to the stream. At the watershed scale, streamflow is the 

component that can be quantified with the greatest certainty. 

Consequently, it is sometimes useful to replace the sum of 

RO + RGB in the water budget with SF and given as:

P = ET + SF + ΔS. (5.3)

Morgan Creek, Maryland (fig. 5.8B) is used here as an 

example of an annual watershed-scale water budget. In the 

Morgan Creek watershed, the average annual precipitation is 

110 cm, which is distributed throughout the year (upper part 

of fig. 5.10). In the pre-agricultural Morgan Creek watershed, 

which was mostly covered with trees and wetlands, it is 

estimated that 78 percent of the total outflow was due to ET. 

Large precipitation events resulted in short periods of high 

streamflow, which occurred in Morgan Creek frequently 

throughout the year (lower part of fig. 5.10). These events 

caused a rapid increase in streamflow by more than an order of 

magnitude, and then returned to the pre-event levels within a 

few days. These rapid increases in streamflow were the result 

of both direct runoff and rapid subsurface flowpaths.

Recharge to groundwater to baseflow (RGB) at the 

watershed scale generally occurs over much longer time 

scales than evapotranspiration and runoff (table 5.2). This 

is important because the longer transit times of RGB help 

to sustain streamflow during periods when there is no 

precipitation. For example, figure 5.10 shows a lack of large 

precipitation events during January and early February, yet 

streamflow is maintained at a relatively constant level over 

the entire period by the discharge of groundwater to the creek. 

In this case, simple analysis of the streamflow data based on 

baseflow separation (Sloto and Crouse, 1996) indicates that 

of the estimated 22 percent of precipitation not lost to ET, 

approximately one-half of this precipitation entered the stream 

by way of rapid pathways and one-half by way of long-term 

RGB. In arid regions—where the RGB pathway is minimal 

and depths to groundwater are greater—streams often go dry 

during periods without precipitation. Therefore, these streams 

are ephemeral.
Water on the Pre-Agricultural Landscape 93

tac11-0566_fig05-10

0

20

40

60

80

10

30

50

70

0

2

4

6

8

1

3

5

7

Precipitation, in millimeters

Streamflow, in cubic meters per second

Year 2003

Streamflow

Precipitation

Jan. Feb. Mar. Apr. May June July Aug. Sept. Oct. Nov. Dec.

Figure 5.10. Precipitation and streamflow for Morgan Creek, Maryland, 2003. Morgan Creek is a 31-km2

watershed in Kent County, Maryland (fig. 5.8B). This relatively flat, coastal landscape was naturally covered 

with a mixture of evergreen and hardwood trees in the upland areas and wetlands in the low-lying areas. Even 

during dry periods of the year, the stream never goes dry. The water at the lowest flows reflects discharge of 

groundwater to the stream, commonly called baseflow (RGB) (Capel and Hopple, 2018).

Field-Scale Water Budgets

In the context of agricultural chemical transport through 

watersheds, it is important to understand how water movement 

results at scales that are comparable to those of typical 

agricultural fields (less than 1 km2

). Field-scale water budgets 

can be constructed in a manner similar to that for watersheds. 

At the field scale, it is often convenient to define the physical 

boundaries of the water budget in the context of the field 

itself. Thus, field-scale water budgets are considered to stop 

at the edge of the field and at the bottom of the root zone 

(approximately 2 m below the land surface, fig. 5.8D) and do 

not include streamflow in the overall budget. Nevertheless, 

water can still be considered to leave the landscape by way 

of three primary pathways—ET, RGB, and RO—so that the 

equation used for watersheds can be applied at the field scale:

P = ET + RGB + RO + ΔS. (5.4)

At the field scale, the timing, intensity, and duration 

of precipitation strongly affect the overall water budget. 

Consequently, it is useful to examine water budgets on daily as 

well as annual time scales. In most cases, precipitation is the 

only parameter in equation 5.4 that is easily measured, and, 

consequently, the other parameters generally are estimated. 

At the field scale, these estimates typically are made by using 

mathematical models where other measured parameters are 

used to determine the water budget (for example, temperature, 

wind speed, relative humidity, soil characteristics, land-surface 

slope; see “Quantifying Water and Sediment Budgets”). As 

examples, daily field-scale water budgets are examined here 

for three natural landscape types: humid grass, humid forest, 

and arid scrubland (figs. 5.11 and 5.12). 
94 Agriculture—A River Runs Through It—The Connections Between Agriculture and Water Quality

tac11-0566_fig05-11

-300

-150

0

150

300

0

20

40

60

80

0

300

600

900

1,200

0

20

40

60

80

0

300

600

900

1,200

0

20

40

60

80

0

300

600

900

1,200

0

20

40

60

80

0

300

600

900

1,200

0

20

40

60

80

J F M M J J A S O DN A J F M M J J A S O DN A J F M M J J A S O DN A

Daily values, all units in millimeters per day

Cumulative values, all units in millimeters

Grass (humid) Forest (humid) Scrubland (arid)

Recharge Runoff Soil storage Evapotranspiration Precipitation

Daily values

Cumulative values

EXPLANATION

Figure 5.11. Estimates of daily precipitation, evapotranspiration, runoff, recharge, and change in soil water for three natural landscape 

types—grass (humid), forest (humid), and scrubland (arid). The scale of the y-axis changes among the water-budget components, 

but is the same across the types for a given water-budget component. The estimates are model results for a single year (2003) for 

the typical climate regimes of central Indiana (humid grass and forest) and central Washington (arid scrubland). For all three cases, 

the modeled landscape was a square with an area of 16.2 hectares, loam soil, and a 3 percent slope. (See “Quantifying Water and 

Sediment Budgets,” Roth and Capel, 2012a).
Water on the Pre-Agricultural Landscape 95

tac11-0566_fig05-12

EXPLANATION

Evapotranspiration

Precipitation

Recharge

Runoff

Grass (humid) Forest (humid) Scrubland (arid)

Figure 5.12. Annual inflows (precipitation) are represented by the left half of the pie diagrams, whereas the outflows 

(runoff, recharge, and evapotranspiration) are represented by the right half of the pie diagrams for three natural landscape 

types—grass (humid), forest (humid), and scrubland (arid). The area of the pie diagrams are proportional to the annual mean 

precipitation (108.7 centimeters for the humid climate and 17 centimeters for the arid climate). It is assumed that all water 

enters as precipitation. The estimates are based on model results (Roth and Capel, 2012a).

Daily water budget values for one or more years can 

be summed to determine annual water budgets. Multi‐year 

averages for the three landscape types are shown in 

figure 5.12. Although the amount of water stored in the soil 

may change from year to year, as mentioned above, the 

long‐term averages for natural landscapes tend to be constant 

and ΔS is, therefore, zero. Differences in both the magnitude 

and distribution of daily precipitation for the humid and 

arid cases are shown in figure 5.11. For the humid case, 

precipitation is distributed relatively evenly throughout the 

year. A substantial number of events exceed 10 millimeters 

per day (mm/d). In contrast, the graph for the arid case shows 

substantial seasonal variations in precipitation, and relatively 

few storm events exceed 10 mm/d. 

ET is the dominant water outflow process for natural 

landscapes at both the field scale and in larger watersheds, 

accounting for nearly two-thirds of the total outflow from 

humid grassland and nearly all outflow from arid scrubland 

(fig. 5.12). ET responds to seasonal weather changes and 

daily precipitation. The immediate increases in ET in both 

the humid and arid cases as a result of recent precipitation 

are shown in figure 5.11. In arid landscapes, most ET occurs 

shortly after precipitation events, when water is readily 

available in the soil. In humid landscapes, where precipitation 

is distributed throughout the year, the short spikes in ET 

following rain events are superimposed on a seasonal trend, 

with maximum ET during the summer and minimum ET 

during the winter. ET in humid settings is controlled largely by 

the growth of vegetation and therefore is strongly related to air 

temperature. Precipitation that falls during the growing season 

is more likely to exit the landscape as ET and less likely to 

exit as RGB or RO (fig. 5.11). 

At the field scale, RGB generally occurs over timeframes 

that are longer than individual precipitation events, because 

of the relatively slow movement of water within the soil 

(table 5.2). As a result, the largest daily RGB values typically 

are associated with multiple, closely spaced precipitation 

events or events of especially long duration (fig. 5.11). 

Because groundwater and streams generally are not included 

in field-scale water budgets, it is commonly assumed that all 

recharged water reaches the groundwater and eventually flows 

into the stream that drains the watershed containing the field. 

In humid grasslands, RGB is most important in the winter 

when the vegetation is dormant (fig. 5.11). Humid forests have 

relatively less recharge compared to humid grasslands because 

ET is greater for forest vegetation because of the deeper, more 

extensive root systems that develop to support large trees 

compared to grass.

Although RO accounts for only a small portion of the 

average annual field-scale water budget, it is the driving 

force for soil erosion, which is an essential process in natural 

landscape evolution. Generally, the few largest precipitation 

events generate most of the runoff and erosion. 
96 Agriculture—A River Runs Through It—The Connections Between Agriculture and Water Quality

At the field scale, stored water is present primarily in the 

form of soil moisture (see “Soil Properties, Soil Water, and 

Soil Erosion”). The amount of water stored in the soil changes 

as a result of precipitation, ET, and recharge to groundwater. 

Water near the soil surface can evaporate from the pore 

spaces, whereas water deeper in the soil can be drawn out 

by plant roots. When the water content of a soil exceeds its 

field capacity, water flows downward by gravity and enters 

the RGB flowpath. The magnitudes of short-term fluctuations 

in stored water are generally greater in humid landscapes 

than in arid landscapes. For both landscapes, there can be 

numerous and significant short-term fluctuations in soil-water 

content (fig. 5.11); the annual changes in soil water content are 

relatively small (less than a few percent).

Quantifying Water and Sediment Budgets

For a small area, precipitation is relatively easy to measure, but the other water-budget components are much 

more difficult to quantify. As a result, hydrologic models commonly are used to estimate the magnitudes of the other 

water-budget components. In these models, the current scientific understanding is captured in mathematical terms 

and equations that describe and estimate the movement of water and any other materials that it may transport across 

and through the landscape. One of these models is the Water Erosion Prediction Project (WEPP) model developed 

by the USDA Agricultural Research Service (Flanagan and others, 1995; U.S. Department of Agriculture, National 

Soil Erosion Research Laboratory, 2010). WEPP is a process-based model that uses the current understanding of soil 

erosion to estimate the quantity of sediment delivered to streams—estimates that have shown acceptable agreement 

with field data across the United States. The WEPP model estimates the water budget (evapotranspiration, recharge, 

runoff, soil-water storage) and the extent of soil erosion for a given climate, soil type, topography, vegetation, and 

land management regime. The model computes daily water balances and is capable of generating estimates for 

multi-decadal periods. Numerical values for parameters quantifying precipitation, temperature, solar radiation, soil 

type, land slope, and land cover are input to drive a system of equations describing interrelated processes such as 

infiltration, runoff, soil compaction, erosion, plant growth, and organic matter decomposition. The potential effects of 

a variety of agricultural management practices—such as tillage, irrigation, and drainage—also can be accounted for. 

The WEPP model was used to generate field-scale soil-water budgets in both natural (Chapter 5) and agricultural 

(Chapter 7) settings for a square field of 16.2 ha (40 acres—a quarter-quarter section) with a loam soil and a uniform 

slope of 3 percent (Roth and Capel, 2012a) These water budgets were generated using 60 years of climate data from 

two areas—a humid site in central Indiana and an arid site in central Washington (fig. 1.3). The water that infiltrated 

below the top 1 m of soil was defined as recharge. For agricultural simulations at the arid site (Washington), the 

typical daily irrigation was added to the precipitation for the model simulations. Because of the abundance of rainfall 

and moisture in the humid site (Indiana), no irrigation was applied during the simulations for that site. Although 

the input parameter values used in the simulations for these two sites were derived from input data from the sites 

themselves, the results presented have not been compared with observations from either location. Instead, they are 

presented as examples to illustrate current scientific understanding for determining field-scale soil-water budgets.
Water on the Pre-Agricultural Landscape 97

Summary

Water moves into, through, and out of natural landscapes 

along expected flowpaths (for example, P, RO, RGB, and 

ET) and over expected timeframes. The timeframes of these 

flowpaths range from minutes to centuries, depending on 

the flowpath and the distance over which the water moves 

(through a field, a small catchment, watershed, or basin). 

The volume of water moving along each flowpath depends 

on a number of factors including climate (precipitation, solar 

radiation, and air temperature), topography, soil type, and 

vegetation type. At the field scale, water can be stored within 

the soil, and at the watershed scale it can be stored within 

surface-water bodies (lakes and wetlands) and subsurface 

environments (soil and aquifers). 

In arid landscapes, ET exceeds precipitation, so nearly all 

water is lost through ET and little is available for either runoff 

or recharge. As a result, in arid landscapes streams are usually 

ephemeral because the water table is too far below the land 

surface to provide baseflow. Precipitation events are seldom 

large enough to induce runoff from the landscape, but the 

largest precipitation events can produce appreciable runoff and 

shape the landscape through soil erosion processes. 

In humid landscapes, precipitation generally exceeds 

ET, but ET is the largest component of outflow of the annual 

water budget. Both RO (fastflow) and RGB (slowflow) can 

be important components of the annual water budgets in 

humid environments. Plentiful recharge keeps the water 

table shallow, which allows baseflow to sustain perennial 

streamflow during dry periods. Runoff can cause intense but 

short-duration increases in streamflow following precipitation 

events. This runoff can cause soil erosion and deliver sediment 

and chemicals to the stream, although landscapes that are 

covered with perennial vegetation are less prone to soil 

erosion processes. 

Streams are integrators of all water flowpaths across 

and through the landscape. The interactions between the 

landscape and these various water flowpaths largely determine 

the quantity and quality of water in the stream. Because it is 

the water from individual field-scale and catchment outflows 

that aggregate to form larger streams and regional rivers, it is 

critical to understand the hydrologic processes at the smaller 

scales to understand the movement of water, sediment, and 

chemicals into the broader environment.
98 Agriculture—A River Runs Through It—The Connections Between Agriculture and Water Quality

Agriculture in the “Delta” Region of Mississippi

The “Delta” Region in northwest Mississippi (fig. 1.3) is located more than 160 km north of the actual 

Mississippi River delta. Prior to the arrival of Europeans, the rich alluvial soil of the Delta supported vast expanses 

of deciduous forests, rivers, bayous, and lakes. This area had fertile soils from annual flooding, a long, frost-free 

growing season, and plentiful rainfall. 

 To make agriculture possible in the Delta, the Mississippi River had to be harnessed by levees to reduce 

and eventually stop the annual flooding. The wetland forests were cleared and drained. Streams were straightened; 

ditches and canals were constructed to move water quickly and efficiently off of the land. Farmers found cotton to 

be a high-value product, but also one that required a considerable amount of labor. This labor was first carried out by 

slaves, and then, after the Civil War, by “sharecroppers” (Zeichner, 1939). In the late 1800s, the railroad reached the 

Delta region, prompting a major increase in the extent of agriculture because of ease of transportation. 

 During the Great Depression, the Agricultural Adjustment Administration paid farmers to remove land 

from production to increase the price of farm products. Anhydrous ammonia was first used here as a soil-applied 

fertilizer in 1932. A small cylinder of anhydrous ammonia was attached to a “Georgia Stock” plow pulled by a gray 

mule named Ike (American Society of Agricultural and Biological Engineers, 2011). The crude apparatus and the 

anhydrous ammonia it applied provided a much needed source of nitrogen for the otherwise rich alluvial soils of 

the Mississippi Delta. Farming became increasingly mechanized during the 1950s, requiring many farm laborers 

to seek employment elsewhere. In the 1970s, the increasing price of soybean prompted the clearing of much of the 

remaining bottomland hardwoods and wetlands to make way for additional cropland. As a result, about one-half of 

the cropland in the Delta was planted in soybean. The remaining land was used to grow rice, cotton, and a variety of 

other crops including corn, peanuts, pecans, sugar cane, sweet potatoes, and pond-raised catfish. By 2010, a major 

increase in corn acreage in the region—driven primarily by demand for corn for the production of ethanol—almost 

eliminated cotton cultivation.

 Because of the uneven seasonal distribution of rain, irrigation was often necessary to meet crop needs, 

maximize productivity, and to ensure against crop loss because of drought. Starting in the 1960s, irrigation water 

was drawn from streams and rivers, and eventually from groundwater (Coupe and others, 2012). Irrigation has 

resulted in lower water levels in the alluvial aquifer and decreased aquifer storage. The decreased storage, in turn, 

has led to substantially decreased surface-water flows at critical times of the year. Many streams in the Delta 

are no longer perennial, now flowing only intermittently during the summer in response to rainfall or irrigation. 

Intrabasin transfers or siphoning water from the Mississippi River are being considered as potential strategies to 

increase the amount of water available for irrigation (Barlow and Clark, 2011). Recent studies by the USGS on the 

effects of agriculture on water quality have focused on pesticides (Coupe and others, 2012; Rose and others, 2018), 

nitrogen in shallow groundwater (Welch and others, 2011), and groundwater/surface-water interactions (Barlow and 

Coupe, 2012).

A B C

The landscape of the “Delta” region of Mississippi. (A) The bottom-land hardwood swamps typical of the time 

before agriculture, (B) typical cotton field, and (C) the view from the air (23 square kilometers). Photographs A

and B by Claire Rose, U.S. Geological Survey, 2011; photograph C from U.S. Department of Agriculture, Farm 

Service Agency, National Agriculture Imagery Program, 2010. 
Agricultural Water and Soil Management 99

6 Chapter —Agricultural Water and Soil Management 

How Have Agriculture Modifications Changed 

the Movement of Water Through Watersheds? 

Successful crop agriculture depends on the availability of 

adequate, but not excessive, water in the plant’s root zone at 

appropriate times of the year. Each crop has its own minimum 

and maximum water needs and tolerances that change seasonally 

(fig. 6.1). Natural differences in the landscape and climate 

across the United States result in areas that have too much water 

available to crops, as well as areas with too little water and areas 

with near ideal water conditions. Agricultural activities have 

altered both directly and indirectly the flowpaths and volumes of 

water moving through the landscape. 

Defined here, “modifications” are all activities in 

agricultural landscapes that affect water flowpaths, water 

budgets, wind movement, soil erosion, and sediment 

yields. Modifications include the implementation of current 

agricultural best management practices (BMPs), but they 

go beyond these to include the practices of cropping, tillage, 

infrastructure alterations (including irrigation, drainage), and 

animal management. Decisions on agricultural modifications 

to the landscape are made by a broad spectrum of people and 

organizations including individual farmers, irrigation districts, 

drainage districts, counties, States, and the Federal Government. 

The term “modifications” is used here to help avoid the 

limitations that commonly used terms (such as BMPs) can 

have on the holistic perspective that virtually all agricultural 

modifications can have an effect on water, and, therefore, the 

broader environment.

Agricultural modifications are implemented for a number 

of purposes (table 6.1). All of the initial modifications to the 

landscape were implemented to create agricultural land (see 

“Initial Modification of the Landscape for Agriculture” in 

Chapter 4). Subsequently, most modifications are implemented 

to sustain or increase the annual crop productivity. Some 

modifications are intended to sustain agriculture for the long 

term by protecting the water and soil resources. In recent 

decades, many types of modifications have been implemented 

to protect the environment by decreasing soil and streambed 

erosion and decreasing the volume and velocity of runoff water, 

thereby increasing the overall stability of riparian and in-stream 

habitat, which can further help reduce erosion and protect 

water quality. As a byproduct, these agricultural modifications 

can create and enhance aesthetic and recreational benefits. 

The USDA administers a number of incentive programs that 

encourage the use of modifications to protect soil and water 

resources. Some of these programs support the use of BMPs, 

whereas other programs encourage the removal of vulnerable 

lands from production (see “U.S. Department of Agriculture 

Conservation Programs” in Chapter 3).

Agricultural modifications are implemented over different 

spatial areas (table 6.1). In most cases, these areas are field- or 

farm-specific. For crops, fields are managed individually. Animal 

feeding operations also are managed at the site-specific level, 

whereas rotational grazing operations are managed at the multiplefield scale. Some modifications are focused on restoring stability 

to streams, wetlands, and fragile portions of agricultural fields 

that border water bodies. Finally, some modifications, particularly 

large public irrigation and drainage projects, are implemented 

at the watershed, county, or regional scale. Within this large 

infrastructure, farmers make decisions as to how individual 

fields will be connected by choosing the irrigation delivery 

method (gravity and sprinkler) or the type of drainage (patterned 

subsurface drains and surface inlets).

A timeframe is associated with each agricultural modification 

(table 6.1). Infrastructural modifications are designed to be longterm (permanent) features of the landscape. Some modifications 

have been in place for more than a century (drainage ditches and 

reservoirs) and are subject to periodic maintenance and expansion. 

Other modifications have time frames of years to decades, based 

on contractual obligations (see “U.S. Department of Agriculture 

Conservation Programs” in Chapter 3) or large investments made 

for their initial installment (terraces, forested buffers). Yet other 

modifications (method of tillage and choice of crop) have time 

frames based on the growing cycle of annual crops (see “A Year 

with Corn in Central Indiana”). After each growing cycle, the 

modification can be repeated or changed. 

Every agricultural modification can affect the environment 

by affecting the movement of water and (or) soil through the 

landscape. When environmental concerns arise from agriculture, 

the field-based, annually changeable modifications are often 

targeted for change to help mitigate the concern. However, greater 

benefits can potentially be gained through changes to the longterm and more spatially expansive landscape modifications. 

Some of the important agricultural modifications and their 

general effect on water budgets and water flowpaths are described

in the rest of this chapter. The estimated changes in the water 

budget and sediment yield of a field with various field-scale 

modifications compared to that of a field growing corn with 

reduced tillage are shown in figure 6.2. The direction of changes 

in water budgets and water flowpaths are largely expected based 

on the purpose of the modification, however the actual magnitudes 

of these changes exhibit a high degree of site-to-site variability. 

The modifications closely related to crops are discussed in four

groups: infrastructure, infield/edge of field, tillage, and cropping. 

Modifications used in animal agriculture are discussed separately.
100 Agriculture—A River Runs Through It—The Connections Between Agriculture and Water Quality

Table 6.1. Agricultural modifications showing purposes, spatial extents, and durations of common landscape modifications for agriculture.

[Shaded cells: Green, crop agriculture; pink, animal agriculture; blank, not applicable. Purpose: 1, primary purpose; 2, secondary purposes; YD, to increase 

yield or other short-term economic incentive; AP, to increase agricultural protection (soil erosion, crop protection) or other long-term sustainability incentives; 

EP, to increase environmental protection. Spatial extent: FD, field scale; ST, stream reach scale; WA, watershed/aquifer scale. Duration: PE, permanent, 

SP, semi‐permanent; AN, annual or less than annual]

Purpose Spatial extent Duration

YD AP EP FD ST WA PE SP AN

Irrigation—Source water

Groundwater-derived irrigation from annually replenished water1

Groundwater-derived irrigation from “mined” water1

Surface water-derived irrigation from within the watershed

Surface water-derived irrigation from outside the watershed

Irrigation—Delivery method

Flood/furrow irrigation

Center pivot irrigation

Sprinkler irrigation

Drip irrigation 1 2 2

Sub Irrigation

Drainage

Surface (constructed) drainage networks

Subsurface drain networks

Field-based subsurface drains

Surface inlet to subsurface drain

Horizontal drains through stream berm

Temporary drainage ditches

Tillage

Conventional tillage

Reduced tillage 1 2

Conservation tillage (including mulch and ridge) 1 2

No till 1 2

Cropping

Crop rotation 2 1

Fallow 2 1

Cover crop 1

Strip cropping 1 2 2

Contour farming 1 2

Naturalization/set aside land

In-field/edge of field modifications

Terraces 1 2

Field (sediment retention) ponds

Land shaping 1 2

Field buffers (filter strips)

Riparian buffers

Fencing 1 2

Wind break/shelter belt

Grassed waterways 1 2

Watershed-scale modifications

Reservoirs 1

Constructed wetlands

Stream restoration/bank stabilization 2 1

Stream straightening/channelization

Animal agriculture

Confined feeding operations

Grazing

Lagoon/waste storage 

Farm ponds

1

Irrigation water derived from groundwater are distinguished here between “mined” groundwater and annually replenished groundwater.” with “A distinction is made 

between “mined” groundwater and annually replenished groundwater.
Agricultural Water and Soil Management 101

tac11-0566_fig06-01

A B

Figure 6.1. Examples of (A) too much water or (B) too little water during the growing season, which results in crop damage. 

Photograph A by Paul Capel, U.S. Geological Survey; photograph B by Tim McCabe, U.S. Department of Agriculture, Natural 

Resources Conservation Service photo gallery, NRCSAR83004, 1983. 

Terraces Buffer

Conservation till

No till

Alfalfa

Soybean

Wheat

Fallow

Prairie grass

Forest

Crop rotation

Cover crop

Reduced contour till

Strip cropping

Change from reduced till corn, in percent

-100

0

100

200

300

Evapotranspiration

Recharge

Runoff

Sediment yield

EXPLANATION

Landscape modification

Figure 6.2. Estimated changes in the water budget and sediment yield of an 

agricultural field for a change in the landscape modification compared to the 

water budget and sediment yield in a field planted in corn with spring reduced 

tillage (chisel) (see “Quantifying Water and Sediment Budgets;” in Chapter 5; 

Roth and Capel, 2012a).
102 Agriculture—A River Runs Through It—The Connections Between Agriculture and Water Quality

A Year with Corn in Central Indiana

Various agricultural activities, water-budget components, and crop growth are shown for a corn field in 

central Indiana for a typical year. A similar sequence of events occurs in most areas, although the timing of each 

activity will be different depending on the climate of the area. The figure illustrates the timing of agricultural 

management activities, changes in the growth of corn, and changes in selected parts of the water budget for the 

field, as estimated by the WEPP model (see “Quantifying Water and Sediment Budgets” in Chapter 5; Capel and 

Hopple, 2018).

EXPLANATION

Precipitation

Evaporation

Recharge

Runoff

Canopy height

Root depth

Herbicides applied

-2

-1

0

1

2

3

0

0

10

10

20

20

30

30

40

50

60

Precipitation, in millimeters

Canopy height, in meters

above land surface

Root depth, in meters

below land surface

Relative fraction

of crop

Evapotranspiration, recharge, and runoff, 

in millimeters

2003

Jan. Feb. Mar. Apr. May June July Aug. Sept. Oct. Nov. Dec.

Primary tillage: 

anytime between 

harvest and planting, 

if used.

Fertilizer: applied 

immediately before planting, 

during planting, or early in 

the growing season.

Fertilizer: occasionally 

applied following 

harvest.

Planting Harvest

Secondary 

tillage prior to 

planting, if 

used.
Agricultural Water and Soil Management 103

Infrastructural Modifications

Irrigation.—When rainfall during the growing season is 

inadequate for favorable crop production, irrigation water is 

applied to soil to supplement natural precipitation. Irrigation 

water can come from many sources, which may be close to 

or at a distance from the water-deficient area. The choice 

as to the source of irrigation water is largely dependent 

on the availability of the water resource, cost of moving 

the water, legal ownership of the water, and its degree of 

salinity (fig. 4.4). Depending on the distance of the source 

and the seasonality of rainfall and snowmelt, the water may 

be channeled directly to the agricultural fields or stored in 

reservoirs for later use. Some irrigation water is pumped from 

shallow or deep aquifers beneath the area of use. Most of 

the agricultural lands in the Great Plains are irrigated from 

the High Plains (Ogallala) aquifer system (McGuire, 2009). 

Other irrigation water is available from surface-water bodies, 

such as rivers, springs, lakes, and reservoirs (fig. 4.4A), and 

moved through canals. The surface-water bodies may be 

within the watershed being irrigated or the water may be 

imported from other watersheds. On an annualized basis, 

the rate of water used for agricultural irrigation that is 

derived from surface water and groundwater is 290,000 and 

200,000 million liters per day (L/d), respectively (Kenny and 

others, 2009). Irrigation for agriculture constitutes 62 percent 

of the total water used in the country (excluding water used for 

hydropower). Irrigation is particularly important in the West, 

where rainfall is insufficient for agricultural needs, and in parts 

of the Southeast, where rainfall during the growing season 

is insufficient. Although, in 2012, only about 17 percent of 

cropland in the United States was irrigated, about one-half 

of the value of all crops sold comes from these areas (U.S. 

Department of Agriculture, Economic Research Service, 2015). 

Irrigation systems are conduits and devices that are used 

to systematically deliver water to uniformly supply an entire 

field with enough water to meet plant needs (neither too 

much nor too little) (fig. 6.3; Dougherty and others, 1995). 

Most irrigation systems require permanent infrastructure 

such as pumping wells, piping, center pivots, canals, or 

reservoirs. Two general types of irrigation—gravity and 

sprinkler—are used. 

In gravity irrigation systems, water is applied directly to 

the upper slope of the field and moves down slope by the force 

of gravity. These systems can be designed to cover the whole 

soil surface (flood irrigation) or just the low areas between the 

rows (furrow irrigation). Because the systems are designed 

so that the entire field is irrigated, excess water (tailwater) 

generally accumulates at the downslope end of the field. This 

excess water becomes runoff that can transport sediment to 

a stream. At one time, gravity irrigation was the dominant 

application method, but this method is less efficient compared 

to sprinkler methods, and therefore is seldom used. 

Diverse techniques are used in sprinkler irrigation 

systems to deliver pumped water to the plants as droplets. 

Center pivot systems are large-volume sprinklers, consisting 

of a single arm (joined pipes) mounted on wheels that rotates 

from a central point to distribute water across the perimeter 

of a circle. Originally, center pivot systems were designed 

to operate on square, quarter-section fields (65 ha), but can 

now cover areas much larger than this (as much as 200 ha; 

Evans and Sneed, 1996). This type of irrigation is especially 

popular throughout the Great Plains. One downside to centerpivot systems is that a substantial amount of water is lost to 

the atmosphere through evaporation. Drip irrigation systems 

deliver droplets of water, but generally with less evaporation 

than with sprinklers. Drip systems generally are used on a 

smaller scale and deliver pressurized water through pipes with 

perforations, adjustable nozzles, and (or) porous materials, 

which dispense the water close to the plants. Micro-drip 

irrigation, in which water is delivered to individual plants, 

is used to conserve water, especially in areas with limited 

water resources (Eisenhauer and others, 2006). These modern 

systems can even include subsurface emitters to limit soil 

evaporation and ensure that the maximum fraction of applied 

water possible is transpired through crop plants.

All irrigation practices purposely increase the amount 

of water that is moving through the agricultural landscape. 

For irrigation to be successful, it must be managed to control 

the amount, frequency, and application rate of the water 

being delivered to the plants yet not lose excessive water to 

evaporation, infiltration, or tailwater runoff, nor cause soil 

erosion. Although water is lost to the environment with every 

irrigation technique, some techniques are more efficient at 

delivering water to the crops than others (Eisenhauer and 

others, 2006). 

Dams and Reservoirs.—A dam is a structure built across 

a stream, river, or estuary to slow or stop water movement. A 

reservoir contains the water impeded by the dam and functions 

to store water for irrigation (and other uses), control peak 

discharge of floodwater, improve inland navigation, provide 

recreation, and (or) produce hydroelectricity. In the arid parts 

of the United States, reservoirs are extensively used to provide 

irrigation to agriculture—14 percent of the 8,121 major dams 

in the United States are used primarily for irrigation. The 

total normal storage capacity for all dams in the Nation is 

130 billion cubic meters (m3) and the storage capacity for 

irrigation dams is 220 million m3 (U.S. Geological Survey, 

2006). The distribution of irrigation reservoirs relative to 

cropland in the United States is shown in figure 6.4. These 

reservoirs are clustered in areas of the Nation where the 

irrigation water is needed to support crops. 
104 Agriculture—A River Runs Through It—The Connections Between Agriculture and Water Quality

tac11-0566_fig06-03

A

C

B

D

E F

Figure 6.3. Irrigation systems come in many forms: (A) center-pivot arm, (B) aerial view of cropland irrigated by center pivots, 

(C) irrigation delivery canal, (D) furrow irrigation, (E) sprinkler irrigation, and (F) micro sprinkler (drip). Photographs A, C, D, and F by 

Paul Capel, U.S. Geological Survey, 2010; photograph B from U.S. Department of Agriculture, Farm Service Agency, National Agriculture 

Imagery Program, 2010; photograph E by Jeff Vanuga, U.S. Department of Agriculture, Natural Resources Conservation Service photo 

gallery, NRCSAZ02015, 2002. 
Agricultural Water and Soil Management 105

tac11-0566_fig06-04

EXPLANATION

Cropland

>200

>100 to 200

Reservoir storage capacity— 

Surface area, in square 

kilometers

>1 to 50

>50 to 100

Figure 6.4. Distribution and storage capacity of reservoirs with the primary purpose of irrigation relative to cropland in the United 

States (U.S. Geological Survey, 2006). 

Drainage.—To allow or enhance plant growth, 

engineered drainage is used to remove excess water in the 

upper soil layer (root zone). Agricultural areas that benefit 

from drainage generally have an overabundance of rainfall 

(or irrigation), slow soil infiltration, and (or) flat topography. 

These conditions oftentimes result in waterlogged soils and a 

shallow water table. These areas were wetland environments 

prior to drainage. In order to convert these areas to land 

suitable for agriculture, drainage techniques are used to 

remove excess water (Bos, 1994), aerate the soil, and improve 

field conditions to allow machinery timely access for tilling, 

planting, and harvesting. Generally, drainage results in 

warmer soil temperatures, reduced surface runoff, improved 

soil structure, and improved root development. Drainage 

systems generally are installed as permanent, integral parts of 

agricultural areas. 

Surface and subsurface are two main types of drainage 

systems (fig. 6.5). Surface drainage comprises a network 

of shallow, open ditches (man-made streams) that convey 

water to larger and deeper collector ditches and eventually 

into a natural stream. In many areas of the country, the rural 

landscape is divided into a grid by the public ditch networks 

adjacent to the roads. In some areas of the Nation with 

flat topography and extensive wetlands, such as the upper 

Midwest, the installation of the surface ditch network was the 

first step in converting these wetlands into agricultural land. 

Subsurface drains control the level of the water table by 

removing excess water from the upper layer of soil (plant root 

zone), usually through a network of perforated pipes (plastic 

or clay tile) placed at a specific depth below the land surface. 

Subsurface drains can be installed as single pipes running 

adjacent to topographically low areas or as a network of pipes 

that are laid parallel to each other. In isolated, topographically 

low areas of a field, surface inlets (perforated stand pipes) are 

installed and connected to the horizontal subsurface drains to 

speed the drainage of temporary ponds that form after storm 

events (fig. 6.5; Roth and Capel, 2012b). Subsurface drains 

deliver water to surface ditches or natural streams through 

infiltration and lateral movement of water along relatively fast 

flowpaths (hours to weeks). However, areas of the field that 

are far from the surface drain have slower flowpaths because 

the distances for lateral movement are greater. 
106 Agriculture—A River Runs Through It—The Connections Between Agriculture and Water Quality

tac11-0566_fig06-05

A

B

C

Figure 6.5. Components of agricultural drainage: (A) surface 

ditch, (B) subsurface drain outlet, and (C) surface inlet. 

Photographs by Paul Capel, U.S. Geological Survey, 2008.

Engineered subsurface drainage has introduced a 

completely new type of water flowpath from the field to the 

stream (fig. 6.6). In watersheds without subsurface drainage, 

all water reaches the stream through a combination of fastflow, 

such as runoff, and slowflow, such as recharge to groundwater 

to baseflow (RGB). Subsurface drainage creates a third source 

of water (drainflow) to a stream that has both slowflow and 

fastflow components. The conceptual model of the continuum 

of two water sources to the stream (fig. 5.9) must be expanded 

to include an additional end-member for landscapes with 

subsurface drainage (fig. 6.6B). All streams can be located 

within this conceptual triangle described by the three endmember sources (slowflow, fastflow, drainflow). This is a 

simplification for many streams, but it is a useful organizing 

tool in assessing the relative contributions of various sources 

of water to a stream.

Streams in small agricultural watersheds can be 

categorized by their characteristic flowpath(s). Figure 6.7

shows rivers and streams in agricultural areas across the 

Nation that are expected to have slowflow, fastflow, and 

drainflow as their dominant flowpaths (corresponding to 

areas near the apexes of the triangle in fig. 6.6B), as well as 

areas that have a mixture of flowpaths (areas in the interior 

of the triangle in fig. 6.6B). Figure 6.7 is based on analysis 

of watershed properties, probable locations of subsurface 

drainage, and analysis of streamflow at gaged streams. 

Although this map is based on limited information, it provides 

a basis to help understand which flowpaths are important 

in various areas of the Nation. Caution must be used when 

using such a map because all flowpaths are affected by local 

conditions that cannot be depicted on a broad-scale map. The 

flowpaths leaving even two adjacent fields could be different 

due to variability in topography, soils, land management, and 

(or) the presence/absence of sub‐surface drainage.

Surface and subsurface drainage reduce the time that 

infiltrating water remains in the soil (root zone) by promoting 

rapid transport of the water after rainfall. Many areas in 

which subsurface drains have been installed are underlain 

by soils with a high clay content; these soils have numerous 

macropores that enhance the rate of water movement to 

subsurface drains (Stone and Wilson, 2006; Smith, 2012).This 

rapid infiltration decreases the amount of water available for 

surface runoff, which in turn decreases the amount of runoff 

and eroded soil (sediment) that is transported to the surface 

ditch or stream. 
Agricultural Water and Soil Management 107

A

B Drainflow

(subsurface drains)

Fastflow

(runoff)

Slowflow

(groundwater)

Maryland

California

Nebraska

Iowa

Indiana

Mississippi

Washington

(irrigation

season)

Washington

(nonirrigation

season)

Groundwater aquifer

Stream

Slowflow

Fastflow

Drainflow

Figure 6.6. Simplified diagrams of (A) water flowpaths from the landscape to the 

stream, including engineered subsurface drainage, and (B) continuum of water 

flowpaths from the landscape and to the stream for watersheds with subsurface 

drainage. Water flowpaths from catchments are described by a triangular space 

defined by the three end-members: slowflow, fastflow, and drainflow.
108 Agriculture—A River Runs Through It—The Connections Between Agriculture and Water Quality

EXPLANATION

Drainflow

Fastflow

Mixed flow

Slowflow

Non-agricultural land

Study locations

Figure 6.7. Areas in crop agriculture that are expected to have slowflow, fastflow, and drainflow as their important 

flowpaths (corresponding to areas near the apexes of the triangle in fig. 6.6B), and areas that have a mixture of flowpaths 

(areas in the interior of the triangle in fig. 6.6B). (From Capel and others, 2018.) 

 The topography of the field and the possible location 

of the drain network outlet largely determines the layout 

of subsurface drains for each field. In some areas of the 

country, public drainage districts have installed networks of 

large‐diameter subsurface drains to provide outlets for many 

fields. The subsurface drainage infrastructure can be modified 

to partly meet the need for environmental protections (slower 

water movement into streams and retention of soil water 

during drought). Controlled drainage, in which a movable gate 

is installed in the drainage outlet, allows manual control of the 

water-table elevation (Gilliam and Skaggs, 1986; Evans and 

others, 1995). When the gate is opened, backed-up subsurface 

drainage water is released and is rapidly transported to the 

outlet. When the gates are closed, the water-table elevation is 

raised, water is stored in the soil, and the rate of subsurface 

drainage is decreased. Subsurface drains also can be used for 

irrigation purposes in dry periods (subirrigation), whereby 

water is pumped into the drainage system to deliver water 

across the lower part of the root zone.

Stream modifications are physical changes made to the 

stream channel (channel geometry) or to the stream corridor 

(in-stream or riparian vegetation). A natural stream corridor 

encompasses a main stream channel and the land adjacent 

to the stream (floodplain), as well as any wetlands and (or) 

tributaries connected to the stream. A natural stream meanders 

through the landscape, eroding sediment from the outer 

portion of these meanders (cut banks), where water velocities 

are greatest, and depositing sediment on the inner portions 

(point bars), where water velocities are too slow to maintain 

the sediment in suspension. Peak flows (highest volume) 

appreciably affect the natural channel’s morphology, of which 

streambank erosion and deposition are integral processes. 

Over time, a stream will meander across the entire width of 

the floodplain, redefining the stream channel and redistributing 

eroded soil and sediment across the adjacent floodplain. 

The meandering of a natural stream can be disruptive to 

row crop agriculture because of the perpetual realignment of 

the stream channel and the erosion of the adjacent land. To 
Agricultural Water and Soil Management 109

address this meandering, many streams adjacent to agricultural 

lands have been channelized. Channelization replaces a 

natural, meandering segment of a stream with a shorter, 

straighter engineered segment that imparts less resistance 

to flowing water. Consequently, flows in the channelized 

stream have more energy than the natural stream, which can 

increase flooding and erosion downstream of the engineered 

segment. In response to the negative downstream effects 

that channelization of streams has had in some locations, 

previously channelized stream reaches are being restored. 

Stream restoration involves restoring the stream channel to 

its natural, meandering course, reinforcing the streambanks, 

re-establishing bank vegetation, and adding wetlands to the 

floodplain to help return the stream’s ecosystems to a more 

natural, sustainable equilibrium, thereby reducing problems 

downstream (Bernhardt and others, 2005).

Constructed wetlands.—These are engineered areas 

in which the water levels have been raised to maintain the 

water table at or above land surface (Hammer and Bastian, 

1989; Lowrance and others, 2006). Eventually (usually within 

a few years) this action results in a vegetated wetland. The 

wetlands are maintained by movement of water through either 

surface water- or groundwater-flow systems. Constructed 

wetlands are designed to mimic and compensate for the loss 

of natural wetlands. Constructed wetlands can intercept and 

remove sediment and chemicals from runoff and drain water 

before the water reaches the stream. These wetlands also can 

provide flood control by slowing down the velocity of water 

and storing it on the landscape. Constructed wetlands provide 

wildlife habitat and create recreational opportunities such 

as nature watching and hiking. In some cases, constructed 

wetlands are used for production of food (for example, rice 

and cranberries). 

The water level and flow in constructed wetlands can be 

controlled. The rate of water delivered to the wetland combined 

with its storage volume determines the transit time of the 

water in the wetland. Often a holding pond is intentionally 

located upstream of the wetland to help maintain the water 

level during dry periods, serve as an additional settling basin 

for sediment, and protect wetland vegetation from water surges 

during storms. The water level in the constructed wetland can 

vary diurnally (because of evapotranspiration) and seasonally 

(because of rainfall patterns). 

In-Field and Edge-of-Field Modifications

Landscape modifications in or at the edge of agricultural 

fields are usually used to reduce water runoff, which also 

reduces soil erosion and the amount of sediment delivered to 

streams (fig. 6.8). 

Terraces (fig. 6.8A) are leveled sections of a hillslope 

that are designed to slow surface runoff, reduce soil erosion, 

and retain soil moisture (Baker, Helmers, and Laflen, 2006). 

Terraces can substantially decrease water runoff and sediment 

yield, and increase infiltration and evapotranspiration. The 

slower water velocity provides a longer time for resulting 

infiltration and evapotranspiration; therefore, terraces are used 

for water conservation in addition to erosion control in semiarid areas. The slower moving water also has lower energy, 

which allows the eroded sediment to settle. Often, a field is 

formed into multiple terraces, giving a stepped appearance. 

The construction of terraces can increase cultivated area and 

allow better access for farm machinery. Different designs can 

be used for releasing excess water from the terraces. Some 

terraces route excess water through grassed waterways or 

surface ditches, whereas others have subsurface drains. For 

those terraces without engineered drainage, excess water is 

lost through infiltration and evapotranspiration. The effects of 

terraces on water-budget components and sediment yield of a 

corn field are illustrated in figure 6.2. 

Buffers and filter strips (figs. 6.8B and 6.8C) are areas 

of perennial vegetation (grasses and (or) trees adjacent to 

agricultural fields) that provides benefits to the environment 

(Baker, Helmers, and Laflen, 2006). Riparian buffers, often 

wooded, are left between a field and a stream to protect the 

stream, whereas strips of vegetation within fields or just along 

field borders are filter strips. In both, the perennial vegetation 

decreases the amount and velocity of runoff, traps sediment, 

and increases infiltration and evapotranspiration. Vegetative 

buffers also provide habitat for many species of animals 

and plants. The effects of buffer strips on water-budget 

components and sediment yield of a corn field are illustrated 

in figure 6.2. 

Grassed waterways (fig. 6.8D) are areas of perennial 

grass vegetation that are placed along ephemeral drainage ways 

within agricultural fields. The thick grass vegetation reduces 

water velocity and protects the soil surface to prevent gully 

formation (Baker, Helmers, and Laflen, 2006). On steeply 

sloping fields, the crop rows run perpendicular to the grassed 

waterway. In particularly wet areas or where the slope is steep, 

the waterways can be lined with concrete or other permanent 

material, rather than grass, to decrease soil erosion. Waterways 

are commonly used as outlets for runoff from terraces.

Land shaping is the movement of soil to shape, grade, 

and smooth the landscape for agricultural purposes (Baker, 

Helmers, and Laflen, 2006). Land shaping is frequently 

practiced to provide a constant slope to the land to improve for 

gravity irrigation. Land shaping also is used to remove surface 

irregularities for more effective use of water by crops, more 

uniform planting depths, and ease of use of farm machinery 

during tilling, planting, and harvesting. This practice can be 

used to improve terrace alignment and field contours and to 

reduce the ponding of water in the field. If land shaping is 

done incorrectly, soil structure can be damaged (compaction 

and clumping in wet soils) or topsoil can be lost (buried), 

resulting in poor crop emergence and (or) water penetration. 
110 Agriculture—A River Runs Through It—The Connections Between Agriculture and Water Quality

tac11-0566_fig06-08

A B C

D E F

Temporary placement

Figure 6.8. In-field landscape modifications include (A) terraces, (B) grass buffer strips, (C) forested buffer strips, 

(D) grassed waterways, (E) contour cropping, and (F) strip cropping. Photographs from U.S. Department of Agriculture, 

Natural Resources Conservation Service photo gallery, A, Lynn Betts, NRCSIA99090, 1999; B, Lynn Betts, NRCSIA03023, 

2002; C, Lynn Betts, NRCSIA00041, 2000; D, Lynn Betts, NRCSIA99446, 1999; E, Tim McCabe, NRCSIA99358, 1999; F, Tim 

McCabe, NRCSIA99355, 1999

Windbreaks are linear plantings of trees and shrubs on 

the edges of agricultural fields or farmsteads. Windbreaks 

protect crops and reduce wind erosion of soils. The reduced 

exposure of crops to wind reduces their evaporative demand, 

and, hence, reduces crop water stress. Windbreaks, especially 

in combination with conservation tillage, help trap snow to 

increase soil moisture in spring in semi-arid areas. When 

planted around livestock areas, windbreaks help minimize 

animal stress and mortality, reduce feed consumption, and 

reduce visual impacts and odors. Windbreaks also help to 

reduce or prevent snow drifts and to spread the snow and, 

thus, spring soil moisture more evenly. 

Field ponds are permanent or semi-permanent structures, 

such as dug basins or impoundments, that fill with water. Field 

ponds are used to capture storm-water runoff from fields, 

decrease the transport of eroded sediment to streams, and 

make water available for irrigation and (or) livestock needs. 

The water-storage capacity of the ponds helps slow down and 

decrease the delivery of water and sediment to the stream.
Agricultural Water and Soil Management 111

Tillage

Tillage (plowing) is the mechanical manipulation of soil 

and plant residue in preparation for planting seeds. Tillage 

mixes soil, crop residue, or applied manure throughout the 

plant root zone. Tillage also destroys weeds before planting, 

and loosens soil to facilitate deeper root penetration. Primary 

tillage breaks up the soil to depths of 30–55 cm and leaves 

a rough soil surface with many clods (U.S. Department of 

Agriculture, Natural Resources Conservation Service, 2010). 

The main primary tillage tools are the moldboard, chisel, and 

disk plows (see “Glossary of Farm Implements”). In many 

areas, primary tillage is not done every year to minimize the 

disturbance of the soil. Secondary tillage follows primary 

tillage to break up the clods and level the soil for planting. 

Secondary tillage is done at a shallower depth (15–30 cm). 

The common secondary-tillage tools include the disk harrow, 

spring-tooth harrow, wire-tooth harrow, and spike‐tooth 

harrow, packer-roller, and rolling basket. From a soil 

conservation perspective, tillage practices are grouped by 

percentage of soil surface covered with plant residue after 

tillage (fig. 6.9). As the percentage of plant residue left on 

the surface increases, less bare soil is exposed to the erosive 

forces of water and wind.

Conventional tillage removes most of the plant residue 

from the surface of the soil—less than 15 percent of crop 

residue cover is left on the land surface. Conventional tillage can 

include primary or secondary tillage techniques. Primary tillage 

is most disruptive and results in a minimum residue cover. 

Conservation tillage encompasses many different 

tillage practices, all of which maintain a crop residue cover of 

30 percent or greater. This relatively large residue cover slows 

the movement of water across the land surface and reduces 

soil erosion. In mulch tillage, the entire field is plowed, and 

the crop residue is mulched (chopped) and evenly distributed 

across the surface.

No till is the conservation tillage practice that results in 

the least soil disturbance. The soil is left almost undisturbed 

during the period between harvesting and planting. When the 

crop is planted, the seeds are planted (mechanically drilled) 

into the soil through the remaining residue. In the no till 

practice, weeds are controlled by herbicides rather than by 

mechanical methods.

The estimated water budgets and sediment yields for a 

field of corn with reduced till, ridge till (conservation tillage), 

and no till (conservation tillage) are compared to their yields 

under conventional tillage in figure 6.2. In comparison to 

conventional tillage (using a chisel plow), all conservation 

tillage practices increased water infiltration, reduced water 

runoff, and reduced sediment yield (see “Effects of Tillage on 

Runoff and Recharge”).

tac11-0566_fig06-09

A

B

C

Figure 6.9. Corn residue on the land surface after 

(A) conventional tillage (<15 percent residue), (B) 

conservation tillage (>30 percent residue), and (C) no 

tillage (no-till). Photographs by Paul Capel, U.S. Geological 

Survey, 2009. 
112 Agriculture—A River Runs Through It—The Connections Between Agriculture and Water Quality

Effects of Tillage on Runoff and Recharge

Surface runoff from a field happens when the rate 

of rainfall or applied irrigation water is higher than the 

rate of infiltration into the soil. Water enters the soil 

through the network of pores that connects the surface 

to the deeper layers of soil (see “Soil Properties, Soil 

Water, and Soil Erosion” in Chapter 5). Land-use and 

soil-management activities can alter the pore-size 

distribution and continuity of the pore system, and, 

consequently, change important hydrological processes, 

such as water infiltration and runoff. The presence, 

size, and effectiveness of soil macropores are especially 

sensitive to soil-management operations, such as 

tillage and the weight of agricultural equipment (such 

as tractors and combines). Each time the soil is tilled, 

the numbers and sizes of soil macropores are increased 

(Arshad and others, 1999). Over time, however, the 

volume of these pores is reduced because of compaction 

from agricultural equipment and rainfall effects. The 

impact of raindrops reduces the macropores near the 

soil surface, whereas farm machinery reduces the soil 

macropores deeper in the subsurface.

After soil is tilled, the surface is poorly protected 

(no living plants and little crop residue). Falling 

raindrops collide with the soil surface, destroying some 

of the surface aggregates. The smaller primary particles, 

which comprise the soil aggregate, are initially 

dispersed, and then are rearranged to form a surface 

crust (0.5–2.0 mm thick) that becomes dense and hard 

upon drying. This process is called “surface sealing” 

(Bissonnais and others, 1989). This crust, which can 

form in all types of soils, reduces water infiltration 

and impedes the emergence of seedlings (fig. 6.10). 

Poor residue cover, high sodium concentration, and the 

presence of aggregates of low stability are important 

tac11-0566_fig06-10

Figure 6.10. Lettuce plants sprouting through a dry soil crust.

contributors to crust formation. Agricultural practices 

that include intensified (that is, frequent primary) 

tillage can accelerate decomposition of soil organic 

matter that maintains soil aggregation. Loss of soil 

organic matter leads to soil that is prone to crusting. 

Soil crusting often indicates a history of excessive 

tillage (except in arid and semi-arid areas where 

naturally high soil sodium concentrations can lead to 

soil crusting).

Soils with crust formation are more susceptible to 

erosion and, consequently, the transference of excess 

water, eroded soil, and agricultural chemicals to the 

stream because of higher runoff volumes. Conservation 

tillage and no till are among the practices that can 

increase surface-soil organic matter and decrease 

surface sealing, reduce soil erosion, and protect the soil 

against the impacts of raindrops.
Agricultural Water and Soil Management 113

Cropping

The different kinds of crops and the sequence of crops 

grown in a given field over a period of time is termed a 

cropping system. Variations in cropping systems include 

choice of crop and crop rotation, cover cropping, contour 

farming, and strip cropping (Reeder and Westermann, 2006). 

Corn, soybean, wheat, cotton, and sorghum are the most 

prevalent crops grown in the United States (Chapter 4), but 

many dozens of other crops are grown. Each crop has its own 

specific effect on the environment because of its water and 

nutrient needs, length of growing season, and extent of ground 

cover by the leaves. At times, some fields are removed from 

production and not planted for a short period (for example, left 

fallow for a growing season) or converted to native grasses or 

trees for conservation purposes. The estimated water budgets 

and sediment yields for a field of corn are compared to those 

for alfalfa, soybean, wheat, fallow, prairie grass, and forest 

in figure 6.2. Each land cover has a different water budget 

and a different sediment yield. Prairie grass, forests, and 

fields of alfalfa all act to decrease runoff and reduce sediment 

yield because these areas are not plowed annually and the 

continuous presence of perennial plant cover protects the soil. 

Crop rotation is the practice of planting different crop 

types (grains, legumes, and grasses) in the same field in a 

seasonal succession to promote long-term improvement of 

soil quality. Crop rotation increases soil organic matter and 

improves soil structure and aggregation, which aid in plant 

root heath (Reeder and Westermann, 2006). Crop rotation 

also improves crop yield (Bullock, 1992). In the Midwest, 

corn-soybean (2-year cycle) or corn-corn-soybean (3-year 

cycle) are common rotations. When soybean or other nitrogenproducing plants (peas, clover, and alfalfa) are used as part of 

the crop rotation, they reduce nitrogen fertilizer needs. 

Cover cropping involves planting fast growing grasses, 

legumes, or small grains (rye, clover, and oats) after the 

harvest of the regular cash crop. The presence of the cover 

crop provides shorter exposure of bare soil, which helps to 

decrease water runoff and protects the surface soil from water 

and wind erosion. The cover crop is plowed back into the soil 

rather than harvested, which increases the organic matter and 

soil fertility.

Contour cropping involves planting rows of crops 

at equal elevations along the contours of the landscape 

(fig. 6.8E). The crop rows planted on the contour create rows 

of furrowed structures across the field, which act like many 

small dams to slow water flow and increase opportunity 

for infiltration. Contour cropping is most effective on long, 

uniform, low-angle slopes, and aids in reducing water runoff 

and soil erosion, which decreases the formation of rills and 

gullies that can form wherever soil erosion is a problem. In 

addition, contour cropping can allow better farm-machine 

access and make planting and harvesting easier. 

Strip cropping is a systematic arrangement of different 

crops (such as corn alternating with soybean) across a field 

(fig. 6.8F). Strip cropping is often used to decrease runoff 

and soil erosion on sloping terrain, where the strips are 

placed parallel to the contours (as in contour cropping). Strip 

cropping also can be used to decrease wind damage and wind 

erosion by planting crops with different heights in rows that 

are perpendicular to the direction of the prevailing wind. 

Animal Agriculture

Animals are an important component of our Nation’s 

agriculture, accounting for more than one-half of the value 

of agricultural products (U.S. Department of Agriculture, 

Economic Research Service, 2016d). Beef and dairy cattle, 

hogs, turkeys, and chickens (broilers and layers) are the major 

products (fig. 2.1B). Each animal requires its own unique 

set of agricultural activities and resources—including space, 

water, food, protection (from pests and weather), and disposal 

of the wastes. Animals are usually raised under conditions 

where they forage for food (pasture and grassland lands) or 

where they are fed (animal feeding operations). Much of 

animal agriculture, outside of arid areas, is interspersed in the 

areas of crop agriculture.

Grazing occurs on farms, grasslands, or rangelands, 

where animals (dairy cows, beef cattle, horses, sheep, goats, 

and others) are allowed to roam freely, feeding on grasses 

and other vegetation (fig. 2.2B). Commonly, grazed land is 

unsuitable for crop production (insufficient rainfall or too 

steep for machinery) and thereby generates higher profits 

through livestock conversion of forage into meat, milk, and 

other products. Overgrazing—livestock feeding that exceeds 

plant growth rate—can lead to soil compaction, loss of 

vegetative cover, and increased water and wind erosion. To 

guard against overgrazing, animals are moved from field to 

field (similar to crop rotations) allowing the most recently 

grazed areas time to recover (rotational grazing).

Fencing of pastures and range areas excludes livestock 

from unwanted areas, including streams and areas adjacent to 

streams. Fencing can help reduce bank erosion and eliminate 

the direct input of animal waste into streams. In some places, 

the construction of fences designed to hold livestock is 

mandated by law.
114 Agriculture—A River Runs Through It—The Connections Between Agriculture and Water Quality

Animal feeding operations are agricultural practices 

in which the feed is brought to the animals (typically dairy 

cows, beef cattle, hogs, chickens, or turkeys) rather than 

having the animals forage (fig. 2.2). The animal feeding 

operations are facilities, either indoor or outdoor, where the 

animals are concentrated (with no available grass or other 

vegetation) for 45 or more days in a 12-month period (U.S. 

Environmental Protection Agency, 2011). There are about 

450,000 concentrated animal feeding operations distributed 

across the United States (U.S. Environmental Protection 

Agency, 2011). Although the spatial footprint (overall area) of 

animal feeding operations is relatively small compared to that 

of crop agriculture, the amount of animal waste generated at a 

facility can have a substantial effect on the surrounding air and 

water quality. Animal waste can be managed in many different 

ways, including on-site treatment, land application (manure 

spreading), and long-term storage (in arid areas).

Lagoons are pond-like water treatment structures 

designed for temporary storage and treatment of liquid animal 

waste until it is used on cropland (fig. 6.11A). The lagoon 

bottom is generally sealed to protect groundwater and surface 

water from contamination by the waste. Most lagoons are 

designed to treat the waste without added oxygen (anaerobic 

decomposition), although other lagoons add oxygen (aerobic 

decomposition). Because the anaerobic lagoons are not 

dependent on maintaining a minimum dissolved oxygen 

concentration, they can be much deeper (usually from 2 to 

5 m) and require less surface area than do aerobic lagoons. 

Aerobic lagoons are designed to provide a higher degree 

of treatment with fewer odors, but anaerobic lagoons can 

decompose more organic matter per unit volume. 

Manure spreading is both a method for disposing of 

animal waste and for returning the beneficial components 

(organic material, nutrients) of the waste back to the soil 

(fig. 6.11B) (see “Manure”). The use of manure to enhance soil 

structure and provide nutrients to crops has been a common 

practice throughout the world for centuries. Manure can be 

spread on fields in liquid and solid forms (see “Glossary of 

Farm Implements”). Manure is usually applied in the fall after 

harvest or in the spring before planting. Manure spreading 

on frozen soil is prohibited in some States to protect surface 

water from contamination by runoff water during snow melt 

(fig. 6.11C).

A B C

Figure 6.11. Manure can be stored and treated in (A) on-site lagoons or (B) spread on fields. (C) Manure spreading on frozen soil 

is prohibited in many states, but has been done as part of a Valentine’s Day message. Photographs A and B from U.S. Department 

of Agriculture, Natural Resources Conservation Service photo gallery. A, Jeff Vanuga, NRCSGA02036, 2002; B, Tim McCabe, 

NRCSIA99216, 1999; photograph C from Albert Lea Tribune, Albert Lea, Minnesota.
Agricultural Water and Soil Management 115

Manure

The use of manure as a nutrient supplement 

for agriculture is a common practice throughout the 

world and is an important part of modern agriculture. 

About 5 percent of the Nation’s crop area receives 

an application of manure annually (MacDonald and 

others, 2009). Agricultural crops benefit greatly when 

manure is used as a fertilizer or soil amendment. 

Manure adds important nutrients (nitrogen, 

phosphorus, and potassium), minerals (calcium and 

magnesium), and organic matter to the soil. When 

added to the soil, organic matter can increase water 

infiltration rates and increase moisture retention. 

Applying manure to fields also is a practical method 

for disposing of animal waste, but manure typically 

contains metals, antimicrobials, hormones, bacteria, 

and pathogens (Boxall and others, 2003; Hanselman 

and others, 2003; Martínez-Carballo and others, 

2007; Andaluri and others, 2012). The use of proper 

storage and application techniques can help to alleviate 

many of the contamination issues associated with 

manure applications. 

Many animals produce manure that is suitable 

for use as agricultural fertilizer. Cattle, swine, and 

poultry are the most common livestock animals, but 

these animals are not evenly distributed throughout the 

United States (fig. 2.1B). This uneven distribution leads 

to differences in the type of manure that is available for 

application at any specific location. The chemical and 

physical composition of manure varies dramatically 

with different animals (fig. 6.12), and that composition 

is to some extent dependent on age, health, diet, and 

location (climate). Manure from certain animals may 

be more beneficial for a particular crop or soil type 

than other animals. Excess of a specific nutrient, for 

example phosphorus, in some manure may require 

alteration of the animal’s diet or addition of dietary 

supplements, for example phytase, to produce 

manure with the desired characteristics to fulfill crop 

requirements (Smith and Joern, 2012).

The improper storage and application of manure 

can be detrimental to the environment. Typically, liquid 

or slurry manures are held in storage lagoons (fig. 6.11) 

or in-barn pits, and dry, solid manure is stored in large 

piles. Liquid manures typically are sprayed on the field 

(see “Glossary of Farm Implements”), injected into 

the soil, or pumped through irrigation systems. The 

tac11-0566_fig06-12

100

75

50

25

0

Manure production, in kilograms per day

per 1,000-kg of animal units

1.2

0.9

0.6

0.3

0

Phosphorus or nitrogen, in kilograms

per day per 1,000-kg of animal units

Beef

cattle

(grazing)

Dairy

cattle

(milking)

Swine Poultry

(layers)

Poultry

(broilers)

Turkeys

Unrecoverable manure

Recoverable manure

Nitrogen

Phosphorus

EXPLANATION

Figure 6.12. Manure and its nitrogen and phosphorus content 

produced by various farm animals. Recoverable manure is the 

manure that can be economically retrieved by the producer 

(Capel and Hopple, 2018).

injection methods have the benefits of increased nutrient 

retention, lower runoff potentials, and reduced odors. 

Solid manure is applied by spreading on the surface of 

the field. Afterwards, the soil is sometimes tilled to mix 

the manure into the soil to preserve nutrients.

Differences in animal distributions throughout the 

United States and in the composition of manure can 

lead to unique environmental issues in different areas. 

For example, the application of manure to fields in arid 

regions can cause a buildup of salts from the manure, 

thereby decreasing soil fertility if adequate rainfall or 

irrigation water is not available to wash the salts from 

the soil. Loss of manure from fields in runoff after excess 

irrigation or a strong storm can degrade water quality of 

runoff-receiving streams through the addition of organic 

matter, nutrients, and bacteria. In areas where there is 

a high density of animal feeding operations, the local 

production of manure may be greater than is needed 

by the crops; over application of manure as a way to 

dispose of it results in excess nutrients (see “Nutrient 

Management Plans” in Chapter 8). This can lead to a 

build-up of soil phosphorus and contamination of surface 

water, or leaching of excess nitrogen as nitrate.
116 Agriculture—A River Runs Through It—The Connections Between Agriculture and Water Quality

Agricultural Landscapes

Each of the agricultural modifications discussed above 

has its own unique effect on the environment, affecting 

movement of wind and water and, ultimately, the transport 

of sediment and agricultural chemicals to the broader 

environment. Many agricultural modifications affect 

individual fields. Some of the modifications, however, also 

can affect surrounding fields and nearby areas. The combined 

effect of all modifications defines the agricultural landscape. 

The combination of the many modifications determines the 

water budget and water flowpaths of the local stream and the 

impacts on the broader environment. An aerial photograph of 

an 11-km2

 agricultural area in north-central Iowa with many of 

the agricultural modifications of the area identified is shown in 

figure 6.13. The next chapter discusses the water budgets and 

local water flowpaths of agricultural-dominated watersheds 

that result from combinations of these modifications.

Corn moldboard 

till

Corn conservation 

till

Soybean no till

Soybean 

chisel till

Center pivot irrigation

Soybean moldboard till

Corn no till

Corn chisel 

till

Corn no till

Corn moldboard till

Corn chisel till Corn moldboard till

Corn moldboard till

Soybean conservation till

Corn chisel 

till

Corn no till

CAFO

Pasture Pasture

EXPLANATION

Subsurface drain outfall

Surface drain outfall

Vertical surface drain

Subsurface drain

Drainage channel

Drains

Pasture

Center pivot irrigation

Building

Buffer strip

Bridge

 Base image from National 

Agriculture Imagery 

Program (NAIP), 2009.

Lagoon

0 200 400 600 800 1,000 Feet

0 100 200 300 Meters

N

Figure 6.13. Aerial photograph of an 11-square kilometer agricultural area in northcentral Iowa with many of its agricultural modifications identified (CAFO, concentrated 

animal feeding operation.) 
Agricultural Water and Soil Management 117

Summary

Crops and animals need water in the proper amount 

and at the proper time to survive and thrive. To meet the 

changing needs of agriculture over time, modifications have 

been made to alter water budgets, water flowpaths, wind 

movement, soil erosion, and sediment yields. Agricultural 

modifications include irrigation, reservoirs, drainage, tillage, 

cropping, in-field and edge-of-field management practices, 

and accommodations for grazing and feeding operations. Crop 

agriculture is more spatially extensive than animal agriculture, 

so the effects of its modifications are seen across a larger area. 

Modifications for animal agriculture have great variability, 

however, because of the type and density of animals raised.

The primary goal of most agricultural modifications is 

to increase productivity. Other modifications have goals of 

protecting and sustaining the soil and water resources and 

(or) protecting the broader environment. These protective/

preventive practices are often referred to as conservation 

practices. Some modifications are annually renewable, 

whereas others last for years to decades, and some have 

become permanent features of the landscape. Whereas most of 

the modifications are implemented at the field or farm scale, 

some are implemented along stream reaches or at the county 

or regional scale.

Landscape modifications can be changed, as needed, for 

the benefit of agriculture and (or) to benefit the environment. 

Changes are determined by an number of factors, including 

Federal, State, and local policy and management decisions, 

as well as farmer decisions on profitability, sustainability, and 

need for operational maintenance. Landscape modifications 

of short duration (annual) or small scale (field) are often the 

easiest (and least expensive) to make, but these changes may 

not be as effective in protecting the environment as changes 

to the long-term (years and decades) and more spatially 

expansive modifications. 

Each agricultural modification has its own unique 

effect on the environment, affecting movement of wind 

and water and ultimately the transport of sediment and 

agricultural chemicals to the broader environment. Many 

agricultural modifications affect individual fields. Some of 

the modifications, however, also affect surrounding fields 

and nearby areas. The combined effect of all modifications 

defines the agricultural landscape. The combination of 

the many modifications determines the water budget and 

water flowpaths of the local stream and the impacts on the 

broader environment. 
118 Agriculture—A River Runs Through It—The Connections Between Agriculture and Water Quality

Agriculture in Central Washington

The floor of the Yakima River Valley lies in the rain shadow of the Cascade Mountains (fig. 1.3) and receives 

only about 18 cm of precipitation each year. When settlers first arrived in the mid-1800s, the area was covered with 

grass, primarily bunchgrasses, and other small desert shrubs. Snowmelt from the upper elevations provided seasonal 

flow in streams, but most of the region was arid and not suitable for crop agriculture. 

The first settlers, mostly cattle or sheep ranchers, built small ditches to route river water to their land. They 

found fertile soils and a climate with long hot days and cool nights throughout summer that was well suited for the 

growth of certain crops like apples. As the population grew, so did the demand for water to supply irrigation needs. 

In the 1880s, work began on a system of irrigation canals to deliver water from the Yakima River to cultivated fields. 

The largest, Sunnyside Canal, was completed in 1980. By the early 1900s, the irrigation system supported thriving 

agriculture including alfalfa, clover, hay, hops, vegetables, and orchard fruits. With substantial quantities of irrigation 

water being applied, standing water occurred in the low-lying areas, creating seasonal wetlands, and concentrating 

alkali in the soils. By 1902, rises in the water table of as much as 23 m were reported (Jayne, 1907), and the need for 

engineered drainage became apparent. Drainage Irrigation Districts were established and construction of a system of 

drainage canals was begun.

The farms in the Sunnyside Canal area of the Yakima Valley typically are small, ranging in size from 1 to dozens 

of hectares that produce a large diversity of crops including corn (both grain and silage), tree fruits, juice and wine 

grapes, asparagus, and alfalfa. Land use also includes dairy and cattle feeding operations. Approximately 10 percent 

of the land is used as pasture (Payne and others, 2007). High-technology drip-irrigation systems, capable of sensing 

water and nutrient needs of individual plants, are being used for high-value crops such as “designer” apples. Drip 

irrigation systems require high initial installation costs but low long-term operation costs.

Recent studies on the effects of agricultural on water quality in the Yakima River watershed have focused on 

nutrient contamination in surface water (Wise and others, 2009; Wise and Johnson, 2011), nutrient contamination in 

shallow ground water (Domagalski and Johnson, 2011; U.S. Environmental Protections Agency, 2012g), and water 

temperature (Voss and others, 2008).

A B C

The landscape of central Washington. (A) The small desert shrubs typical of the time before agriculture, 

(B) typical cropped fields with the Sunnyside irrigation canal, and (C) view from the air (23 square kilometers). 

Photographs A and B by Paul Capel, U.S. Geological Survey, 2010; photograph C from U.S. Department of 

Agriculture, Farm Service Agency, National Agriculture Imagery Program, 2010. 
Water on the Modified Agricultural Landscape 119

7 Chapter —Water on the Modified Agricultural Landscape 

How Has Agriculture Changed the Movement of 

Water Through Watersheds? 

Approximately 13 percent of the land in the United States 

is used to grow crops (not including hay, fig. 2.1A) (Baker and 

Capel, 2011). When these natural lands were converted from 

natural to agricultural use, the natural water cycle was altered. 

Ongoing agricultural modifications continue to alter the water 

flowpaths through the landscape. Although many agricultural 

modifications are field based, their combined effects have an 

appreciable effect on the water budget and the local flowpaths. 

These modifications also affect how the agricultural landscape 

interacts with the broader environment. An understanding of 

how agricultural modifications have changed and continue to 

change the water cycle and water flowpaths is an important 

foundation for effective policy and management decisions.

Crops thrive where the soil is fertile and well drained, 

land slope is gentle, and the right amount of water is available 

during the growing season. Where these factors are part of 

the natural landscape, only minor changes were necessary 

to convert the natural landscape to productive agriculture. 

However, where the natural landscape was not well suited 

for agriculture, extreme changes were needed for agriculture 

to succeed. 

Generally, modifications to the natural landscape to 

support agriculture have proven successful throughout the 

United States. However, the modifications to the natural 

landscape have, in some cases, negatively impacted the 

environment. Whereas many of these consequences of 

agricultural modifications were intended—increased water 

input in arid areas, optimization of soil moisture, altered 

runoff patterns—some were unintentional and their long-term 

effects unforeseen.

One of the important ways that agricultural landscape 

modifications have changed the environment is by altering 

water flowpaths. Water moves through catchments along 

expected flowpaths that connect the various components 

of the hydrologic systems—atmosphere, soil, groundwater, 

and surface water. For a particular catchment, each flowpath 

starts at a specific location where water from precipitation or 

irrigation enters the catchment and ends at the point where 

the water leaves the catchment. For example, a flowpath may 

start at a point in a field where irrigation water contacts the 

land surface and end at a point where the local stream flows 

out of the catchment. At the catchment scale, streams are 

considered as the primary integrators of water landscapes, 

and shallow groundwater is considered largely as a conduit 

through which water flows toward streams. In some locations 

around the country, some of the water that infiltrates into 

the soil recharges deep groundwater systems that eventually 

discharges to distant streams, lakes, or the ocean. In general, 

this deep, regional groundwater system is not relevant to the 

catchment scale. 

Depending on its route between the starting and ending 

points, each flowpath has a characteristic transit time. Transit 

times range from short (minutes and hours) to long (decades 

and centuries) periods of time (table 7.1). The degree to which 

agricultural modifications change water movement depends 

on which flowpaths were important before agricultural 

development and which flowpaths are important as a result of 

the agricultural modifications. (The transit times associated 

with natural water flowpaths are summarized in table 5.1.)

The effects of agricultural modifications on water 

budgets and water flowpaths for three typical pre-agricultural 

settings are discussed in the following sections. Specific 

location examples show that a combined understanding of the 

underlying natural hydrology and the local effects of various 

agricultural modifications provide the basis for understanding 

the manner by which water moves through agricultural areas.
120 Agriculture—A River Runs Through It—The Connections Between Agriculture and Water Quality

Table 7.1. Potential flow paths and associated transit times in an agricultural catchment.

[Red cells indicate the typical time ranges for each flowpath for agricultural catchments. Blank cells indicate that these flowpaths or time scales are not important 

for this type of catchment]

Flowpath

Transit time

Minutes Hours Days Months Years Decades Centuries

Flowpaths to the atmosphere

Evaporation

Plant transpiration

Flowpaths to shallow groundwater

Macropore flow through soil

Matrix flow through soil

Seepage from wetlands

Flowpaths to small streams

Local groundwater flow

Subsurface drain flow

Runoff

Macropore flow through soil to stream

Flowpaths to rivers and oceans

Regional groundwater flow

Where the Natural Landscape is 

Well Suited to Agriculture 

Where sunlight and rainfall during the growing season 

are adequate, the soil is fertile and well drained, and the 

topography is gentle, only slight modifications are necessary 

to adapt natural landscapes to agricultural production. As 

agriculture was developed and expanded across the country, 

the areas that were naturally well suited to agriculture 

typically were the first to be cultivated. Although these 

areas required relatively little modification to become 

agriculturally productive, some fundamental changes 

that were necessary have had important hydrologic and 

environmental consequences.

When natural land was converted to cropland, the first 

fundamental change to the landscape was preparation of the 

land surface by clearing native vegetation and plowing the soil 

(see “Initial Modification of the Landscape for Agriculture” 

in Chapter 4). Plowing and other tillage practices removed 

vegetation, broke the soil surface, and loosened the soil in 

preparation for planting crops. Sometimes, the land surface 

was altered by leveling, smoothing, or grading in order to 

reduce surface ponding or to reroute runoff. These landshaping modifications typically increase the rate at which 

water is routed from fields to streams and, therefore, can 

increase peak streamflow during storm events.

The second fundamental change to the landscape 

resulted when land was converted to cropland; the native 

vegetation was replaced with crops. The amount of water used 

by seasonal crops typically is less than that used by native 

perennial vegetation, and the decrease in evapotranspiration 

means that additional water is available (Scanlon and others, 

2005; Zhang and Schilling, 2006; Schilling and others, 

2008). This additional water can either run off to a nearby 

stream, or move through the soil surface, recharge the shallow 

groundwater, and eventually enter the stream as baseflow. 

The increased groundwater recharge can result in a rise in the 

water table and increased baseflow to the stream. 

Typically, when a watershed that is well suited to 

agriculture is developed, the overall effect on the water 

budget is to decrease the volume of water that leaves as 

evapotranspiration, increase the volume that flows to the 

stream, and increase the overall streamflow that leaves the 

watershed (fig. 7.1). In most of these watersheds, the water 

flowpaths are the same or similar to those present prior to 

agriculture. Morgan Creek, Maryland, provides a specific 

example of a watershed that is naturally well suited to 

agriculture (see “Hydrologic Consequences of Agriculture in 

a Watershed Where Natural Conditions Were Well Suited to 

Agriculture” in Chapter 7).
Water on the Modified Agricultural Landscape 121

tac11-0566_fig07-01

Precipitation

Evapotranspiration

Streamflow

Outputs

Catchment water budget—Arrow sizes 

indicate relative magnitudes of the flowpaths

EXPLANATION

Inputs

Internal flowpaths 

Runoff

Figure 7.1. The natural water cycle in a setting that is well suited to agriculture. Under natural conditions, a large 

amount of the precipitation in the catchment is consumed by evapotranspiration, a small amount of the excess 

precipitation is moved to the stream by overland flow paths, and the remainder moves through the soil surface, 

recharges the groundwater, and enters the stream as baseflow. 
122 Agriculture—A River Runs Through It—The Connections Between Agriculture and Water Quality

Hydrologic Consequences of Agriculture in a Watershed Where Natural 

Conditions Were Well Suited to Agriculture

The Morgan Creek watershed covers about 31 km2 on the Delmarva Peninsula in eastern Maryland (see 

“Agriculture in Maryland’s Eastern Shore” in Chapter 4; Hancock and Brayton, 2006). The topography of the 

watershed is nearly level with some gently rolling relief. The soils are porous and well drained. The climate is 

humid subtropical, with an annual average precipitation of approximately 1.1 m that falls throughout the year 

(fig. 5.12). Groundwater seeps in the floodplain adjacent to Morgan Creek form small tributaries that sustain a 

substantial baseflow in Morgan Creek throughout the year.

The topography, soils, and climate of the Morgan Creek watershed make it well suited to agriculture. The 

region has been agriculturally productive since the mid-17th century, when tobacco was a key crop. Today, more 

than 70 percent of the land in the watershed is used for agriculture. The major crops are corn and soybean.

Agricultural practices in the watershed include a variety of conservation tillage practices, cover crops, 

grassed waterways that direct overland runoff and route it to Morgan Creek or its tributaries, the construction 

of small retention ponds on many farms to control erosion and trap sediment, and small amounts of irrigation. 

Forested buffer zones along streams are common in the area.

About 70 percent of the precipitation that falls in the Morgan Creek watershed is consumed by 

evapotranspiration (fig. 7.2). The remainder of the precipitation either infiltrates to the subsurface, flows into 

retention ponds, or follows overland flowpaths to Morgan Creek or its tributaries. Water that infiltrates to the 

subsurface travels from the land surface through the soil layers to recharge groundwater, eventually discharging 

as baseflow in Morgan Creek. Water that flows into retention ponds is mostly returned to the atmosphere as 

evaporation because these ponds are designed to minimize seepage to the subsurface and typically do not overflow 

except during large storm events. About 30 percent of the precipitation falling in the watershed eventually flows 

out of the watershed as streamflow in Morgan Creek, and about 60 percent of the total flow enters the creek as 

baseflow from groundwater. Surface runoff accounts for the remainder of the total streamflow. A small amount of 

water flows out of the watershed as regional groundwater (fig. 7.2).

Because the natural landscape and climate conditions of the Morgan Creek watershed were so well-suited to 

agriculture, successful farming in the area required only minor changes to the landscape. As a result, agriculture 

has had a minor effect on the watershed’s water cycle. One of the key effects of agricultural development here—a 

decrease in evapotranspiration when native forest and other perennial vegetation was cleared and replaced with 

annual crops—has been counterbalanced by increased evaporation from the retention ponds scattered throughout 

the watershed. Although the amounts of water are likely distributed differently among the various flowpaths 

compared to pre-agricultural times, most water follows the same major flowpaths (table 7.2).
Water on the Modified Agricultural Landscape 123

tac11-0566_fig07-02

Precipitation

100%

Evapotranspiration

70%

Streamflow

28%

Runoff 5%

Outputs

Catchment water budget—Arrow sizes indicate 

relative magnitudes of the flowpaths

EXPLANATION

Inputs

Internal flowpaths 

2003

Inflows = outflows = 112 cm

Evapotranspiration Precipitation

Groundwater

out

Streamflow

Inflows Outflows

B

A. Morgan Creek, Maryland

Figure 7.2. Diagrams showing (A) important water flowpaths and 

(B) estimated water budget for the Morgan Creek, Maryland, watershed, 

2003. The relation between precipitation and streamflow is shown in 

figure 5.9 (Capel and Hopple, 2018).
124 Agriculture—A River Runs Through It—The Connections Between Agriculture and Water Quality

Table 7.2. Potential flowpaths and associated transit times in Morgan Creek, Maryland.

[Shading: Blue cells indicate natural conditions; red cells indicate conditions after agricultural development; blank cells indicate that these 

flowpaths or time scales are not important for this type of catchment]

Flowpath

Transit time

Minutes Hours Days Months Years Decades Centuries

Flowpaths to the atmosphere

Evaporation

Plant transpiration

Flowpaths to shallow groundwater

Macropore flow through soil

Matrix flow through soil

Seepage from wetlands

Flowpaths to streams

Local groundwater flow

Subsurface drain flow

Runoff

Macropore flow through soil to stream

Flowpaths to rivers and oceans

Regional groundwater flow

Where the Natural Landscape is Not Well 

Suited to Agriculture 

As the Nation’s population grew and expanded across 

the country during the late 1800s and early 1900s, agriculture 

expanded into many places that were not necessarily well 

suited to farming (fig. 2.1). Natural conditions in these areas 

were less ideal for agriculture as compared to the prime lands 

previously farmed. Substantial modifications were necessary 

for agriculture to succeed in these non-ideal areas.

Of all the environmental factors (physical determinants) 

that govern whether or not a location is suitable for 

agriculture—climate, soils, slope, and rainfall—the right 

amount of water at the right time is one of the most important 

(Chapter 4). Because the amount of water is one of the few 

factors that is feasible to modify at a large scale, the water 

budgets and water flowpaths of large areas across the country 

have been changed to yield productive farmland by removing 

the constraints of either too much water or too little water. 
Water on the Modified Agricultural Landscape 125

Where There is Too Much Water.—In many locations, 

especially in the East and Midwest, precipitation exceeds the 

water needs of vegetation. Under natural conditions, water 

tables are shallow and wetlands and (or) waterlogged soils 

develop in low-lying areas if the topography is fairly flat and 

soils are fine grained (fig. 7.3). In order to convert these areas 

to productive agriculture, engineered drainage systems were 

necessary to remove water ponded on the land surface and to 

drain the waterlogged soil. Prior to European settlement, there 

were about 1 million km2 of wetlands in the conterminous 

United States (Dahl and Allord, 1996). Less than one-half of 

this wetland area remained by the mid-1980s, and most of the 

drained areas had been converted to agricultural use (Dahl 

and Johnson, 1991; Gollehon and Quinby, 2006). In 2010, 

about 25 percent of the cropland in the country was drained 

(fig. 4.8). Wetlands and ponds generally serve to store water, 

at least temporarily, and, thus, slow the rate of movement 

and increase the transit time of water. When engineered 

drainage systems are installed, ponds and wetlands may be 

partly or completely drained and water storage within the 

watershed is reduced. In most cases, the draining of ponds 

and (or) wetlands is intentional and these drained areas 

become available for growing crops. However, wide-scale 

drainage systems have far-reaching effects, and, in some cases, 

wetlands or ponds are unintentionally drained.

Evapotranspiration

Runoff

Precipitation

Outputs

EXPLANATION

Inputs

Internal flowpaths 

Catchment water budget—Arrow sizes 

indicate relative magnitudes of the flowpaths

Figure 7.3. Natural water cycle in a catchment too wet for agriculture. In a catchment a with shallow water table and 

fine-grained soils, infiltration of rainfall is precluded by the limited capacity of the waterlogged subsurface to accept 

more water. Much of the rainfall reaching the land surface therefore follows overland flowpaths to local topographic 

lows including ponds and streams. This added zone of temporary water storage slows the delivery of storm event water 

to streams, and, thus spreads the stream response over time and decreases peak outflows. In addition, for small storm 

events, soil storage may be sufficient to store all event water and there may be no outflow. 
126 Agriculture—A River Runs Through It—The Connections Between Agriculture and Water Quality

The purpose of drainage is to move water quickly and 

efficiently off the landscape. Subsurface drainage removes 

moisture from the soil and prevents the water table from rising 

above the drain level for long periods of time. The water from 

the subsurface drains generally empties into a surface drain 

(ditch) that carries the water to a natural stream. Catchments 

in these areas are defined by the spatial network of connected 

surface and subsurface drains rather than by topographical 

divides. Prior to agricultural drainage, many of these areas 

did not contain perennial streams; many were wetlands that 

released surface water only during particularly wet periods. 

Streamflow at the outlet of artificially drained catchments 

is the composite of flow from all drains. Individual fields 

within the catchment respond differently to rainfall on the 

basis of their topography, soil texture, and extent of soil 

macropores. The distance that water travels from individual 

drain outlets to the catchment outlet varies depending on the 

distance of the field from the outlet and the characteristics of 

the surface channels that carry water from the drain outlet to 

the watershed outlet. If discharge from multiple drains takes 

approximately the same time to reach the catchment outlet, 

these discharges will be additive, leading to relatively high 

peak flows. By contrast, discharge from various zones of the 

catchment can reach the outlet at different times, which will 

result in a lower peak but a greater duration of streamflow.

The large-scale effect of cropping and agricultural 

drainage in the Mississippi River Basin have increased 

streamflow as a function of the percentage of land in row 

crops (fig. 7.4; Raymond and others, 2008). The increased 

streamflow is the result of numerous landscape modifications 

that allow crop production in naturally wet areas. Overall, 

these modifications substantially change water budgets and 

water flowpaths by reducing evapotranspiration (replacing 

perennial vegetation with seasonal crops), decreasing 

water storage in wetlands and shallow soil, and increasing 

the volume and velocity of runoff (through surface and 

subsurface drainage). Leary-Weber Ditch, Indiana, provides 

a specific example of a watershed that is not naturally well 

suited to agriculture because of the abundance of water (see 

“Hydrologic Consequences of Agriculture in a Watershed 

Where Natural Conditions Were Too Wet for Agriculture”).

0.08

0.06

0.04

0.02

0

-0.02

0.10

-0.06

-0.04

Percentage of watershed in cropland

Change in discharge at average precipitation, in meters per year

0 20 40 60 80 100

Independent watersheds

Watersheds with nested watersheds

EXPLANATION

Figure 7.4. As the percentage of cropland increases 

in the sub-watersheds within the Mississippi River 

Basin, there is a trend toward increases in streamflow 

at average precipitation. The change was calculated by 

averaging the time periods before 1966 and after 1987 

(modified from Raymond and others, 2008). 
Water on the Modified Agricultural Landscape 127

Hydrologic Consequences of Agriculture in a Watershed Where Natural 

Conditions Were Too Wet for Agriculture

The Leary-Weber Ditch watershed covers about 7.1 km2 in the Sugar Creek watershed in east-central 

Indiana (see “Agriculture in Central Indiana” in Chapter 8; Lathrop, 2006). The topography in the area is flat and 

the soils are poorly drained. The area has a humid continental climate and receives about 90 cm of precipitation 

each year.

Prior to the establishment of farms in the area in the early 1800s, wetlands covered much of the land 

surface. When rain fell on this landscape, a diffuse system of overland flow rivulets likely formed to carry 

water to local depressions and, when these overflowed, water flowed into Sugar Creek. The earliest farmsteads 

were established on the highest land to avoid the waterlogged soils. Gradually, however, an extensive network 

of subsurface drains and surface drainage ditches, such as Leary Weber Ditch, were dug to carry the water to 

nearby streams such as Sugar Creek. In recent years, nearly 90 percent of the Leary-Weber Ditch watershed is 

used to grow corn and soybean and all of this land is drained by engineered surface and subsurface drainage 

systems (fig. 4.9).

About 60 percent of the precipitation in the Leary-Weber Ditch watershed is consumed by 

evapotranspiration (fig. 7.5). Much of the surplus water infiltrates the soil and is intercepted by the subsurface 

drain network that carries it to Leary-Weber Ditch. A small part of the excess precipitation falls on areas adjacent 

to the ditch and follows overland flowpaths directly into the ditch. Under certain hydrologic conditions, the 

predominant flowpaths carrying water to the ditch can change markedly. For example, during intense storm 

events (greater than 2 cm/h), and when soil conditions are wet prior to rainfall, it is estimated that overland 

flow comprises about 40 percent of the flow in the ditch (Baker, Stone, and others, 2006). The contribution to 

subsurface drain flow from water quickly moving through soil macropores may be 50 percent or higher during 

high-intensity storm events (Kumar and others, 1997; Stone and Wilson, 2006).

Flow in Leary-Weber Ditch is usually continuous during winter and spring, when rainfall is plentiful and 

crops are small or not present (fig. 7.5). From mid-summer to early autumn, however, streamflow often ceases 

because no water is available after crop requirements are met. As a result of the large changes to the natural 

hydrologic system, agricultural development in the Leary-Weber watershed has greatly affected the watershed’s 

water cycle (table 7.3). Overall, the predominant flowpaths are much faster after agricultural development than 

under natural conditions, which can enhance chemical transport and streambank erosion.
128 Agriculture—A River Runs Through It—The Connections Between Agriculture and Water Quality

tac11-0566_fig07-05

Evapotranspiration

62%

Precipitation

100%

Drains

Runoff 10%

Outputs

EXPLANATION

Inputs

Internal flowpaths 

Catchment water budget—Arrow sizes 

indicate relative magnitudes of the flowpaths

0

60

120

0 180

1

2

3

Evapotranspiration

Precipitation

Groundwater

out

Groundwater in

2003

Inflows = outflows = 93 cm

Inflows Outflows

B

A. Leary-Weber Ditch, Indiana

Streamflow, in cubic meters

Precipitation, in millimeters

per second

Streamflow

Precipitation

2003

Jan. Feb. Mar. May June July Aug. Sept. Oct. Dec.Nov. Apr.

C

Streamflow

Figure 7.5 Diagrams and graph showing (A) important hydrologic flowpaths, (B) estimated water budget, and (C) relation 

between precipitation and streamflow for the Leary-Weber Ditch, Indiana, watershed, 2003 (Capel and Hopple, 2018).
Water on the Modified Agricultural Landscape 129

Table 7.3. Predominant flowpaths and associated transit times in Leary-Weber Ditch, Indiana.

[Shading; Blue cells indicate natural conditions; red cells indicate conditions after agricultural development; blank cells indicate that these 

flowpaths or time scales are not important for this type of catchment]

Flowpath

Transit time

Minutes Hours Days Months Years Decades Centuries

Flowpaths to the atmosphere

Evaporation

Plant transpiration

Flowpaths to shallow groundwater

Macropore flow through soil

Matrix flow through soil

Seepage from wetlands

Flowpaths to streams

Local groundwater flow

Subsurface drain flow

Runoff

Macropore flow through soil to stream

Flowpaths to rivers and oceans

Regional groundwater flow

Where There is Not Enough Water.—Large regions 

in the Western United States have temperatures, soils, and 

topography that are well suited to agriculture, but have too 

little precipitation during the growing season to support crops 

(fig. 4.3). Under natural conditions, virtually all of the small 

amount of precipitation that falls on arid landscapes either 

evaporates or is used by native plants that are adapted to lowmoisture conditions. Often there is not enough precipitation 

to sustain perennial streams at the catchment scale (fig. 7.6). 

Occasionally, however, rain or snowmelt events produce 

enough water to exceed the infiltration capacity of the soil. 

The result is surface runoff to temporary (ephemeral) streams 

in low-lying areas. Although the water infiltrates and seeps 

into the soil matrix during these events, most of this water is 

eventually consumed by evapotranspiration before reaching 

the water table. The water table tends to be quite deep in 

arid regions.

Various sources of irrigation water are used and the 

methods used to apply this water to cropland and pasture 

varies widely. Where surface water is the source of irrigation, 

practices range from occasionally rerouting local stream 

water onto nearby fields to importing large volumes of water 

from distant areas by extensive pipeline or canal systems. 

Where groundwater is the source of irrigation water, practices 

range from occasionally pumping that water to supplement 

precipitation to regularly pumping large volumes to supply 

virtually all crop needs.
130 Agriculture—A River Runs Through It—The Connections Between Agriculture and Water Quality

tac11-0566_fig07-06

Precipitation

Evapotranspiration

Outputs

EXPLANATION

Inputs

Internal flowpaths 

Catchment water budget—Arrow sizes 

indicate relative magnitudes of the flowpaths

Figure 7.6. Natural water cycle in a catchment too dry for agriculture. Under natural conditions, almost all of the small 

amount of precipitation that falls in arid and semi-arid landscapes either evaporates or is used by native plants that are 

adapted to low-moisture conditions. Often there is not enough precipitation to sustain perennial streams at the smallcatchment scale. Occasionally, however, rain or snowmelt produces enough water to exceed the infiltration capacity of 

the soil and the resulting overland flow forms temporary streams in topographic lows. Although the water that infiltrates 

and seeps through the soil matrix during these events is often greater than the short-term evapotranspiration potential, 

the water table tends to be quite deep in arid regions and, in most cases, excess moisture is eventually consumed by 

evapotranspiration before reaching the water table. 

In all cases, irrigation increases the volume of water 

delivered to the land surface during the growing season. Much 

of this added water is consumed by the plants and lost through 

evapotranspiration, but commonly more water is applied than 

can be used by crops. Consequently, flowpaths across the land 

surface or into the subsurface carry more water than under 

natural conditions. To carry the increased volume of water, 

man-made flowpaths, such as drainage ditches, have been 

developed in some locations. In other locations, the response 

of natural flowpaths to extreme rainfall storm events in the 

natural (pre-agriculture) landscape has become more important 

and sometimes perennial. The large amounts of irrigation 

water applied frequently and over the long term markedly 

change the water budget and water flowpaths. 

Furrow and flood irrigation purposely deliver water 

at a rate faster than it can infiltrate the soil and be used by 

the plants. This water delivery results in surface runoff 

(tailwater) that is typically routed away from cropped areas 

by drainage ditches. These field-scale ditches carry water to 

natural streams. If no natural streams were present, a network 

of ditches was excavated to collect and route the water to a 

natural stream.

Irrigation water that infiltrates the soil, but is not 

consumed by plants, moves downward and recharges 

groundwater. This increased recharge, relative to natural 

conditions, can raise the water table (fig. 7.7). Under these 

conditions, the water table can intersect topographically lowlying areas of the catchment and discharge as surface water. 

Perennial streams and surface ditches, supported by baseflow 

and irrigation runoff, can form where previously not present. 

The surface channels have flow characteristics that can be 

appreciably different than those of natural streams (fig. 7.8). 

In some cases, the water table rises so close to land surface 

that waterlogging of near-surface soil is extensive and artificial 

drainage systems must be installed to allow agriculture 

to continue.
Water on the Modified Agricultural Landscape 131

tac11-0566_fig07-07

0

10

20

30

40

Groundwater level, in meters below land surface

1950 1955 1960 1965 1970 1975 1980

Year

Figure 7.7. Rise in the water-table elevation resulting from 

long-term irrigation with surface water in the Columbia 

Plateau, Washington. The rise began when the Columbia Basin 

Irrigation Project was implemented in the late 1950s. The 

water table rose approximately 20 meters over a period of less 

than 10 years. Installation of a drainage system in the 1970s—

primarily surface ditches—prevented further rise of the water 

table (Capel and Hopple, 2018) .

0

5

10

15

20

25

0.00

0.15

0.30

0.45

1917 1918 1919

Water year (October 1–September 30)

Water year (October 1–September 30)

1920 1921

Annual yield, in centimeters

per water year

Annual yield, in centimeters

per water year

0

5

10

15

20

25

0.00

0.15

0.30

0.45

1999 2000 2001 2002 2003

A. Non-irrigated catchment (Satus Creek, WA, USGS site 12508500)

B. Irrigated catchment (Granger Drain, WA, USGS site 12505450) 

Daily catchment yield,

in centimeters per day

Daily catchment yield,

in centimeters per day

Annual yield

Daily catchment yield

EXPLANATION

Figure 7.8. Effect of imported irrigation water on catchment water yield. (Catchment water yield is the streamflow 

divided by catchment area.) The peak yields in (A) the non-irrigated catchment (Satus Creek, Washington) are 

substantially higher than in (B) the irrigated catchment (Granger Drain, Washington). The substantial amount of 

sustained baseflow in the irrigated catchment leads to a higher annual water yield compared to the non-irrigated 

catchment and can contribute to a greater transport of agricultural chemicals (Capel and Hopple, 2018). 
132 Agriculture—A River Runs Through It—The Connections Between Agriculture and Water Quality

As more efficient methods for delivery and application of 

irrigation water have been developed and implemented (U.S. 

Department of Agriculture, 2010), the changes to the water 

cycle caused by irrigation have been substantially reduced. 

Although these more efficient methods are increasingly used, 

conversion is proceeding slowly because they are expensive 

and, at this time, are economically feasible only for a few 

crops and environmental conditions (Schuck and others, 

2005). Another confounding factor is the nature of water rights 

in the Western States, where “prior appropriation” water rights 

generally prevail. These rights are contingent on the holder 

putting the water to beneficial use (Anderson and Woolsey, 

2005). All or parts of these water allocations may be lost if 

the water is not regularly used. Because of the non-use clause 

in the language, water users have a legitimate concern about 

losing some of their water rights by using less water in the interest 

of conservation. In the Eastern States, “riparian water rights” 

generally prevail. These rights are granted based on ownership of 

the land that physically touches the stream, lake, or other water 

body, and cannot be lost through non-use of the water.

In addition to the effects of irrigation on the catchment in 

which the irrigation is applied, extensive, long-term irrigation 

can deplete the stream or aquifer that serves as the source 

reservoir from which irrigation water is withdrawn. When 

surface water is withdrawn for irrigation, the flow in the 

source stream declines. Although excess irrigation water is 

eventually returned to the stream in most cases, the volume of 

water has been appreciably reduced by evapotranspiration and 

the net effect is reduced flow in the source stream (fig. 7.9). 

5.6

14

34

40

96

48

28

11

25

2.8

2.8

31

Yakima

Richland

Project Canal

Sunnyside Canal

Granger Drain

Coulee Dr Sulphur Cr Wasteway

Marion Drain

Sub Drain 35

and return

Chandler Diversion

Richland

Canal

Columbia Cana

K

l

ennewick Irrigation District Canal

Wapato Irrigation

East

Toppenish

Drain

DID

#7

Joint

Dr 2

Drain

35

Satus

Drain

Satus Creek

Toppenish Creek

YAKIMA

RIVER

COLUMBIA

RIVER

Naches River

Streamflow, in cubic meters per second

Columbia River

Yakima River and tributaries

Drainage and irrigation canals

28

EXPLANATION

0 5

5

10 Miles

0 10 Kilometers

Figure 7.9. Typical irrigation-season streamflow and irrigation canal flow for the lower part of the Yakima River Basin, Washington. The 

widths of flow lines, which are proportional to flow rate, show that irrigation withdrawals from surface water have appreciable effects 

on streamflow (Modified from Wise and others, 2009).
Water on the Modified Agricultural Landscape 133

When groundwater serves as the source of irrigation 

water, long-term pumping can lower the water table. When the 

water table is lowered, baseflow resulting from groundwater 

discharge to streams can decrease, or losses from stream 

channels to groundwater can increase. Under either of these 

scenarios, streamflow is reduced and can change the stream 

habitat. The DR2 Drain watershed in Washington provides a 

specific example of a watershed that is not naturally suited to 

agriculture because there is not enough available water (see 

“Hydrologic Consequences of Agriculture in a Watershed 

Where Natural Conditions Were Too Dry for Agriculture”).

In many areas of the United States, the water-table 

decline is substantial, extensive, and long term (fig. 7.10). 

In a 2004 compilation, groundwater withdrawals for 

irrigation accounted for about two-thirds of total groundwater 

withdrawals in the United States (Hutson and others, 2004). 

In California’s Central Valley, both surface water and 

groundwater are used extensively for irrigated agriculture, 

and declines in groundwater levels up to 120 m have resulted 

(Faunt, 2009). These water-level declines have resulted 

in considerable land subsidence because of compaction 

of aquifer materials (Galloway and others, 2000; Reilly 

and others, 2008). On the Columbia Plateau, pumping has 

removed water from storage in the aquifers and resulted in 

water-level declines of up to 100 m locally and declines of 

more than 30 m over extensive areas (Morgan and others, 

2008). In the High Plains aquifer of the Central United States, 

groundwater‐level changes from predevelopment to 2007 

ranged from a rise of 26 m in Nebraska to a decline of 71 m in 

Texas, and the total water in storage in the aquifer declined by 

more than 300 billion m3 over that period (McGuire, 2009). 

EXPLANATION

Other cropland

Cropland in areas that have 

groundwater-level declines

Figure 7.10. Areas of groundwater-level decline relative to the extent of cropland. The areas of cropland in which 

groundwater levels have declined does not necessarily mean that irrigated agriculture is the principal cause of the 

decline. Crops that are irrigated with water from sources in decline may be vulnerable (Reilly and others, 2008).
134 Agriculture—A River Runs Through It—The Connections Between Agriculture and Water Quality

Summary

Water movement through an agricultural watershed is 

governed by the combination of natural hydrological factors 

and the various agricultural modifications. The collective 

effects of the modifications change the natural, pre‐agricultural 

water budgets and water flowpaths. The extent of this change 

depends largely on how suitable the natural landscape of 

the watershed was for agriculture. Areas with fertile soil, 

adequate drainage, and adequate rainfall during the growing 

season require relatively few modifications to become 

agriculturally productive. Generally in these areas, conversion 

of the natural landscape to agricultural use results in a 

decrease in the volume of water that leaves the watershed as 

evapotranspiration, an increase in runoff and (or) recharge, and 

an increase in the annual streamflow that exits the watershed. 

Despite these water budget changes, the flowpaths that water 

follows are the same or similar to flowpaths present prior to 

agricultural modifications.

In areas where there is too much water in the soil for 

agriculture (wetland areas), engineered drainage systems 

are used. The drainage systems have become a permanent 

component of the landscape and have substantially altered the 

water budget and water flowpaths. The purpose of drainage 

is to move water quickly and efficiently off the landscape. 

Subsurface drainage removes moisture from the soil and 

prevents the water table from rising above the drain level for 

long time periods. The water from subsurface drains generally 

empties into surface drains (ditches), which carry the water 

to a natural stream. Overall, these modifications substantially 

change the water budgets and water flowpaths by reducing 

evapotranspiration (replacing perennial vegetation with 

seasonal crops), decreasing water storage in wetlands and 

shallow soil, and increasing the volume and velocity of runoff 

(through surface and subsurface drainage).

In some areas, precipitation during the growing season 

is inadequate to meet crop requirements, even though other 

climatic and landscape factors are suitable for agriculture. 

In these areas, water for agriculture is supplied by irrigation. 

Irrigation increases the input of water to the catchment and 

creates new water flowpaths or expands the flowpaths that 

were present before agricultural uses began. The source of the 

irrigation water can be either surface water or groundwater. 

Surface-water sources can be either within or outside of 

the watershed where it is used. Long-term pumping from 

groundwater sources for irrigation can lower the water table. 

The decline can be substantial and affect the sustainability of 

the groundwater resource. If groundwater is pumped from a 

shallow aquifer that is hydraulically connected to a stream, 

streamflow can decrease due to the reduction or cessation 

of baseflow. Irrigation imported from external surface-water 

sources can increase groundwater recharge and cause the 

water table to rise to the extent that subsurface drainage is 

needed to drain the soil. Large amounts of irrigation water 

applied frequently and over the long term (years and decades) 

can markedly change the water cycle in an area. 

Water moves through catchments along largely expected 

flowpaths that connect the various components of the 

hydrologic systems—atmosphere, soil, groundwater, and 

surface water. For a particular catchment, each flowpath 

starts at the specific location where water from precipitation 

or irrigation enters the catchment and ends at a specific 

location where the water leaves the catchment. Different 

flowpaths predominate in different fields, and, depending on 

the route followed between the starting and ending points, 

each flowpath has a characteristic transit time, ranging from 

minutes to centuries.

Some fraction of the water that enters an agricultural 

landscape moves into the broader environment through 

runoff or recharge. This water that moves through surface and 

subsurface flowpaths can transport sediment and agricultural 

chemicals. The movement of these chemicals off farms and 

into the broader environment is largely governed by the 

movement of the water. 
Water on the Modified Agricultural Landscape 135

Hydrologic Consequences of Agriculture in a Watershed Where Natural 

Conditions Were Too Dry for Agriculture

The DR2 watershed covers approximately 5.5 km2 in the Granger Drain watershed in south-central 

Washington (see “Agriculture in Central Washington” in Chapter 6; Payne and others, 2007). The watershed is 

arid, receiving an average of only about 17 cm of precipitation annually. Most of the precipitation falls during 

November through January. The driest months are during the summer growing season (fig. 7.11). 

Land use in the DR2 watershed is approximately 90 percent agricultural—orchards, vineyards, pasture, 

corn, and vegetable crops and dairy feeding operations. All agricultural operations are dependent on irrigation. 

The large demand for irrigation water throughout the region is met by withdrawals from the Yakima River 

that are transported by an extensive system of canals. At the individual field scale, water is applied to crops by 

various methods, including rill, drip, and sprinkler irrigation systems.

By 1902, substantial quantities of irrigation water were being regularly applied throughout the region. 

The water table rose more than 20 m (Jayne, 1907). Construction of a system of drainage ditches, such as DR2, 

began in 1907, as well as installation of subsurface drainage. The regular application of irrigation water since 

this time has maintained a water table that is high enough to sustain year-round flow in the lower reaches of 

many drainage ditches. Thus, more than a century of irrigated agriculture has created surface-water and shallow 

groundwater-flow systems that were not present prior to development.

Relative to natural conditions, irrigation has increased water input to the DR2 watershed approximately 

tenfold (fig. 7.11; McCarthy and Johnson, 2009). This increased input results from irrigation applied directly 

to the fields within the watershed as well as from excess irrigation water applied elsewhere that flows as 

groundwater into the watershed. Leakage alone from the irrigation water delivery canal equals precipitation. 

About one-half (45 percent) of the total water input to the watershed is consumed by evapotranspiration. A small 

amount of water leaves the watershed as regional groundwater flow, and the remainder—about 40 percent—

leaves the watershed as streamflow in DR2.

During the non-irrigation season (typically from mid-October to mid-March), flow is sustained in DR2 

by baseflow. During the irrigation season, approximately one-third of the flow in DR2 is from baseflow. The 

remaining two-thirds consists of a mixture of spill from the irrigation system (water transported directly from 

the irrigation delivery system to the stream without being released to the landscape), overland flow such as 

tailwater ditches, and applied irrigation water that infiltrates and then travels through shallow subsurface 

flowpaths, such as subsurface drains, without being part of the groundwater system. As a result of the large 

changes to the natural hydrologic system, agricultural development in the DR2 watershed has had a large effect 

on the watershed’s water cycle (table 7.4).
136 Agriculture—A River Runs Through It—The Connections Between Agriculture and Water Quality

tac11-0566_fig07-11

Runoff 10%

Precipitation

10%

Irrigation and

canal leakage

60%

Evapotranspiration

45%

Prior to agricultural development

Inflows = outflows = 17 cm

2003

Inflows = outflows = 183 cm

Precipitation Evapotranspiration

Inflows Outflows

Evapotranspiration

Precipitation Groundwater

out

Groundwater

in

Streamflow

Inflows Outflows

Irrigation

Canal leakage

0

10

20

0.0 30

0.1

0.2

0.3

Streamflow, in cubic meters

Precipitation, in millimeters

per second

Streamflow

Precipitation

2003

Jan. Feb. Mar. May June July Aug. Sept. Oct. Dec.Nov. Apr.

B C

Outputs

EXPLANATION

Inputs

Internal flowpaths 

A. Granger Drain, Washington

Catchment water budget—Arrow sizes 

indicate relative magnitudes of the flowpaths

Figure 7.11. Diagrams and graph showing (A) catchment water budget with important hydrologic flowpaths, 

(B) estimated water budget after and before agricultural development, and (C) relation between precipitation and 

streamflow for the DR2 watershed, Washington, 2003 (Capel and Hopple, 2018).
Water on the Modified Agricultural Landscape 137

Table 7.4. Predominant flowpaths and associated transit times in DR2 watershed, Washington.

[Shading: Blue cells indicate natural conditions; red cells indicate conditions after agricultural development; blank cells indicate 

that these flowpaths or time scales are not important for this type of catchment]

Flowpath

Transit time

Minutes Hours Days Months Years Decades Centuries

Flowpaths to the atmosphere

Evaporation

Plant transpiration

Flowpaths to shallow groundwater

Macropore flow through soil

Matrix flow through soil

Seepage from wetlands

Flowpaths to streams

Local groundwater flow

Subsurface drain flow

Runoff

Macropore flow through soil to stream

Flowpaths to rivers and oceans

Regional groundwater flow
138 Agriculture—A River Runs Through It—The Connections Between Agriculture and Water Quality

Agriculture in the Central Valley of California

The San Joaquin River Valley in central California (fig. 1.3) is bounded by the Sierra Nevada to the east, the 

Coast Ranges to the west, the Tehachapi Mountains to the south, and the Sacramento–San Joaquin Delta to the 

north. The topography of the region is a gently sloping plain, with rivers and their seasonal tributaries carrying 

flow from the surrounding mountains. Pre-settlement vegetation included extensive stands of saltbush scrub, desert 

grassland, alkali scrub, and wetlands. Annual rainfall ranges from 46 (north) to 25 cm (south), and the climate is 

characterized by warm dry summers and moist winters. 

In the early 1800s, a Spanish expedition arrived in Merced County and found the grass plains ideal for grazing 

livestock. The Spanish Army officer in charge of the expedition named the river Rio de Nuestra Señora de la 

Merced (The River of our Lady of Mercy), from which the county derives its name. By 1855, about 500 pioneers 

raised cattle in the area. Settlers arrived rapidly and cereal production became important; soon managing livestock 

was no longer profitable. In 1870, the Central Pacific Railroad constructed tracks down the San Joaquin Valley, 

providing transportation for agricultural products.

In 1919, the Merced Irrigation District was formed. The Exchequer Dam on the Merced River was selected as 

the District’s first project and was completed in 1926 to provide flood control and water for irrigation and power 

generation. In 1967, New Exchequer Dam was completed to expand Lake McClure Reservoir capacity. In the same 

year, McSwain Dam was completed downstream as a regulating reservoir. The addition of canals and irrigation 

appreciably changed the crops farmers were able to produce. Crop diversification included orchards, vineyards, 

fruits, almonds, corn, and grains. The Merced Irrigation District operates most of the irrigation infrastructure, which 

supplies water to about 625 km2

 of farmland with about 4,000 sets of control gates and 1,300 km of canals (Merced 

Irrigation District, 2013).

In the 1990s, Merced County saw a reorganization of irrigation practices, providing better service to growers 

while managing water more efficiently and cost effectively. Flow management projects in the upper watershed, 

where surface water is unavailable, developed drip irrigation and automated micro-sprinkler irrigation systems to 

increase efficiency. In the lower watershed, where surface water is plentiful, flood systems and permanent sprinklers 

dominate. Dairy accounts for about one-third of the total value of agricultural production, almond orchards account 

for 45 percent of agricultural land, followed by corn and grain at 16 percent, and vineyards at 12 percent. A variety 

of fruits are grown in Merced County including peaches, figs, and wine and raisin grapes.

Nitrate in groundwater is one of the greatest water-quality issues of concern in the California’s Central Valley 

watershed (Burow and others, 2008; van der Schans and others, 2009; Landon and others, 2011). Because of a 

shallow water table, attributable in part to irrigation with surface water, and a high percentage of sandy soils, 

the shallow groundwater of the eastern San Joaquin Valley has elevated concentrations of nitrate (greater than 

10 milligrams per liter [mg/L] as nitrogen). Much of the aquifer contains enough dissolved oxygen to prevent 

significant transformation of nitrate to other forms of nitrogen and allow the mobilization of naturally occurring 

trace elements into recharge from excess irrigation (Jurgens and others, 2010).

A B C

The landscape of the Central Valley of California. (A) The grasslands typical of the time before agriculture, 

(B) an almond orchard, and (C) the view from the air (23 square kilometers). Photograph A from Bureau 

of Land Management, BLM CA160, Seeds of Success, 2012; photograph B from U.S. Department of 

Agriculture, Natural Resources Conservation Service photo gallery, NRCSCA06014; photograph C from 

U.S. Department of Agriculture, Farm Service Agency, National Agricultural Imagery Program, 2010. 
Chemicals in Crop and Animal Agriculture 139

—Chemicals in Crop and Animal Agriculture 8 Chapter

How Do Agricultural Chemicals Affect 

Water Quality?

Agricultural activities are defined here as all farming 

procedures that fulfill the requirements for the production of 

crops or animals—space, soil, water, nutrients (for plants), 

food (for animals), protection (from pests and weather), and 

disposal of the wastes. Many of these agricultural activities 

are accomplished through modifications to the landscape or 

applications of chemicals. Landscape modifications affect 

the water flowpaths (Chapter 6). Some fraction of the water 

that is introduced into the agricultural landscape can carry 

agricultural chemicals, sediment, and microorganisms into the 

environment. (Even though sediment and microorganisms are 

not chemicals, they share many of the same source, transport, 

and fate processes.) The water, sediment, chemicals, and 

microorganisms can give rise to a number of environmental 

concerns (table 2.2). Water-quality concerns related to 

agriculture usually result from elevated levels of chemicals, 

sediment, and (or) microorganisms as compared to the natural 

condition. Any level of a man-made (synthetic) chemical is an 

elevated level compared to the natural condition.

More than 7,000 chemicals are associated with plant 

and animal agriculture (Capel and others, 2017). Some of the 

important chemicals or chemical groups and their source(s) 

are presented in table 8.1. Many chemicals are purposely 

used in agricultural activities because they directly benefit 

crop or animal production. These chemicals include the active 

ingredients in fertilizers, lime, pesticides, hormones, and 

antimicrobials. Other chemicals are produced by the crops or 

animals as waste by-products, such as methane and manure. 

Some are produced by chemical or microbiological reactions 

in the soil and water. These chemicals include pesticide 

transformation products, nitrogen oxides (see “Nitrogen” in 

Chapter 2), and dissolved organic carbon. Some chemicals 

are naturally stored in the soil and water, such as selenium, 

arsenic, and salts, and can be mobilized by agricultural 

activities, such as irrigation. Finally, some chemicals 

are introduced into the agricultural landscape through 

“piggy‐backing” on purposely applied chemicals. Animal 

manure, used as a fertilizer on crops and as a soil amendment, 

can contain chemicals used for the animals (hormones, 

antimicrobials) and microorganisms like coliform bacteria 

(E. coli) (see “Manure” in Chapter 6). 

Some chemicals are predominantly used in or produced 

from plant agriculture and others predominantly in animal 

agriculture. Some chemicals are common to both activities 

(table 8.1). In the case of manure, it is a by-product of animal 

agriculture and purposely used as a fertilizer and soil amendment 

in plant agriculture. For those chemicals that are purposely used, 

their reasons for use is generally to increase yield, to protect 

crops or animals, to increase soil quality, to dispose of waste, to 

facilitate the effectiveness of another chemical with a purposeful 

use, or to protect the environment (table 8.1). 

Agricultural Chemicals

There are two broad categories of chemicals that 

are important to agriculture—those that provide nutrients 

(fertilizers and manure) and those that protect the crops 

and (or) animals from pests (pesticides). The water-quality 

concerns from these categories are due in part to their effects 

on the environment, and in part to their quantity of use and 

(or) release in agriculture. 

Fertilizers provide the necessary nutrients for plants to 

grow. Nutrients are the essential elemental building blocks 

of the plant cells. Three major nutrients with limited natural 

supply—nitrogen, phosphorus, and potassium—are often 

added to crops either as synthetic fertilizers or as manure 

(table 8.2). As many as seven other essential plant nutrients, 

mainly trace metals that are needed by plants in small amounts 

(micronutrients), are added to fertilizer as needed: manganese, 

boron, copper, iron, chlorine, molybdenum, and zinc. The 

other elemental building blocks of plants (carbon, oxygen, 

hydrogen, and sulfur) generally are abundant in the air, water, 

and soil. If one of these plant nutrients is in short supply, 

plant growth will be limited and yield will be reduced. The 

techniques, timing, and rates of fertilizer application depend 

on the nutrient content of the fertilizer, the crop and yield goal, 

soil type and texture, climate, and the equipment available to 

the applicator (see “Glossary of Farm Implements”). Nitrogen 

and phosphorus can have adverse water-quality impacts upon 

entering the broader hydrologic environment (see “Nitrogen” 

in Chapter 2).

Nitrogen is involved in protein synthesis and chlorophyll 

formation. Nitrogen is a component of DNA. Plants with 

adequate nitrogen show healthy vigorous growth, strong root 

development, dark green foliage, and substantial seed and 

fruit formation. The use of synthetic nitrogen fertilizers has 

increased steadily in the last 50 years, rising to the current 

rate of 1 billion metric tons of nitrogen per year (fig. 3.8). The 

spatial distribution of nitrogen use on cropland in the United 

States is shown in figure 8.1A. 
140 Agriculture—A River Runs Through It—The Connections Between Agriculture and Water Quality Table 8.1. Selected common chemicals used in and produced by crop and animal agriculture.

[Shaded cells: Green, chemicals primarily associated with crop agriculture; pink, chemicals primarily associated with animal agriculture; blue, chemicals associated with both crop and animal agriculture; blank, not applicable. Source(s) into the agricultural system: PU, purposeful use; PB, “piggy-backed” on something with a purposeful use; IS, in situ production; MN, mobilized naturally occurring substance; WP, waste product of agriculture. Benefit(s) from purposeful use: YD, increases yield; AP, increases agricultural protection (for example, soil erosion, crop production); SQ, improves soil quality; FP, facilitates the effectiveness of another purposeful chemical; WD, disposes of waste; EP, increases environmental protection. Abbreviations: MSMA, monosodium methyl arsenate; DDT, dichlorodiphenyltrichloroethane; DDE, dichlorodiphenyldichloroethylene]

Source(s) into the 

agricultural system

Benefit(s) from 

purposeful use Brief description, examples

PU PB IS MN WP YD AP SQ FP WD EP

Antibiotics, antimicrobials Protects animals from bacteria that cause disease (at therapeutic dose). Improves animal health,

which translates to more rapid and efficient growth (at subtherapeutic dose). Oxytetracycline,Penicillin G, lincomycin.

Arsenic Used in herbicides: Used in herbicides, such as MSMA. Used in livestock growth promoters, such

as roxarsone.

Bacteria, viruses

A

A

A Microorganisms are abundant in all agricultural environments, only a few are pathogens. Escherichia

coli, Bacillus thuringiensis.

Dust Fine particulate matter that can become airborne. Fungicides Provides crops protection from fungal diseases. Azoxystroban, captan, metalaxyl.Gypsum Supplies sulfur and calcium for plant nutrition. Improves poor soil structure caused by high sodium

in the soil.

Herbicides Provides crop protection from weeds that compete for space, sunlight, and nutrients. Atrazine,

metolachlor, gylphosate.

Hormones Promotes growth and increases meat and milk production. Bovine somatotropin (bST) for dairy.

Testosterone, trenbolone for beef.

Inert ingredients in pesticide formulations Chemicals added to the active ingredient in pesticide formulations to improve handling and

application. Surfactants, gums, clay, oils, pheromones. 

Insecticides Provides crop and animal protection from insects. Carbaryl, chlorpyrofos, DDT, permethrin.Lime Adjusts the pH of the soil to make phosphorus more accessible to plants. Calcium oxide.Manure Waste from livestock that can be used as fertilizer and soil amendment.Methane By-product of digestion by cattle and decomposition of manure.Nematicides Provides crop protection from soil nematodes (roundworms). 1,3-Dichloropropene, metam sodium.Nitrogen Essential nutrient for crop and animal growth. Nitrate, ammonia, urea, nitrous oxide.Organic matter From plant and animal wastes, oftentimes used as a soil amendment. Manure, compost, sludge. 

Polyacrylamide (PAM) Erosion control agent for furrow irrigation. Phosphorus Essential nutrient for crop and animal growth. Total phosphorus, orthophosphate.Potassium Essential nutrient for crop and animal growth. Potash.Salinity Ions dissolved from rocks and minerals. Dissolved solids, salt.Sediment Mobilized soil particles in surface waters. Sand, silt, clay, turbidity.Trace elements B B B Elements that are found in low abundance in the Earth’s surface. Some can be required for growth

(micronutrients), some are toxic. Selenium, mercury, copper, zinc, boron, uranium.

Transformation products of pesticides and 

other organic chemicals

Chemicals formed by the transformation of a chemical. Metolachlor sulfonic acid, deethyl atrazine, 

DDE.

Volatile organic compounds (VOCs)

C

C

C A class of chemicals with a tendency to exist in the vapor phase. Methyl bromide, propane, carbon

tetrachloride, methane.

A Some current-use pesticides are active or inactive bacteria. There are bacteria and viruses in animal manure that are applied to the fields. There are bacteria and viruses naturally occurring in soils.B Some current-use pesticides contain arsenic, manganese, zinc, and other trace elements. Historically, pesticides also contained mercury. Selenium, cobalt, boron, and others are micro-nutrients for 

both plants and animals. Many different trace elements can be found in manure. Mobilization of naturally occurring trace elements is due to the dissolution of soils and rocks, often dependent on the level of dissolved oxygen in the water. This is of particular importance for arsenic, selenium, iron, and manganese. 

C Many different volatile organic compounds (VOCs) are included as inert ingredients in pesticide formulations. Some pesticides, called fumigants, are VOCs (methyl bromide). Many different 

VOCs are created during the storage and treatment of animal waste (including odor-causing chemicals). Many different VOCs are produced by animals (including methane in cows).
Chemicals in Crop and Animal Agriculture 141

Table 8.2. Mass of selected chemicals (active ingredients) used in crop and animal agriculture in the United States, 2009.

[From U.S. Food and Drug Administration, 2010; Grey and others, 2012. Abbreviations: Mg, megagram (1 Mg = 1 metric ton); –, not applicable]

Mass used 

(Mg)

Percent of 

group total 

Percent of individual 

pesticide categories

Crop agriculture

Nutrients 17,427,000 100.0 –

Nitrogen (as N) 10,372,000 59.5 –

Phosphorous (as P) 1,264,000 7.3 –

Potassium (as K) 2,327,000 13.4 –

Micronutrients 3,464,000 19.9 –

Soil Amendments – – –

Gypsum (as Ca2SO4) 1,267,000 – –

Manure 594,000 – –

Total Conventional Pesticides 301,000 100.0 –

Fungicides 22,000 7.3 –

 Chlorothalonil 3,780 1.3 17.2

 Hydrated lime 2,530 0.8 11.8

Mancozeb 2,500 0.8 11.3

 79 other fungicides 13,200 4.4 60.1

 Herbicides1 242,000 80.2 –

 Glyphosate 99,500 33.0 41.1

Atrazine 29,200 9.7 12.1

 Metam 15,900 5.9 6.6

 154 other herbicides1 97,000 32.2 40.1

Insecticides 12,100 4.0 –

 Chlorpyrifos 3,180 1.1 26.4

 Acephate 1,360 0.5 11.3

 Clothianidin 570 0.2 4.7

 80 other insecticides 6,950 2.1 57.6

Nematicides and fumigants 25,400 8.4 –

 Dichloropropene 14,400 4.8 56.9

 Chloropicrin 5,060 1.7 19.9

 Methyl bromide 3,120 1.0 12.3

 7 other nematicides and fumigants 2,760 0.9 10.9

Other Pesticidal Agents

Petroleum oil 30,800 – –

Sulfur 26,200 – –

Animal agriculture

Antimicrobial Agents 13,100 100.0 –

Tetracyclines (for example, tetracycline) 4,610 35.3 –

Ionophores (for example, laidlomycin) 3,740 28.6 –

Others (for example, bacitracin) 2,230 17.0 –

Macrolides (for example, erythromycin) 862 6.6 –

Penicillins (for example, penicillin) 611 4.7 –

Sulfas (for example, sulfadimethoxine) 518 4.0 –

Aminoglycosides (for example, streptomycin) 340 2.6 –

Lincosamides (for example, lincomycin) 116 0.9 –

Cephalosporins (for example, cephapirin) 41 0.3 –

1

Includes plant growth regulators.
142 Agriculture—A River Runs Through It—The Connections Between Agriculture and Water Quality

>7,500

Not agricultural land 

>0 to 1,000 

>1,000 to 2,500

>2,500 to 5,000

>5,000 to 7,500 

>1,500

Not agricultural land 

>0 to 300 

>300 to 500

>500 to 1,000

>1,000 to 1,500 

>100

Not agricultural land 

>0 to 10 

>10 to 25

>25 to 50

>50 to 100 

A. Nitrogen

B. Phosphorus

C. Herbicides

tac11-0566_fig08-01 p1

EXPLANATION

Estimated 2002 nitrogen (as N) inputs from 

 fertilizer, manure, and atmosphere, in 

 kilograms per square kilometer 

EXPLANATION

Estimated 2002 phosphorus (as P) inputs from fertilizer, 

 and manure, in kilograms per square kilometer 

EXPLANATION

Estimated 2002 total use for 110 herbicides 

 (active ingredients, glyphosate not included),

 in kilograms per square kilometer 

Figure 8.1. Application of (A) nitrogen fertilizer, (B) phosphorus fertilizer, (C) herbicides (excluding glyphosate), 

(D) glyphosate, and (E) insecticides for the United States. 
Chemicals in Crop and Animal Agriculture 143

EXPLANATION

>100

Not agricultural land 

>0 to 10 

>10 to 25

>25 to 50

>50 to 100 

Estimated 2002 total use for glyphosate (active 

ingredient), in kilograms per square kilometer

EXPLANATION

>25

Not agricultural land 

>0 to 3 

>3 to 5

>5 to 10

>10 to 25 

Estimated 2002 total use for 65 insecticides (active 

ingredients), in kilograms per square kilometer 

D. Glyphosate

E. Insecticides

tac11-0566_fig08-01 p2

Figure 8.1.–—Continued

Many different chemical forms of inorganic nitrogen 

(N) are used in crop agriculture. Anhydrous ammonia 

(82 percent N) is a liquid stored under pressure. Anhydrous 

ammonia becomes a gas when exposed to air and, therefore, 

must be injected into the soil. Ammonium nitrate (34 percent 

N) is a solid typically applied in granular form to pasture lands 

and specialty crops such as citrus. Urea (46 percent N) also 

is a solid applied in granular form. It can be combined with 

ammonium nitrate in water to make a urea ammonium nitrate 

(UAN) solution. It usually takes less than 2 weeks for urea 

to be transformed to ammonia. In contrast to these inorganic 

fertilizers, the nitrogen content of manure is much less (see 

“Manure” in Chapter 6). 

Phosphorus (P) is an essential component of DNA and 

proteins and is essential in energy production within plant 

cells. Abundant phosphorus stimulates early root growth in 

plants and hastens maturity by stimulating flower blooming 

and seed formation. There are three common phosphorus 

fertilizer products: mono-ammonium phosphate (MAP) 

(11 percent N, 52 percent P), di-ammonium phosphate (DAP) 

(18 percent N, 46 percent P) (together called ammoniated 

phosphates), and triple superphosphate (46 percent P), a highly 

concentrated form. The spatial distribution of phosphorus 

fertilizer use on cropland in the United States is shown 

in figure 8.1B. 
144 Agriculture—A River Runs Through It—The Connections Between Agriculture and Water Quality

Potassium (K) is involved in photosynthesis, protein 

synthesis, regulation of plant stomata, and numerous other 

critical cell functions. Potassium also increases plant 

resistance to drought and disease. Muriate of potash (from 

60 to 62 percent K) is the most common potassium fertilizer. 

Broadcasting, the uniform application of chemicals to 

the surface of fields, can be done either by a ground rig or by 

airplane. The fertilizer infiltrates into the soil with water or is 

plowed into the soil. Broadcast applications generally are used 

for large field areas, when time and (or) labor are limiting, or 

when it is important to obtain a uniform distribution across 

the field. Banding applications are narrow bands of fertilizer 

applied in furrows between the rows. This type of application 

is often used to stimulate early plant growth and increase 

yield. Banding is especially important in no-till cropping 

systems where crop residues or winter cover result in lower 

soil temperatures and higher moisture levels that can reduce 

plant heartiness. A sidedressing fertilizer, usually supplemental 

nitrogen, is applied after the crop has emerged (early to midgrowth period of the crop). Sidedressed fertilizer is commonly 

used in sandy, wet, and irrigated soils where nitrogen loss 

from the root zone can be high. In irrigated areas, nitrogen and 

potassium can be added to the irrigation water (chemigation). 

Phosphorus is not normally applied in this manner because 

it forms insoluble compounds that tend to clog the irrigation 

system. Finally, nutrients can be applied directly to the leaves 

of the maturing plant (foliar application). These nutrients 

are absorbed and used by the plant within minutes after 

application. Foliar application can supplement nutrient needs 

at a critical time for the plant, but it is not a substitute for the 

normal fertilizer application to soil. 

For the major crops, fertilizer application can take place 

before planting in the spring, at the time of planting, or after 

planting. In addition, manure is commonly applied to fields 

in the fall in many parts of the country. Nutrient management 

plans are developed so that the amount applied is based 

on the crop’s nutritional needs minus the nutrients already 

available in the soil (see “Nutrient Management Plans”). 

Nitrogen is added at least once a year to most crops, although 

lesser amount are applied to legumes, which fix nitrogen 

from the atmosphere. In many vegetable and corn cropping 

systems, a split application of nitrogen is often used. In 

areas where the loss of nitrogen from the field is potentially 

high, multiple applications may be needed. Phosphorus and 

potassium are stored in the soil longer than nitrogen and 

generally are applied on a less frequent basis than nitrogen. 

The micronutrients are applied, when needed, at the time of 

planting. Agricultural lime is applied any time prior to crop 

emergence to adjust the soil pH and aid in the uptake of 

phosphorus by the plants.

Pesticides are a diverse group of chemicals that provide 

protection to crops and animals from various pests. Some 

pesticides are used to protect the crops and (or) animals 

from weeds (herbicides and plant growth regulators), 

insects (insecticides), fungi (fungicides), soil nematodes 

(nematicides), and bacteria (antibiotics and antimicrobials) 

(table 8.2). These compounds generally are thought to provide 

a wide range of benefits, including increased production 

and quality and a reduction in losses from infestations and 

diseases. Pesticides can have adverse water-quality impacts 

upon entering the broader hydrologic environment (Gilliom 

and others, 2006; see “Human and Ecosystem Health Effects 

of Agricultural Chemicals” in Chapter 2).

About 350 million kilograms (kg) of conventional 

pesticides (active ingredients) have been used in the 

United States each year over the past several decades 

with use leveling off at about 300 million kilograms per year 

(kg/yr) since the late 1990s (fig. 3.8, table 8.2). In 1997, about 

900 pesticidal active ingredients in more than 20,000 different 

pesticide products were registered for use in the United States 

(Aspelin and Grube, 1999). Each year some new pesticides 

are introduced and old pesticides are withdrawn from use. In 

the mid-1990s, the introduction of genetically modified (GM) 

crops caused a substantial change in the use of conventional 

pesticides, both insecticides and herbicides, on agricultural 

crops. Use of many conventional insecticides in cotton and 

corn has decreased due to the increased use of genetically 

modified varieties of these crops with the inclusion of the 

Bt gene. The use of conventional herbicides on soybean, corn, 

and cotton also has changed because of the introduction of 

plants that are genetically resistant to a specific herbicide, such 

as glyphosate (see “Changes in Conventional Pesticide Use 

Due to Development of Genetically Modified Crops”). The 

use of herbicides, glyphosate, and insecticides on cropland is 

shown in figure 8.1.

Pesticides can be applied using a wide variety of 

methods. As with fertilizers, the appropriate application 

method depends on the kind of pesticide being used, the 

formulation type (granular, emulsified, aqueous, or other), the 

crop and type of planting system, and the equipment used by 

the applicator. Herbicides are commonly applied to either the 

soil or the weed foliage (depending on the mode of action), but 

insecticides and fungicides are often directed at microhabitats 

within the foliage canopy. Preventive use of herbicides, 

insecticides, and fungicides require that the entire plant or soil 

area be treated. The foliage density and crop spacing often 

dictate which application method will offer the best coverage. 

Pesticide application methods include broadcast (from tractor 

or airplane), incorporation (injection), chemigation (mixed 

with irrigation water), directed/banded application, and 

treatment of the seed (see “Glossary of Farm Implements”). 

Precision application of pesticides provides a technology that 

targets areas so that the minimum effective amount of the 

chemical can be applied.
Chemicals in Crop and Animal Agriculture 145

Nutrients are important to both crop and animal agriculture. In crop agriculture, nutrients are essential for 

optimum production of food, feed, fiber, and fuel, and must be replenished. In animal agriculture, nutrients are 

contained in the animal feed, and an appreciable amount of these nutrients is passed through the animal as waste 

(manure). Since at least the early 1980s, the term “nutrient management” has been commonly used in agriculture, 

and has developed into a system of practices to ensure the appropriate use of nutrients for production while 

minimizing impacts on water quality.

The USDA Natural Resources Conservation Service (NRCS) developed a Conservation Practice Standard for 

nutrient management. This standard defines nutrient management as “managing the amount, source, placement, 

form, and timing of the application of nutrients and soil amendments” (U.S. Department of Agriculture, Natural 

Resources Conservation Service, 2011). The standard also outlines five elements of purpose behind nutrient 

management: (1) budget and supply nutrients for plant production; (2) properly utilize manure or organic 

by-products as a plant nutrient source; (3) minimize agricultural non-point-source pollution of surface and 

groundwater resources; (4) protect air quality by reducing nitrogen emissions (ammonia and nitrogen oxides 

compounds) and the formation of atmospheric particulates; and (5) maintain or improve the physical, chemical 

and biological condition of soil. The NRCS standard “...applies to all lands where nutrients and soil amendments 

are applied.” The NRCS is a non-regulatory agency, so any involvement by farmers in nutrient management 

is done without regulatory concern (U.S. Department of Agriculture, Natural Resources Conservation Service, 

2011). The standard was designed as technical guidance for implementing a nutrient management plan. Even 

so, there are many reasons that farmers elect to manage nutrients on their farms, for environmental benefits, risk 

management, or economic advantages.

According to the NRCS, a nutrient management plan should: (1) create a nutrient budget for nitrogen, 

phosphorus, and potassium, considering all potential sources of nutrients; (2) establish realistic crop yield 

goals; (3) minimize the movement of chemicals to surface and groundwater by including in a plan the source, 

amount, timing, and method of application; and (4) avoid the application of nutrients to areas with established 

minimum application setbacks (for example, sensitive areas like sinkholes, wells, gullies, ditches, and inlets to 

surface drains).

The USDA has set the goal that all animal feeding operations have a voluntary manure management plan—a 

Comprehensive Nutrient Management Plan (CNMP)—that includes agricultural best management practices 

to help manage manure and other animal by-products and to control soil erosion. A CNMP includes both the 

production area (where the animals are concentrated and fed and the storage areas for manure and feed) and the 

land where manure is applied.

A nutrient management plan can be seen as a “nutrient budget” for a given field or farm. Even though not 

required by law, nutrient management plans are beneficial to producers and the environment to properly manage 

nutrients on the farm, reduce nutrient loss to the environment, reduce the nutrient inputs required for a cropped 

field, and improve long-term soil quality.

Nutrient Management Plans
146 Agriculture—A River Runs Through It—The Connections Between Agriculture and Water Quality

Changes in Conventional Pesticide Use Due to Development of 

Genetically Modified Crops

Genetically modified (GM) varieties of corn, soybean, and cotton have largely replaced conventional varieties 

of these crops (see “Genetically Modified Crops” in Chapter 3). By far the most widely used application of GM 

technology has been the development of crops that are tolerant to a specific herbicide, or that produce insecticidal 

properties within the plant itself, or both (stacked GM crops). GM crops have the potential to save the grower time 

and money by reducing the amount and variety of conventional pesticides used. Over the past two decades, the 

use of conventional pesticides in the United States has been strongly affected by the use of GM crops (Coupe and 

Capel, 2015). This change in pesticide use for soybean and corn is illustrated in figure 8.2

Herbicide-tolerant GM crops are made to tolerate a single, broad-spectrum herbicide, primarily glyphosate, 

but more recent technologies include resistance to glufosinate and imazethapyr. For insect-resistant GM crops, the 

whole plant has been made to produce a toxin to certain classes of insects.

For soybean, there has been a substantial increase (about 9-fold) in the use of glyphosate since the 

introduction of the herbicide-tolerant GM crop in 1996. Over the same period, the mass of all conventional 

herbicides used on soybean has decreased (about 6-fold). Soybean has minimal problems with insecticides, 

however, so no insect-resistant GM soybean have been developed. The use of conventional insecticides on soybean 

is small, but may be on the rise in recent years (Coupe and Capel, 2015).

Since glyphosate-tolerant GM corn was introduced in 1996, herbicide use has only decreased about by 

one‐half. The amount of glyphosate used has increased substantially and continually, but is still only one-half of the 

total amount of conventional herbicides—even though about 90 percent of the corn planted is of the glyphosatetolerant GM varieties. The use of conventional herbicides on GM corn is still substantial and has not been reduced 

to nearly the same extent as for soybean. The use of conventional insecticides on corn has substantially decreased 

(about 8-fold) since the introduction of the Bt corn. 

Changes in the use of insecticides and herbicides on cotton have been similar to that for corn since the 

introduction of the GM cotton crop in 1995. The use of total conventional insecticides used has decreased about 

5-fold and the use of total conventional herbicides has decreased by about one-half, but the combined herbicide 

mass is still greater than the mass of glyphosate use, even though about 90 percent of the cotton crop has GM 

herbicide tolerance (Coupe and Capel, 2015).

0

10

20

30

40

50

1990 1995 2000 2005 2010

Year

Mass applied, in millions of kilograms

0

20

40

60

80

120

100

1990 1995 2000 2005 2010

Year

Herbicide mass applied, 

in millions of kilograms

A. Soybean B. Corn

0

2

4

6

8

12

10

Insecticide mass applied, 

in millions of kilograms

Total herbicides (except glyphosate) Glyphosate Total insecticides

EXPLANATION

Figure 8.2. Application of total conventional insecticides, total conventional herbicides (except 

glyphosate), and glyphosate on (A) soybean and (B) corn in the United States, 1992–2009 (Coupe and 

Capel, 2015).
Chemicals in Crop and Animal Agriculture 147

Most pesticides are not applied in their pure form, but 

rather are formulated with carriers (inert ingredients) that 

allow for a uniform application at a specific dose. Water is the 

most common diluent, but various solvents, mineral oils, and 

vegetable oils also are used. Pesticides also can be adsorbed to 

solid materials, such as chalk, clays, rice hulls, and nut shells. 

These liquid formulations and solid carriers are all designed 

to add stability to the pesticide mixture, make them easier to 

handle, easier to apply, and maximize their efficacy. A variety 

of inert chemicals (about 1,500 different chemicals; Capel and 

others, 2017) are used in the formulations of pesticides for 

specific purposes. Included among these inert chemicals are 

wetting and sticking agents (surfactants and gums) added to 

help the pesticide formulation produce the optimum droplet 

size and to help make contact with the plant surface. Other 

inert ingredients are attractants (pheromones and syrups) added 

to attract insects to the pesticides, thickeners (polymers and 

clays), and defoaming agents (silicones) added to create the 

correct consistency for application. For some insecticides, 

a synergist (piperonyl butoxide) is added to increase 

effectiveness of the active ingredient.

Chemicals in animal agriculture are both consumed 

to simulate growth and protect against pests and disease, 

and produced as waste by-products. The essential nutrients 

for growth (carbon, nitrogen, phosphorus, potassium, and 

micronutrients) come from feed and other supplements, all of 

which are largely derived from plant materials. Animal feed 

largely comes from natural grasslands or agricultural crops. 

Grazing animals (largely cattle) obtain most of their food 

from grasslands. Other animals, which are concentrated in 

feeding operations (fig. 8.3), obtain all or part of their nutrition 

from feed that is derived from crops, such as corn, soybean, 

sorghum, and hay. The animal feed is sometimes supplemented 

with additional nutrients (particularly phosphorus), 

micronutrients, and vitamins. Agricultural animals, particularly 

those in concentrated feeding operations, also receive 

chemicals to protect them from pests (insecticides) and 

disease (antimicrobials; table 8.2). Some animals also receive 

hormones and low-level antimicrobials to promote faster 

growth (U.S. Food and Drug Administration, 2002, 2010). 

Animals produce waste. A number of environmental 

concerns arise from the handling, storage, utilization, 

and disposal of animal waste (table 2.2; see “Manure” in 

Chapter 6). When applied to agricultural land appropriately, 

manure adds organic matter and nutrients to the soil. However, 

tac11-0566_fig08-03

Figure 8.3. Antimicrobials are widely used in concentrated 

animal feeding operations to improve health and growth rates 

(Wisconsin). Photograph by Bob Nichols, U.S. Department of 

Agriculture, Natural Resources Conservation Service photo 

gallery, NRCSWI00015, 2000. 

manure also can be over applied and supply excess nutrients 

that can affect water quality. Manure also can contain trace 

elements, antimicrobials, hormones, bacteria, and pathogens 

(Boxall and others, 2003; Hanselman and others, 2003; 

Martínez-Carballo and others, 2007; Andaluri and others, 

2012). These substances can be distributed throughout the 

fields on which the manure is applied. For the most part, the 

pathogens die in the soil and the chemicals are transformed, 

but some small fraction persists and can move into the 

environment. Improper manure handling and application have 

resulted in contamination of streams and human food supplies. 

The contamination of streams by manure can lead to low 

dissolved oxygen concentrations that can result in fish kills. 

Animal agriculture also generates waste gases directly 

from the animals or from off-gassing of the manure. These 

waste gases cause two distinct types of environmental 

concerns. Methane (directly from cattle) and nitrous oxide 

(from the decomposition of manure) are greenhouse gases 

that can contribute to global climate change. Hundreds of 

other gaseous chemicals from animal agriculture (including 

hydrogen sulfide, ammonia, and volatile fatty acids) have 

a strong odor and some are irritants when breathed. These 

chemicals can cause substantial local concerns for human 

health and aesthetics. 
148 Agriculture—A River Runs Through It—The Connections Between Agriculture and Water Quality

Environmental Behavior of Agricultural Chemicals

Each agricultural chemical behaves uniquely in the 

environment, but all are subject to common chemical, 

biological, and hydrological processes. Upon entering the 

environment, a chemical can undergo three different types of 

change processes. A chemical can undergo a change as to its 

local, molecular-scale environment (distribution processes), 

its location (transport processes), or its chemical form 

(transformation processes). These processes, taken together, 

can be used to help interpret and predict the environmental 

behavior and fate of a chemical.

Distribution processes determine the extent to which 

an agricultural chemical is distributed among various 

environmental phases—water, air, sediment, and (or) biota. 

The distribution of an individual chemical is based on the 

characteristics of the chemical, the characteristics of the 

environmental phases, such as water and soil, and the relative 

volumes of the different environmental phases. The water 

solubility of a chemical largely controls the distribution 

between water and biota and between water and sediment. 

(Sediment is used herein as a general term to describe 

all environmental particles such as suspended sediment, 

stream-bottom sediment, soil, and aquifer solids). The vapor 

pressure of a chemical largely controls the distribution 

between air and sediment, whereas Henry’s Law constant 

(vapor pressure divided by water solubility) largely controls 

the distribution between air and water. The equilibrium 

distribution of a variety of agricultural chemicals among air, 

water, and sediment is shown in figure 8.4. These results are 

simple generalizations, but are useful in helping to make 

the connections between a chemical’s properties, phase 

distribution, and environmental transport. Environmental 

characteristics, such as pH, redox, temperature, salinity, 

and type of soil surface, also affect the phase distribution of 

chemicals. As an example, the solution pH has an important 

effect on the distribution of chemicals and chemical compounds 

with acid/base properties, such as ammonia and glyphosate.

Transport processes move chemicals from one location 

to another—from the soil surface, through the layers of soil 

to a subsurface drain, to an agricultural ditch, to a major river, 

and ultimately to the ocean. Chemical (and particle) transport 

is accomplished through the energy provided by the movement 

of water or wind. Most of the material presented in Chapters 4

through 7 has focused on the movement of water through 

and out of agricultural areas and serves as the foundation 

for understanding chemical transport into and through 

the environment.

The environmental phase in which a specific chemical 

accumulates is important to its environmental behavior and 

transport. Chemicals that are strongly distributed toward 

sediment (upper left part of fig. 8.4) are largely transported in 

the environment with the soils and sediment, and accumulate 

tac12-0566_fig8-04

Percent of chemical in water

0 20 40 60 80 100

Percent of chemical associated with sediment

0

20

40

60

80

100

DDT

Phosphate

Chlorpyrifos Glyphosate

Trifluralin Testosterone

Malathion Triallate

Potassium

Linuron

Sulfate

S-Cl-pyridizine

Heptachlor Metolachlor

Arsenic III

Chloride

Carbofuran Atrazine

Nitrate

Carbon

tetrachloride

in soils, streambed sediments, and suspended sediments. This 

group of chemicals includes phosphorus, some pesticides 

(DDT, chlordane, permethrin, and chlorpyrifos), and some 

trace elements (lead and copper). A subset of these chemicals 

also accumulate in biotic tissues (DDT and chlordane). Some 

chemicals are present predominantly in water (lower right part 

of fig. 8.4) and, generally, are moved by the flow or movement 

of water. This group of chemicals includes chloride, nitrate, 

atrazine, and metolachlor. The environmental movement of the 

permanent gases and other chemicals with high vapor pressures 

is generally controlled by the movement of the air.

Transformation processes change the structural form of 

a chemical to produce a new chemical(s). Every chemical has 

a unique three-dimensional arrangement of atoms. Chemical 

structure can range from simple, such as that of molecular 

nitrogen (N≡N), to complex structures like proteins. A change 

in this three-dimensional arrangement of elements creates a 

different chemical(s) with different environmental behavior, 

and oftentimes different environmental concerns. Chemical 

transformations are induced by external forces (energy from 

the environment) acting on the chemical, including biological 

(plants, animals, or microorganisms), chemical (reactions with 

other chemicals), and physical (sunlight and heat) forces. 

Figure 8.4. Estimated equilibrium distribution of various 

agricultural chemicals among air, water, and sediment in 

and above a stream. The chemicals at the origin of the graph 

(0 percent in the water, 0 percent associated with sediment) are 

gases with high vapor pressures and predominantly are present 

in the environment in the gas phase. Most chemicals settle along 

the line that connects 100 percent in the water with 100 percent 

associated with sediment. These chemicals are distributed 

between water and sediment, and do not readily move into the air. 

A few chemicals, such as the herbicide trifluralin, settle below this 

line, indicating that some of the chemical in the air, so all three 

phases are important in their environmental distribution (Capel 

and others, 2018).
Chemicals in Crop and Animal Agriculture 149

Many elements combine with other elements to form a 

large number of different chemicals. As an example, some 

of the various forms of nitrogen are shown in figure 2.4 (see 

“Nitrogen” in Chapter 2). Each of these forms of nitrogen is 

a different chemical, but the element nitrogen is always the 

same, regardless of whether it is in the form of ammonia, 

nitrate, nitrous oxide, or urea. Natural processes can transform 

one form of nitrogen into all its other forms. Together, these 

processes allow nitrogen to cycle among its various forms 

and guarantee that no single form becomes a “dead end” 

that accumulates in the environment. The important nitrogen 

forms (boxes) in the agricultural environment and the 

transformations that connect them (arrows) are illustrated in 

figure 8.5. 

110566fi0805

Transformation reactions

Chemical symbols

 Name Symbol

EXPLANATION

B Bacteria, fungi, and yeasts

P Plants

C Chemical reactions

Combustion of biomass

Water

Air

Biomass

Component(s) of the hydrologic system where 

nitrogen species accumulates—color of box 

denotes dominant component, color of box 

outline denotes other important component

Industrial processes that produce 

nitrogen species for fertilizer 

Transient species, does not tend to 

accumulate in soil

Applied to soil as plant nitrogen source 

(fertilizer)

NO2

NO3

-

NO2

-

Norg

NO

N2O

N2

N2H4

(NH2)

2CO

NH2OH

NH3

+5

+4

+3

+2

+1

0

-1

-2

-3

Oxidation state of nitrogen species

B B

C

C C

B

B

B

B

B

B

B

B

B

B

B

B,P

Nitrate NO3-

Nitrogen dioxide NO2

Nitrite NO2-

Nitric oxide NO

Nitrous oxide N2

O

Molecular nitrogen N2

Hydroxylamine NH2OH

Hydrazine N2

H4

Organic nitrogen Norg

Ammonia NH3

Urea (NH2)

2

CO

Figure 8.5. Cycle of nitrogen in the agricultural environment showing the transformations between the various 

chemical forms of nitrogen, arranged in rows by their oxidation state. (Capel and Hopple, 2018.)
150 Agriculture—A River Runs Through It—The Connections Between Agriculture and Water Quality

Carbon forms strong bonds with itself and many 

other elements. This bonding provides the possibility of 

creating new synthetic carbon-containing chemicals. When 

one of these carbon-containing chemicals is transformed, 

the carbon remains as carbon, but the complex, threedimensional structure generally becomes simpler until it is 

eventually transformed to carbon dioxide or methane. The 

transformation of the herbicide glyphosate is illustrated in 

figure 8.6. The complex chemical structure of glyphosate 

is produced industrially. After glyphosate is released 

in the environment, it is transformed through a series 

of intermediate chemicals (transformation products or 

degradates) until the carbon dioxide endpoint is reached. 

Some of the transformation products have short (hours and 

weeks) environmental lifetimes, whereas others persist much 

longer (months and decades). For example, glyphosate has 

an environment lifetime of about 7 months in water, but 

aminomethylphosphonic acid (AMPA), its first transformation 

product, has a longer lifetime and has been observed to 

accumulate in some of the hydrologic components (Coupe 

and others, 2011). Because there are no natural processes 

that produce synthetic chemicals, such as glyphosate, 

transformations are always in the direction from complex 

synthetic chemicals to carbon dioxide. Many intermediate 

chemicals can be formed and transformed in the process. 

The range of environmental lifetimes for some common 

agricultural chemicals is illustrated in figure 8.7. For chemical 

elements, this range is the lifetime of a certain specific 

chemical form (for example, nitrate being transformed 

into other nitrogen forms, fig. 8.5). For carbon-based 

molecules, this is the lifetime of the specific chemical being 

transformed into another chemical (for example, glyphosate 

transformed to AMPA, fig. 8.6). The extent and rate of the 

transformation is oftentimes dependent on the chemical 

and biological characteristics of the environment. As an 

example, nitrate is shown twice in figure 8.7. Nitrate has a 

relatively short lifetime in environmental settings with little 

or no oxygen (such as many groundwater environments), 

but a comparatively long lifetime in environments that are 

rich in dissolved oxygen (such as groundwater containing 

dissolved oxygen).

tac11-0566_fig08-06

O OC

H H

O

C

O

H2

N P OH

OH

H

N

O O

P OH

OH

HO

H

H

H H

H NC

Glyphosate

Aminomethylphosphonic

acid (AMPA)

Methylamine

Formaldehyde

Carbon dioxide

Figure 8.6. Transformations of glyphosate in the environment 

(Wiersema and others, 2012). 

Legacy chemicals have long environmental lifetimes 

(years and decades) and are observed long after their sources 

have decreased in concentration or been eliminated. The 

legacy chemicals detected in groundwater generally are 

long-lived compounds that are largely dissolved in water and 

move with water through the groundwater system. The legacy 

chemicals detected in surface water generally are long-lived 

compounds that strongly bind to sediment and accumulate 

in bottom sediments or are taken up by living organisms and 

accumulate in their tissues. Sediments, from eroded soil and 

streambanks, that have accumulated in stream channels and 

streambeds are also a legacy concern.
Chemicals in Crop and Animal Agriculture 151

tac11-0566_fig08-07

Percent of chemical associated with water

0 20 40 60 80 100

Environmental lifetime, in days

0

200

400

600

7,000

Tylosin Malathion Nitrate

Chlorpyrifos Carbofuran

Metolachlor

Heptachlor Methyl Para Linuron

Glyphosate Metolachlor ESA

Oxytetracycline Atrazine

Sulfate

Nitrate

Chloride

Figure 8.7. Environmental lifetime and the percentage of 

the chemical in the water for various agricultural chemicals. 

Legacy chemicals (those with long environmental lifetimes) 

typically found in surface water are grouped on the upper 

left side of the graph (minimally in the dissolved phase) and 

legacy chemicals typically detected in groundwater are 

grouped on the upper right side of the graph (largely in the 

dissolved phase). Nitrate is readily removed in subsurface 

environments with no or low dissolved oxygen (reduced 

conditions, blue arrow), but has a long lifetime in subsurface 

environments with higher dissolved oxygen concentrations 

(oxic conditions, red arrow) (Capel and others, 2018).

DDT, chlordane, and other organochlorine insecticides 

were widely used in agriculture from the 1940s through the 

1970s. Many of these chemicals have long environmental 

lifetimes (years to decades) and are strongly associated with 

sediment in water. None of the organochlorine insecticides 

have been used in this country for decades (except for 

lindane), but they are still being detected in bed sediment and 

fish tissue samples from agricultural streams across the Nation 

(Gilliom and others, 2006). DDT (and its transformation 

products DDE and DDD) and (or) other organochlorine 

insecticides are still present in fish tissues and bed sediments 

(detected in up to 90 and 50 percent of the samples analyzed, 

respectively). These chemicals have been replaced in recent 

decades by other insecticides that are less persistent and 

less accumulative, but the organochlorine insecticides will 

continue to be present in fish tissues and sediments, as well as 

air, water, and human tissues, for decades to come. 

Nitrate and some pesticides and their transformation 

products are common legacy chemicals in groundwater 

(Gilliom and others, 2006; Dubrovsky and others, 2010; 

Puckett and others, 2011). Eighty-three percent of the studies 

of shallow groundwater in agricultural areas collected one 

or more samples (of 20–30 wells sampled) with a nitrate 

concentration greater than 10 mg/L as nitrogen (the Federal 

drinking water Maximum Contaminant Level [MCL] value). 

In subsurface environments with no or low dissolved oxygen 

(reduced conditions), nitrate is transformed relatively quickly 

(hours to weeks) and is seldom observed. In subsurface 

environments containing dissolved oxygen (oxic conditions), 

however, nitrate has a slow rate of transformation and, 

therefore, persists as a legacy chemical. Groundwater 

contributions of nitrate to streams are substantial. At least 

one-third of the total annual load of nitrate in two-thirds of 

148 small streams studied across the Nation was derived from 

baseflow (Dubrovsky and others, 2010).

The potential for chemicals and their transformation 

products to remain in the environment and become legacy 

chemicals can be predicted based on chemical properties and 

an understanding of the hydrologic system. The environmental 

lifetime of the chemical and the percent of the chemical in 

the water for a number of agricultural chemicals are shown in 

figure 8.7.
152 Agriculture—A River Runs Through It—The Connections Between Agriculture and Water Quality

Agricultural Chemicals in Shallow Groundwater

Throughout this report, streams are treated as the primary endpoint of water-quality concerns. Groundwater is 

only considered as a flowpath for water and chemicals as they travel from the land surface to the stream. In some 

areas, however, groundwater is intercepted by wells and pumped to the surface for human consumption, irrigating 

crops, or watering livestock. The drinking water for about 44 million people in the United States is from private 

wells (Belitz and others, 2016). It is, therefore, important to consider groundwater as more than just a pathway for 

water to travel from land surface to streams.

In agricultural areas, shallow groundwater is susceptible to contamination from agricultural chemicals 

and animal manure. Shallow wells in irrigated areas with well-drained soils are particularly vulnerable to 

contamination from the land surface (Nolan and Hitt, 2006). Wells with inadequate, leaking, or otherwise damaged 

casings are especially at risk (Eberts and others, 2013).

Nitrate is commonly detected in groundwater that underlies agricultural areas, frequently at concentrations 

exceeding the Federal drinking water Maximum Contaminant Levels (MCL) of 10 mg/L as nitrogen (U.S. 

Environmental Protection Agency, 2009) (fig. 8.8). Commercial fertilizer and animal manure are agricultural 

sources of nitrate. Water-soluble pesticides, mostly herbicides, and the compounds into which they degrade also 

are found in shallow groundwater underlying agricultural areas. In addition to the currently used pesticides and 

pesticide transformation products (fig. 8.8B), groundwater also may be contaminated by chemicals that are no 

longer in use (Steele and others, 2008). Animal waste also may be a source of bacteria, salts, and pharmaceuticals 

such as antimicrobials and hormones in shallow groundwater. Concentrated animal feeding operations (CAFOs), in 

which large numbers of animals live in restricted spaces, can be substantial sources of these chemicals (Burkholder 

and others, 2007).

0

5

10

15

20

Concentration, in milligrams per liter as N

A B

EXPLANATION 

90th percentile

10th percentile

50th percentile (median)

25th percentile

75th percentile

Maximum contaminant

 level

10 

0 

20 

30 

Atrazine 

Deethylatrazine 

Simazine 

Metolachlor 

Prometon 

Metribuzin 

Alachlor 

Tebuthiuron 

Carbofuran 

Cyanazine 

Frequency of detection in shallow wells in 

agriculutral areas at concentrations of 

greater than or equal to 

0.1 microgram per liter, in percent 

Figure 8.8. (A) Range of nitrate concentrations measured in shallow groundwater in agricultural areas throughout 

the United States (Dubrovsky and others, 2010). The red line indicates the Maximum Contaminant Level (MCL) of 

nitrate allowed by the U.S. Environmental Protection Agency for public drinking water. (B) Pesticides and pesticide 

transformation products detected in shallow groundwater in agricultural areas throughout the United States (Gilliom 

and others, 2006). 
Chemicals in Crop and Animal Agriculture 153

Summary

Thousands of chemicals are connected with crop and 

animal agriculture. Many chemicals are purposely used 

because they directly benefit agriculture—increase yield, 

protect crops or animals, increase soil quality, dispose of 

waste, facilitate the effectiveness of another chemical that 

has a purposeful use, or protect the environment—whereas 

others are produced by the crops and animals or by chemical 

or microbiological reactions as waste by-products. Some 

chemicals that are naturally stored in the soil and water are 

mobilized by agricultural activities. Finally, some chemicals 

are introduced into the agricultural landscape through 

“piggy-backing” on purposely applied chemicals. Nutrients 

(fertilizers and manure) and chemicals that protect the crops 

and (or) animals from pests (pesticides and antimicrobials) are 

purposely applied, but can cause water-quality impacts. The 

water-quality concerns from these chemicals result, in part, to 

their effects on the environment, and, in part, to their quantity 

of use and (or) release in agricultural activities. 

Every chemical associated with agriculture has its own 

unique behavior in the environment; each agricultural chemical 

is subject to common chemical, biological, and hydrologic 

processes. These are processes that can be generalized to help 

interpret and predict the environmental behavior and fate of a 

specific chemical. Upon entering the environment, a chemical 

can undergo three different types of processes that affect 

and change its behavior and fate. The chemical can undergo 

a change as to its location (transport processes), its local 

(molecular-scale) environment (distribution processes), or its 

chemical form (transformation processes). 

Many environmental concerns are the result of an excess 

of a chemical (or sediment or pathogens) compared to its 

natural background concentration. Agricultural activities 

can introduce an excess concentration of naturally occurring 

chemicals to the environment through tillage, manure, and 

fertilizer applications, or changes in the pre-agricultural 

flowpaths of water. Any presence of man-made chemicals in 

the environment, such as synthetic pesticides, antimicrobials, 

and surfactants, is in excess of the natural concentration 

(zero). The excess concentration of a chemical does not 

always affect the environment, but it is an indication that 

agricultural activities are affecting the environment.
154 Agriculture—A River Runs Through It—The Connections Between Agriculture and Water Quality

Agriculture in Central Indiana

Central Indiana’s Till Plain was formed 17,000 years ago when the Laurentide ice sheet that covered parts 

of the Midwest retreated, leaving behind thick rich soils and productive aquifers. Forests, prairies, and wetlands 

were all part of the central Indiana landscape (fig. 1.3). Swamps formed in areas of seasonally standing water. 

Beech-maple forests were common. Prairies dotted the northern and western fringes of the area, where bison 

herds roamed. Central Indiana forests also were inhabited with mountain lions, wolves, black bear, and elk. By 

the early 1800s appreciable populations of these species were gone from the area. The deep fertile soils that 

make up this region generally are poorly drained, nearly level, and loamy. The climate is characterized as humid, 

continental with hot summers and cold winters. The annual precipitation is about 90 cm and is fairly evenly 

distributed throughout the year. 

Some of the earliest farmers in Indiana were Native Americans from the Mississippian culture who built 

large permanent communities supported by agriculture. The first Europeans in Indiana were French fur traders, 

who arrived in the late 1600s. The early pioneers were subsistence farmers who lived off their land and provided 

for themselves. With the construction of the National Road in the 1820s (U.S. Highway 40), more settlers arrived. 

Pressure for arable land increased; more forest was cleared, and lowland areas were drained either by ditches 

or subsurface drains to make land more suitable for crops. Within a generation the population soared from 

5,600 people in 1800 to nearly 1 million by 1850. Without improved drainage, farming would not be possible in 

many areas. By 1910, more than 80 percent of the land had been cleared for agriculture. Woodlands remained 

mostly along the riparian corridors. Farmers planted the land in corn, wheat, oats, potatoes, flax, and apples and 

raised cattle, sheep, and hogs. Corn production steadily increased, and in the 1940s soybean was introduced as a 

rotation crop with corn. After World War II, a trend toward high yield, intensive agriculture increased the use of 

commercial fertilizers and pesticides. The average corn yield in the 1940s was about 2,500 kilograms per hectare 

(kg/ha); in 2010, the average hybrid corn yield was about 9,000 kg/ha. In 2010, about 65 percent of the land in 

Indiana was in farmland with the average size farm about 100 ha. Currently, corn and soybean are principal crops, 

and hogs, dairy, and chickens are the primary livestock raised in central Indiana. Recent trends show a small but 

increasing area used for specialty agriculture including tomatoes, apples, floriculture, and organic agriculture.

Recent studies by the USGS on the effects of agriculture on water quality in central Indiana have focused 

on the transport of nutrients and pesticides in watersheds where subsurface drains are present (Baker, Stone, and 

others, 2006; Baker and others, 2007; Stone and Wilson, 2006). There have been numerous USGS studies on the 

biological response to nutrients in Indiana streams (Caskey and others, 2007, 2010; Caskey and Frey, 2009; Frey 

and others, 2011). Other studies have focused on the effects of agricultural management practices on water quality 

in Indiana watersheds (Bracmort and others, 2004, 2006; Kladivko and others, 2004; Harper and Hartke, 2009), 

and the link between agricultural chemicals and birth defects (Winchester and others, 2009).

A B C

The landscape of central Indiana. (A) The wetlands and beech forests typical of the time before agriculture, 

(B) typical cropped fields, and (C) the view from the air (23 square kilometers). Photograph A by Marion 

Jackson, Natural Heritage of Indiana (Hoot Woods, Owen County, Indiana), 1997; photograph B by Jeffrey 

Martin, U.S. Geological Survey, 2008; photograph C from U.S. Department of Agriculture, Farm Service 

Agency, National Agricultural Imagery Program, 2010.
Connections Between Agriculture and Water Quality 155

—Connections Between Agriculture and Water Quality

How can we go from understanding these connections to informing agricultural management 

decisions and setting realistic expectations?

This chapter provides real-life examples of how understanding the connections between agricultural activities and the 

movement of water, chemicals, and sediment can be used to inform agricultural management decisions to improve and protect 

the quality of our Nation’s streams and groundwater.

9 Chapter
156 Agriculture—A River Runs Through It—The Connections Between Agriculture and Water Quality

Agricultural activities impact water quality 

at many scales 

The importance of considering the response of the 

ecosystem as a whole to modifications in agricultural 

activities—even those intended to improve water quality—is 

illustrated by the recent history of the Yakima River Basin. 

In the late 1990s, changes in irrigation practices, from furrow 

to sprinkler and drip methods, and the use of a soil stabilizer, 

polyacrylamide (PAM), were widely implemented in the 

Yakima River Basin in central Washington. These changes 

resulted in a substantial reduction of suspended sediment—

loads were reduced by more than 90 percent in many areas 

such as in the Granger Drain, Washington (fig. 9.1A). This 

resulted in a dramatic improvement in stream-water clarity.

Prior to irrigation improvements, nuisance blooms of 

aquatic plants were limited by the turbidity caused by the 

sediment in agricultural runoff. Beginning in 2001, however, 

the increase in stream-water clarity enabled the growth of 

large, dense patches of aquatic plants in the nutrient-rich 

Yakima River during the spring and summer (fig. 9.1B). This 

growth is an aesthetic nuisance in the river; plant decay causes 

a decrease in dissolved-oxygen concentrations, which can 

harm fish. 

Over the past few decades, the quality of groundwater has 

decreased in many of the Nation’s agricultural areas, including 

the regional aquifer systems that underlie the Central Valley of 

California, Basin and Range, Rio Grande, Columbia Plateau, 

Snake River Plain, Florida, northern glacial areas, Mississippi 

Embayment, Northern Atlantic Coastal Plain, and the High 

Plains (DeSimone and others, 2009). Agricultural activities 

can introduce agricultural chemicals and mobilize naturally 

occurring trace elements and dissolved solids (table 8.1) that 

are transported to the groundwater. Elevated concentrations 

of nitrate, herbicides, insecticides, fungicides, and their 

transformation products; chloride and other dissolved solids; 

and trace elements, such as selenium, arsenic, and uranium, 

have been observed. In some areas of these regional aquifers, 

the water table has declined due to extensive withdrawals that 

exceed recharge. The combined effects of decreased water 

quality and declining water tables can have substantial—

perhaps irreversible—impacts. The sustainable availability 

of water and limitations on the uses of water for drinking or 

irrigation due to poor water quality are important long-term 

concerns in many of these aquifer systems. 

The High Plains aquifer is the principal aquifer that 

underlies parts of eight States on the Great Plains (fig. 9.2; 

Gurdak and others, 2009). Much of the landscape overlying 

the aquifer is used for crop and animal agriculture supported 

by extensive irrigation. The water table of the High Plains 

aquifer has declined in many areas over the past few decades 

because water withdrawal has been greater than recharge. 

Some of the excess irrigation water infiltrates into the soil 

and transports agricultural and natural chemicals to the 

groundwater. Elevated concentrations of dissolved solids, 

nitrate, and pesticides are observed at and below the water 

table in many areas. Nitrate concentrations greater than 

background levels are observed throughout the aquifer. 

Although the transit times of water from the land surface to 

the water table is generally slow (decades to centuries), there 

are local areas, such as beneath natural depressions in the land 

surface, that have transit times to the water table on the order 

of months to years. 

Pre-1998 Post-1998

Sediment load, in tons per day

0

50

100

150

200

250

300

EXPLANATION 

Outlier

Outlier

90th percentile

10th percentile

50th percentile (median)

25th percentile

75th percentile

A. Granger Drain, Washington B

Figure 9.1. Changes in irrigation practices contributed to a significant reduction in the sediment load after 1998 and an increase 

in water clarity. The increased water clarity combined with nutrient-enriched waters contributed to the growth of large patches 

of dense aquatic vegetation in the spring and summer that resulted in decreased oxygen levels in the stream (Capel and Hopple, 

2018). Photograph B by Kurt Carpenter, U.S. Geological Survey. 
Connections Between Agriculture and Water Quality 157

A

C

B

TEXAS

COLORADO

NEW

MEXICO

NEBRASKA

OKLAHOMA

WYOMING SOUTH DAKOTA

KANSAS

TEXAS

COLORADO

NEW

MEXICO

NEBRASKA

OKLAHOMA

WYOMING SOUTH DAKOTA

KANSAS

TEXAS

COLORADO

NEW

MEXICO

NEBRASKA

OKLAHOMA

WYOMING SOUTH DAKOTA

KANSAS

0 50 100 150 KILOMETERS

0 50 100 150 MILES

0 50 100 150 KILOMETERS

0 50 100 150 MILES

0 50 100 150 KILOMETERS

0 50 100 150 MILES

Base information from U.S. Geological Survey digital data, 1:100,000

Albers Equal-Area projection, Standard Parallels 29 ̊30' and 45 ̊30', central meridian –96 ̊

Base information from U.S. Geological Survey digital data, 1:100,000

Albers Equal-Area projection, Standard Parallels 29 ̊30' and 45 ̊30', central meridian –96 ̊

Base information from U.S. Geological Survey digital data, 1:100,000

Albers Equal-Area projection, Standard Parallels 29 ̊30' and 45 ̊30', central meridian –96 ̊

104 ̊ 100 ̊ 96 ̊

104 ̊ 100 ̊ 96 ̊

42 ̊

34 ̊

38 ̊

42 ̊

34 ̊

38 ̊

104 ̊ 100 ̊ 96 ̊

42 ̊

34 ̊

38 ̊

Nonirrigated agricultural land

Irrigated agricultural land

EXPLANATION

EXPLANATION

0 to 20

21 to 40

41 to 60

61 to 80

81 to 100

No data

Percent probability 

of nitrate concentration 

greater than 4 milligrams 

per liter

0 to 15

15 to 30.5

30.5 to 61

61 to 91

91 to 122

122 to 152

>152

Depth to water table, 

in meters

EXPLANATION

Figure 9.2. Areas of the High Plains aquifer that are 

vulnerable to elevated nitrate concentrations have 

been identified through the use of field observations 

and statistical models (A) (Gurdak and Qi, 2006). 

Overall, about 21 percent of the High Plains aquifer 

has a probability of greater than 60 percent of 

elevated nitrate concentrations (>4 mg/L nitrate as 

nitrogen). Areas that have more agricultural land 

(irrigated and nonirrigated) (B), shallower depths to 

water table (C), and less clay in the unsaturated zone 

are areas with the greatest likelihood of elevated 

nitrate concentrations. In the northern region of the 

High Plains aquifer, the organic matter content of 

the soil is important. Areas with less organic matter 

in the soil have a greater likelihood of elevated 

nitrate concentrations. 
158 Agriculture—A River Runs Through It—The Connections Between Agriculture and Water Quality

Every summer a large hypoxic (low or no dissolved 

oxygen) zone forms in the Gulf of Mexico that extends along 

the Texas and Louisiana coastlines (fig. 9.3), threatening an 

ecosystem that supports valuable commercial and recreational 

fisheries—almost 80 percent of the U.S. landings of shrimp 

come from these prized waters. When dissolved-oxygen levels 

are less than 2 mg/L, less mobile or immobile animals such 

as mussels are often killed. Hypoxia also results in loss of 

habitat, displacement of fish, and decrease in reproductive 

ability in some fish species (Rakocinski and others, 1997; 

Craig and Crowder, 2005; Conley and others, 2009). This 

hypoxic zone is the second largest in the world. In 2015, the 

zone covered about 16,770 km2, an area about the size of 

Connecticut and Rhode Island combined.

The annual size of the hypoxic zone is related to the 

annual amount of nitrogen and phosphorus delivered to the 

Gulf of Mexico by the Mississippi River. Each of the 31 States 

draining this vast watershed contributes different amounts 

of nitrogen and phosphorus. These nutrients stimulate algal 

growth in the Gulf of Mexico and consume the oxygen in 

the water when they die and decay (Rabalais and others, 

2002). A USGS water-quality model, calibrated using more 

than 850 monitoring sites, helps identify which areas and 

sources are contributing the largest amounts of nutrients to the 

Gulf of Mexico (fig. 9.3; Robertson and Saad, 2013) Three 

States—Iowa, Illinois, and Indiana—represent 11 percent 

of the drainage area, but contribute about 40 percent of the 

total nitrogen to the Gulf of Mexico. The major sources of 

total nitrogen delivered to the Gulf of Mexico include farm 

fertilizer (41 percent), atmospheric deposition (26 percent), 

urban areas and wastewater treatment plants (14 percent), and 

confined animal manure (10 percent; fig. 9.3). 

The Mississippi River/Gulf of Mexico Hypoxia Task Force 

has set a goal of reducing the size of the hypoxic zone to less

than 5,000 km2 by 2035 (U.S. Environmental Protection Agency, 

2015). Twelve States along the main stem and major tributaries 

of the Mississippi River have developed nutrient reduction 

strategies to support this goal. Some of these States have used 

the results from the USGS water-quality model to prioritize 

which watersheds to target for nutrient reduction actions. 
Connections Between Agriculture and Water Quality 159

26%

7%

7%

41%

9%

10%

A

B C

Inputs from 

legume crops

Inputs from 

atmospheric

 deposition 

Inputs from 

agricultural

 manure Inputs from 

agricultural 

fertilizers

Inputs from 

wastewater 

treatment plants

Inputs from 

urban areas

Figure 9.3. A large hypoxic area, also called a dead zone, develops in the Gulf of Mexico each summer. When oxygen levels are less 

than 2 milligrams per liter, less mobile or immobile animals such as mussels cannot move out of the hypoxic areas and often die during 

widespread hypoxic events. (A) The Gulf of Mexico hypoxic zone in 2015 covered about 16,770 square kilometers (6,474 square miles, an 

area about the size of Connecticut and Rhode Island combined, during the week of July 28–August 3, 2015). Nutrients from sources such 

as farm fertilizers, animal manure, and urban areas contribute nutrients that stimulate the growth of algae that die off and deplete oxygen 

levels in the Gulf of Mexico. (B) Areas contributing the largest amounts of nitrogen to the Gulf of Mexico are located in the corn and 

soybean growing region from Kansas to Ohio. (C) The relative importance of various watershed sources of nitrogen to the Gulf of Mexico. 

Agricultural activities contribute 60 percent of the total nitrogen. Source: (A) Nancy N. Rabalais, Louisiana Universities Marine Consortium 

and R. Eugene Turner, Louisiana State University, written commun., 2016; National Oceanic and Atmospheric Administration (2015); (B and 

C) Robertson and Saad (2013).
160 Agriculture—A River Runs Through It—The Connections Between Agriculture and Water Quality

Nitrogen and phosphorus, used in fertilizers, are transformed into different chemical forms as they 

move through the environment, but they do not disappear

Fertilizers containing nitrogen and phosphorus are 

vital for agriculture. Once released into the environment, 

these elements are assimilated by crops, stored in the soil, 

or transported elsewhere. The nitrogen and phosphorus are 

transformed into many different chemical forms, but they 

do not disappear. They continue to be present somewhere, 

in some form. Generally, there is excess nitrogen and (or) 

phosphorus, an amount in excess of what the crop needs and 

what is available to the crops, some of which can move into 

the hydrologic environment. Many forms of nitrogen and 

phosphorus are found in all components of the hydrologic 

systems, including surface water, bed sediment in surface 

water, groundwater, air, rain, soil, soil water, and biota. By 

improving the estimates of the amounts of nitrogen and 

phosphorus actually needed by the crops and by optimizing 

application methods, the amounts of applied fertilizers could 

be decreased, and, therefore, decrease the excesses available 

for transport beyond the field (Sharply and others, 2005). 

Accounting for the annual inputs and outputs of nitrogen 

in agricultural watersheds provides insights on the amount of 

nitrogen that moves into the groundwater and streams (fig. 9.4; 

Essaid and others, 2016). Nitrogen is applied to most cropped 

fields as either chemical fertilizer or manure. Some crops, 

such as soybean and alfalfa, also can transform atmospheric 

nitrogen gas to other forms of nitrogen that are usable by 

the plants. A small amount of nitrogen (nitrate) is deposited 

from the atmosphere through rain and snow. A large fraction 

of nitrogen is assimilated by the plants and removed from 

the field by way of harvest or transferred to the soil through 

the crop residue. A fraction is returned to the atmosphere, 

largely as nitrogen gas and nitrous oxide (fig. 8.5). Typically, 

a sizeable excess of nitrogen—ranging from about 18 to 

40 percent of the output—remains, some of which is stored 

in the soil, transported with recharge to groundwater, or 

transported with runoff to streams. The fraction of the excess 

nitrogen that moves into groundwater and streams is the 

source of most of the water-quality concerns from nitrogen. 

0 

20 

40 

60 

80 

100 

120 

140 

160 

180 

200 

Nitrogen, in kilograms per hectare

Maryland Indiana Iowa Mississippi Nebraska Washington 

EXPLANATION

Input

Atmospheric deposition

Fixation

Fertilizer

Manure 

Excess

Volatilization

Crops

Pasture/hay

Output

Figure 9.4. Accounting for the annual inputs and outputs of nitrogen in agricultural watersheds 

provides insights on the “excess” component, which is the amount of nitrogen that moves into the 

groundwater and streams and contributes to water-quality concerns. (Essaid and others, 2016.) 
Connections Between Agriculture and Water Quality 161

Nitrogen, as a fertilizer, is most often applied in the forms 

of urea, ammonia, and (or) organic nitrogen (manure), but 

it is readily transformed into nitrate—a highly mobile form. 

Nitrate was observed in every component of the hydrologic 

system in the Leary-Weber Ditch watershed, in central 

Indiana (fig. 9.5; Baker, Stone, and others, 2006). Nitrate was 

detected in relatively high concentrations in the subsurface 

drain water and the stream water. Nitrate also was present, 

although at much lower concentrations, in rain, groundwater, 

and surface runoff. These findings show that the movement of 

nitrate to this central Indiana stream is more likely through the 

subsurface drains than through groundwater or surface runoff. 

The nitrate observed in the various hydrological components 

in the watershed represents a fraction of the total excess 

nitrogen calculated in figure 9.5 for this same watershed.

0.1

1

10

Nitrate concentration, in milligrams per liter

15

0.05

Surface

runoff

water

GroundwaterStream

water

Subsurface

drain water

Rain

Nitrate EXPLANATION 

50th percentile (median)

25th percentile

75th percentile

Water transit time

Shorter Longer

Integrator

of water

flowpaths

Figure 9.5. The “excess” nitrogen in this watershed is infiltrating through the 

soil, into the subsurface drains, and into the stream. Understanding how and when 

nitrate is moving into the groundwater and stream is critical to developing costeffective nutrient-reduction actions. (From Baker, Stone, and others, 2006.) 
162 Agriculture—A River Runs Through It—The Connections Between Agriculture and Water Quality

applied metolachlor because the two transformation products 

have greater mobility and longer persistence compared to the 

applied metolachlor. 

Transit time is a key factor controlling which forms 

of metolachlor are detected in the various hydrologic 

components. The median values for metolachlor as a 

percentage of total metolachlor (sum of parent metolachlor, 

metolachlor sulfonic acid, and metolachlor oxanilic acid) 

tend to become smaller as the transit time of the water 

in the hydrologic component increases (Rose and others, 

2018). The water transit times of each of the components 

are arranged in figure 9.6 from shortest to longest—surface 

runoff, rain, subsurface drain water, and groundwater. The 

stream integrates the water from all other components. 

Over the course of a year, surface runoff is the primary 

flowpath delivering applied metolachlor to the stream, 

whereas groundwater is the primary flowpath delivering the 

transformation products to the stream.

Pesticides and their transformation products are detected throughout the environment

Many pesticides and (or) their transformation products 

can be found in all components of the hydrologic system, 

including surface water, bed sediment, suspended sediment, 

groundwater, air, rain, soil, and biota. The transformation 

products have different behaviors and health effects compared 

to the parent compound. 

Metolachlor has been used as a corn and soybean 

herbicide for more than three decades. It is applied to the soil 

surface before the plants emerge from the soil. Metolachlor 

has two transformation products—metolachlor sulfonic acid 

and metolachlor oxanilic acid—that are widely detected 

throughout the environment. One or more of these chemicals 

were detected in all six hydrologic components in the 

Leary-Weber Ditch watershed, in central Indiana, where 

metolachlor is commonly applied to corn (fig. 9.6). The 

highest concentrations of metolachlor were in the stream and 

the surface runoff water just after application. In deep soils 

and groundwater, however, the concentrations of the two 

transformation products were higher than the concentrations of 

1.0

0.06

0.04

0.02

0.0

0.08

Surface

runoff

water

GroundwaterStream

water

Subsurface

drain water

Rain

EXPLANATION 

50th percentile (median)

25th percentile

75th percentile

Total metolachlor concentration, 

in micrograms per liter

Water transit time

Shorter Longer

Integrator

of water

flowpaths

Figure 9.6. Metolachlor, an herbicide commonly applied to corn and 

soybean fields, and its transformation products were detected in all 

hydrologic components of the Leary Weber Ditch system, Indiana. The highest 

concentrations were detected in the stream and the surface runoff water. In 

deep soils and groundwater, however, the long-lived metolachlor transformation 

products were detected at higher concentrations than the more quickly 

transformed metolachlor. Over the course of a year, surface runoff is the primary 

flowpath delivering applied metolachlor to the stream, whereas groundwater is 

the primary flowpath delivering the transformation products to the stream. The 

measurement of metolachlor and the two long-lived transformation products 

provides a good understanding on the levels of this herbicide in this hydrologic 

system. (From Rose and others, 2018).
Connections Between Agriculture and Water Quality 163

Metolachlor and its two transformation products are 

some of the most frequently detected pesticides in surface and 

groundwater (fig. 8.8; Gilliom and others, 2006), although 

metolachlor transformation products typically are detected more 

frequently in groundwater than the parent metolachlor. The 

generalized percentage of metolachlor, relative to its annual 

application present in each hydrologic component, is shown in 

figure 9.7. One hundred percent of the input of metolachlor is 

from application; the chemical does not occur naturally. About 

10 percent of the metolachlor volatilizes into the atmosphere. 

Most of this amount is transformed by sunlight, but a small 

amount—about 0.3 percent of the total—returns to the land 

surface in precipitation. The largest fraction of the metolachlor 

(about 90 percent) moves into the shallow soil where it is either 

taken up by plants or transformed by microorganisms. About 

0.4 percent is transported to streams during storm or irrigation 

events and another 0.4 percent is stored in the shallow soil. A 

small amount—typically less than about 0.02 percent—moves 

into groundwater and eventually may be discharged to streams. 

Figure 9.7. Although less than 1 percent of the pesticide 

applied moves off the field into streams and groundwater, 

metolachlor is found in all hydrologic components in many 

agricultural areas where it is applied. The detailed studies 

on the behavior of metolachlor provide insights into the 

environmental mobility of many other pesticides that have 

not been studied as extensively, some of which are far more 

toxic. The values are the typical percentage of the annual 

application. After years of study, a generalized mass budget 

of a yearly application of metolachlor can be calculated. 

However, not enough is known about the transformation 

products to do similar mass budget calculations. 

Understanding the behavior and transport of pesticide 

transformation products continues to be an important topic of 

investigation. (From Rose and others, 2018.)
164 Agriculture—A River Runs Through It—The Connections Between Agriculture and Water Quality

Agricultural chemicals can be transported in the atmosphere

Natural processes and agricultural activities continually 

introduce water, dust, chemicals, and microorganisms into the 

atmosphere. Interactions between the agricultural activities 

on the landscape and the atmosphere can cause various 

environmental impacts. These impacts vary by chemical, 

by scale—from local (odors from concentrated animal 

feeding operations) to global (climate change and global 

dispersion of pesticides), and by environmental impacts (such 

as human health effects from inhalation, accumulation of 

greenhouse gases, loss of soil, and disruption of ecosystems 

and of neighboring agricultural fields). Animal agriculture, 

particularly concentrated feeding operations, release many 

odor- and irritation-causing chemicals (for example, hydrogen 

sulfide, ammonia), methane (a greenhouse gas), and dust into 

the air. Activities associated with crop agriculture introduce 

dust, nitrogen, pesticides, and volatile-organic compounds 

(from pesticide formulations) into the atmosphere through 

spray drift, wind erosion, and volatilization. Once in the 

atmosphere, these compounds can be transported locally to 

globally with the winds, deposited back onto the landscape, 

and (or) undergo transformation reactions induced by 

sunlight. Pesticides and dust particles have been observed to 

move globally through the atmosphere from their points of 

origin to their points of deposition (Welch and others, 1991). 

Organochlorine insecticides (for example, DDT, lindane, and 

toxaphene) have been transported to the Arctic Ocean where 

they have accumulated in the ocean biota (Bidleman and 

others, 1989). Pesticides transported through the atmosphere 

sometimes have had a negative effect on neighboring crops 

and vegetation (Seiber and others, 1993). 

A mixture of herbicides, insecticides, and fungicides 

are commonly present in the air during the growing season. 

During the 2007 growing season in an agricultural area in 

Mississippi, from 3 to 11 of 16 monitored herbicides (median 

7 herbicides) were detected in the air (fig. 9.8; Majewski and 

others, 2014). Glyphosate was detected in every weekly air 

sample, although at low concentrations (Chang and others, 

2011). Widely used pre- and post-emergent corn, cotton, and 

soybean herbicides (pendimethalin, metolachlor, and atrazine) 

were detected early in the growing season. Propanil, used 

only on rice, appeared in the air later in the growing season 

(Majewski and others, 2014). 

tac11-0566_fig09-08

Apr. May June July Aug. Sept.

2007

Concentration, in nanograms per cubic meter

0

5

10

15

20

Glyphosate 

Pendimethalin

Metolachlor

Atrazine

Propanil

Other herbicides

EXPLANATION

Figure 9.8. A mixture of herbicides were commonly 

detected in the air during the growing season in an 

agricultural area in Mississippi. The atmosphere is a 

largely unrecognized source of current-use pesticides to 

aquatic ecosystems and humans in agricultural and nonagricultural areas. Atmospheric transport of pesticides 

is one way that aquatic biota, wildlife, and humans 

are exposed to pesticides. The health effects from this 

exposure route are poorly understood (Capel and Hopple, 

2018). 

 Five common forms of nitrogen can be present as gases 

(molecular nitrogen [N2], nitrogen oxides [NO, NO2, N2O] 

and ammonia [NH3]; fig. 8.5). Agriculture-related activities, 

including release of nitrogen fertilizers from soil, manure 

management, and burning of agricultural residues, consistently 

contribute about 70 percent of the Nation’s nitrous oxide 

(N2O) emissions into the atmosphere (U.S. Environmental 

Protection Agency, 2012a). Nitrous oxide is an extremely 

potent greenhouse gas that accounts for about 5 percent of 

the Nation’s total greenhouse gas emissions from human 

activities. Nitric oxide (NO) and nitrogen dioxide (NO2) 

are reactive gases that contribute to acidic precipitation 

and ozone layer destruction (fig. 2.4). Ammonia is released 

into the atmosphere during fertilizer application, from 

fields throughout the growing season, and through the offgassing of livestock waste. Ammonia is a weak greenhouse 

gas and, at higher concentrations, is toxic and an irritant 

(U.S. Environmental Protection Agency, 2012a). 
Connections Between Agriculture and Water Quality 165

Water flowpaths determine how quickly chemicals are transported from fields to streams

Water and its movement over and through agricultural 

landscapes is the primary mechanism by which agricultural 

chemicals move throughout the various components of the 

hydrologic system. The timing and intensity of precipitation 

and (or) irrigation and the nature of the primary water 

flowpaths have a large effect on the timing and magnitude 

of the delivery of an agricultural chemical to a stream, to 

groundwater, or to the atmosphere. Agricultural modifications, 

such as drainage and irrigation, substantially change the water 

flowpaths compared to the natural hydrologic system, and can 

quickly move agricultural chemicals to the stream.

Flowpaths from fields have characteristic transit times. 

Transit times when water moves to streams by way of fastflow 

(surface runoff) are characteristically short (minutes to hours). 

Transit times to streams by way of slowflow (subsurface 

flowpaths) can be much longer (months to decades). These 

slower subsurface connections create a “lag time” between the 

time when agricultural activities occur on the landscape and 

when their effect is observed in other parts of the hydrologic 

system. Transit times to streams through drainflow have both 

slowflow and fastflow components. Subsurface drains are 

designed to move water quickly out of the soil root zone. 

Initially after a period of rain, subsurface drainage creates 

relatively fast flowpaths to the stream (through infiltration 

and lateral movement) for areas near the drains. However, 

areas of the field that are far from the subsurface drains have 

slower flowpaths because the distances for lateral movement 

are greater. Water can move vertically to drains from hours to 

days, and laterally from hours to months. 

In some watersheds, especially those with steep slopes 

and (or) clayey soils, surface runoff (fastflow) is the most 

important flowpath. The streamflow in these watersheds 

responds quickly to rain events, usually within minutes to 

hours. Chemicals (total phosphorous, some trace elements, and 

some pesticides) and sediment are primarily transported to the 

stream with the surface runoff. Connections often are minimal 

between the stream and the groundwater. In Bogue Phalia, 

Mississippi, streamflow and nitrate increased and decreased 

quickly following a rainfall period that occurred 3 days before 

the streamflow peak (fig. 9.9). The nitrate concentration 

increased abruptly with the increase in streamflow and actually 

reached a maximum concentration before peak streamflow. The 

nitrate was moved quickly off the landscape in this watershed 

where surface runoff is an important flowpath. Nitrate 

concentration was lower during the periods between storms. 

In watersheds with gentle slopes and permeable 

soils, groundwater generally is an important contributor to 

streamflow (slowflow). Rain water can infiltrate through the 

soil and recharge the groundwater. This water eventually 

is discharged to the stream. The total time between when 

rain falls and when the water appears in the stream can be 

0

0.3

0.6

0.9

1.2

0

6

12

18

24

Nitrate concentration, in milligrams per liter

Streamflow, in cubic meters per second

2013

Aug. 31 Sept. 2 Sept. 4 Sept. 6 Sept. 8 Sept. 10

Fastflow transport of water and nitrate

Bogue Phalia, Mississippi

Streamflow

Nitrate concentration

EXPLANATION

Figure 9.9. Fastflow is an important pathway for the delivery 

of nitrate to Bogue Phalia, Mississippi. Many conservation 

management practices, such as buffer strips and conservation 

tillage, are effective in preventing sediment and sedimentassociated chemicals from reaching the stream. However, these 

practices are less effective at controlling dissolved chemicals such 

as nitrate (Capel and Hopple, 2018). 

months to decades. These streams also receive some fastflow 

from periods of heavy or intense rain. Chemicals that readily 

dissolve in water are moved with recharge to groundwater. The 

groundwater and dissolved chemicals move slowly through 

the subsurface to the stream. There can be a long lag time 

(months to decades) between when the chemical is applied at 

the land surface and when it appears in the stream. 

Groundwater with high concentrations of nitrate 

discharges into Chesterville Branch on Maryland’s Eastern 

Shore (fig. 9.10). During the growing season, the highest 

concentrations of nitrate occur during the periods of low flow 

between storms; stream nitrate concentrations decrease when 

streamflow increases and low-nitrate runoff dilutes the nitrate 

in the stream. For the single storm shown in figure 9.10, the 

increase in streamflow is from rain that occurred the day of 

the streamflow peak. Low streamflow prior to and after the 

peak is from groundwater. The nitrate concentration decreased 

with increased streamflow and actually reached a minimum 

concentration just after peak streamflow, then increased again 

as the streamflow decreased. The high nitrate concentrations 

come from the groundwater. The increased flow in the stream 

is water moving by fast flowpaths after the rain. The storm 

water, which moved quickly off of the landscape, contained 

less nitrate, thus diluting the nitrate concentration in the 

stream. The high concentrations of nitrate in the stream during 

periods of low flow were from nitrogen that was applied to 

agricultural fields years to decades before. 
166 Agriculture—A River Runs Through It—The Connections Between Agriculture and Water Quality

tac11-0566_fig 09.10

Streamflow

Nitrate concentration

EXPLANATION

0

3

6

9

12

15

18

0

3

6

9

Nitrate concentration, in milligrams per liter

Streamflow, in cubic meters per second

2013

June 18 June 19 June 20 June 21 June 22 June 23

Slowflow transport of water and nitrate

Chesterville Branch, Maryland

Figure 9.10. The increase in nitrate concentration following 

the peak streamflow comes from slowflow groundwater—

the legacy from nitrogen that was applied to agricultural 

fields years to decades previously. Thus, benefits of current 

conservation actions may not be realized in a stream for 

several decades (Capel and Hopple, 2018).

Subsurface drains, which create an artificial water 

flowpath, are engineered to quickly move water out of the 

soil and into the stream. The drains, located below the root 

zone, control the level of the groundwater. The water flowing 

in the drains is a combination of recent excess rainfall/

irrigation (fastflow) and (or) groundwater (slowflow). The 

flow in the drains can respond to rainfall or irrigation within 

hours. Chemicals that readily dissolve in water (nitrate, salts, 

herbicides, and herbicide transformation products) can move 

quickly through the soil root zone into the drains and directly 

to the stream. The chemicals moving through subsurface 

drains bypass many of the soil’s natural removal processes. 

For the 2-day storm in the South Fork of the Iowa River in 

north-central Iowa, the increase in streamflow was from rain 

that started 2 days prior to the peak of streamflow (fig. 9.11A). 

Much of the water moved off of the landscape within a few 

days, but some water took almost a week to get to the stream 

through the network of surface and subsurface drains. Nitrate 

decreased with the initial increase in streamflow, but then 

increased and peaked a day after the streamflow peak. This 

was followed by a decrease in concentration, which was at 

a slower rate compared to the decrease in streamflow. In the 

context of the entire growing season (fig. 9.11B), the nitrate 

concentrations in the drainflow-source stream in north-central 

tac11-0566fig 09-11

Nitrate concentration, in milligrams per liter

Streamflow, in cubic meters per second

Apr. 24 Apr. 28 May 2 May 6 May 10

2011

Apr. 1 May 31 July 30 Sept. 28

South Fork of the Iowa River at Providence, Iowa

0

0

6

12

18

24

0

10

20

30

40

0

10

10

15

20

5

20

30

40

Streamflow

Nitrate concentration

EXPLANATION

A

B

Figure 9.11. (A) Subsurface drainage quickly drains wet 

soils, but this engineered flowpath bypasses much of the 

capacity of the soil to remove nitrate. In this watershed, 

the nitrate that moves through the top layers of the soil 

quickly reaches the subsurface drain and is routed to the 

stream. (B) The nitrate concentration remained elevated 

throughout the entire growing season due to the frequent 

rains and the delivery of water from the drained landscape 

(Capel and Hopple, 2018).

Iowa remained high due to the frequent rains and the steady 

delivery of water from the drained landscape. The nitrate 

concentrations in this stream were substantially greater than 

the other two example hydrologic settings and in agreement 

with previously studies (Dubrovsky and others, 2010).
Connections Between Agriculture and Water Quality 167

groundwater discharging to the Tomorrow River is 27 years. 

More than an estimated 95 percent of nitrate in this stream 

is derived from groundwater (fig. 9.12; Tesoriero and others, 

2013). Figure 9.12A represents hydrologic conditions in 2009. 

The applications of nitrogen fertilizer yielded a concentration 

of 12 mg/L as nitrogen in the shallow groundwater. The water 

in the stream had a nitrate concentration of 3 mg/L, indicating 

that stream water was a mixture of groundwater with elevated 

nitrate concentrations and water from shorter flowpaths with 

lower concentrations of nitrate near the stream. Figure 9.12B

represents expected hydrologic conditions in 2019, if the land 

had been entered into the Conservation Reserved Program 

(CRP) in 2009. Nitrate concentrations in this stream would 

likely increase, not because of recent fertilizer applications, 

but because groundwater discharging to the stream in 2019 

would have recharged the aquifer during a period of more 

intensive fertilizer application two to three decades earlier. 

Starting in about 2020, the nitrate concentration in the shallow 

groundwater under the field would begin to decrease due to 

the CRP. It would be approximately three decades, however, 

before the land-use change and lower applications of nitrogen 

would result in lower nitrate concentrations in the stream.

Agricultural activities to improve water quality may not be observed for years or decades

Expected improvements in water quality from new 

management practices in a field with subsurface flowpaths 

(long transit times) may not be realized for years to decades 

because the concentration of nitrate in the older groundwater 

discharging to the stream is reflective of historical inputs and 

agricultural activities. The agricultural chemicals stored in the 

shallow groundwater constitute a legacy (fig. 8.7), and can 

be released to the stream for years to decades after they were 

used on the fields. This result has been observed for nitrate and 

(or) herbicides in streams and wells used for drinking water 

(McMahon, Böhlke, Kauffman, and others, 2008; McMahon, 

Burow, Kauffman, and others, 2008; Puckett and others, 2011; 

Ator and Denver, 2012; Kalkhoff and others, 2012; Tesoriero 

and others, 2013). In contrast, improvements in water quality 

from new management practices in a field with rapid flowpaths, 

such as runoff and subsurface drainage (short transit times), are 

likely to be observed within months or a few years. 

Long groundwater flowpaths, which have long lag 

times between when nitrate is used at the land surface and 

its delivery to the stream, occur in the Tomorrow River 

watershed in central Wisconsin (Tesoriero and others, 

2013). This stream receives approximately 80 percent of 

annual streamflow from groundwater. The average age of 

EXPLANATION

0 2 4 6 8 10 12

Nitrate concentrations, in milligrams per liter as nitrogen

A. 2009—Agricultural watershed B. 2019—Agricultural practices changed to fallow lands

20 years Travel time from water table

10 years

20 years Travel time from water table

10 years

Stream 

Agriculture

transitioned to

fallow Water table flowpath Water table Stream 

Agricultural

flowpath

Figure 9.12. Nitrate concentrations in the stream during 2009 (A) were a mixture of older water with elevated concentrations of 

nitrate that entered the groundwater system in the 1980s under agricultural fields and recent water with lower concentrations entering 

from areas bordering the stream. If the land were entered into the Conservation Reserve Program starting in 2009, the input of nitrogen 

to the fields would have decreased (see “U.S. Department of Agriculture Conservation Programs” in Chapter 3). Nevertheless in 2019 

(B), the nitrate concentrations in the stream would be expected to be greater because the nitrogen applied at the land surface during 

the 1980s to 2000s continued to move through groundwater and discharge into the stream. This figure is based on the conditions at 

Tomorrow River, Wisconsin.
168 Agriculture—A River Runs Through It—The Connections Between Agriculture and Water Quality

 Each location on the landscape has a unique set of 

hydrologic flowpaths and a unique effect on the environment; 

not all locations contribute water, sediment, and chemicals 

in an equal manner. Some locations on the landscape are 

more susceptible to the movement of sediment and chemicals 

(“critical contributing areas”) and, thus, disproportionately 

affect the water quality. The identification of the critical 

contributing areas can assist water managers in targeting limited 

management resources at both field and watershed scales.

In many areas of the glaciated Midwest, the relatively 

flat landscape contains shallow topographic depressions 

that are underlain by fine-grained soils. After snow melt or 

heavy rainfall, temporary ponds can form in the depressions; 

therefore, surface inlets (fig. 6.5C) connected to the horizontal 

subsurface drainage network have been installed to quickly 

remove water from the temporary ponds. The water that enters 

the surface inlet moves through the horizontal subsurface 

drains into a surface ditch or stream. The area of the field 

containing the topographical depression is connected directly 

to the stream at the drain outlet. The ponded water that 

drains directly to the stream makes the depressions critical 

contributing areas (fig. 9.13; Roth and Capel, 2012b). This 

surface inlet-to-stream connection creates a direct flowpath 

that bypasses all natural removal process and all conservation 

practices designed for trapping sediment (Roth, 2010; 

Feyereisen and others, 2015). 

At a regional scale, there are also parts of the landscape 

that disproportionately affect the water quality. In the 

Chesapeake Bay watershed (fig. 9.14), excess nitrogen from 

fertilizer and manure moves through the soil to groundwater 

and eventually discharges to the tributaries of the Bay. The 

relative importance of this pathway is unevenly distributed 

across the watershed, partly because of the type of underlying 

aquifer and the amount of land in agriculture. The highest 

concentrations of nitrate in shallow groundwater are in 

agricultural areas with carbonate and sandy aquifers. 

Strategic, long-term water-quality monitoring and 

modeling provide a basis for sound management decisions. 

Some parts of the landscape disproportionately affect water quality

Figure 9.13. The temporarily ponded depression on the right edge 

of the photograph has a surface inlet connected to a horizontal 

subsurface drain that empties into the surface ditch. Agricultural 

chemicals and sediment are delivered directly to the stream 

through the surface inlet. The gray/brown areas show additional 

depressions where saturated soils prevented crop growth. 

tac11-0566_fig09-13

EXPLANATION

Subsurface drain outfall

Vertical surface drain

Horizontal subsurface drain

Surface ditch

Base image from National Agriculture 

Imagery Program, 2008

0 200 400 600 Feet

0 100 200 Meters Temporarily ponded water
Connections Between Agriculture and Water Quality 169

tac11-0566_fig09-14

EXPLANATION

Concentration of nitrate in groundwater,

in milligrams per liter nitrate as nitrogen

< 1.0

1.1–2.0

2.1–3.0

3.1–4.0

> 4.0

Figure 9.14. The concentration of nitrate in groundwater 

is not evenly distributed across the Chesapeake Bay 

watershed due to differences in land uses, nitrogen use 

as fertilizer, and type of underlying aquifer. The spatial 

distribution of the nitrate concentration in groundwater 

has been estimated with a statistical model based 

on chemical monitoring data, land use, and type of 

underlying aquifer (Terziotti and others, 2017). Annual 

loads of nitrogen delivered from groundwater to streams 

were calculated based on the nitrate concentrations in 

streams during baseflow conditions. 
170 Agriculture—A River Runs Through It—The Connections Between Agriculture and Water Quality

Long-term monitoring data provide critical quantitative 

information on past, present, and future water-quality 

conditions. These data can be used to track changes in water 

quality in response to past and present agricultural activities, 

quantify the effectiveness of implemented changes, and inform 

the development of new agricultural strategies and policies. 

Sustained monitoring provides insights into long-term trends 

and can provide understanding that may be obscured in 

short-term monitoring efforts because of changing climate, 

hydrology, and inputs of chemicals and sediment. Early 

recognition of trends is critical for implementing changes 

to prevent deterioration of water resources, and ongoing 

monitoring provides the best assurance for early recognition. 

Monitoring is the foundation for the development of 

hydrologic mathematical models that simulate and help 

understand the complex interactions within a hydrologic 

system. Water-quality models are used to simulate the flow of 

water, chemicals, and sediment through a hydrologic system 

for distances ranging from meters to thousands of kilometers. 

Models can use available streamflow and water-quality 

monitoring data to extrapolate observed water-quality conditions 

to unmonitored areas and to quantify the magnitude of different 

sources. Models also can be used as decision-support tools to 

evaluate the effects of alternative management practices on water 

Strategic, long-term water-quality monitoring and modeling provide a basis for sound management decisions

quality. Additionally, models can be used to identify which types of 

data are most important to collect and where monitoring gaps exist.

Long-term monitoring provides a rare glimpse into 

changes in stream nitrate levels over the last 60 years

Only a few of the Nation’s rivers have been monitored 

for nitrate for long periods of time, more than a few decades 

(Stets and others, 2015). These rare, long monitoring records 

provide insights into long-term trends in nitrate concentration, 

and into the human and natural drivers that control these trends. 

Monitoring data collected for almost six decades in the Maumee 

River in northwestern Ohio show a strong relationship between 

human activity (nitrogen fertilizer and manure application in the 

watershed) and increases in the average nitrate concentration 

(fig. 9.15). The Maumee is one of 22 rivers that have 

observations ranging back to the 1950s. Most of the watersheds 

had their greatest increases in nitrate concentration over the 

period of 1945–1980, whereas during the more recent period of 

1981–2008 the concentrations have increased at a slower rate or 

have somewhat stabilized (fig. 9.15). In most cases, these increases 

in nitrate concentration were related to the increase in fertilizer use, 

agricultural expansion, and urban development. 

0 

2 

4 

6 

8 

10 

0 

2 

4 

6 

8 

10 

1950 1960 1970 1980 1990 2000 2010 

Nitrogen applied from fertilizer and manure, 

in grams nitrogen per square meter per year 

Nitrate concentration, in milligrams per liter as nitrogen 

Year 

Nitrate concentration

Nitrogen applied from fertilizer and manure 

EXPLANATION

Figure 9.15. The Maumee River flows through the intensely row-cropped area of northwestern Ohio into Lake Erie. The trend in nitrate 

concentration in the Maumee River showed a rapid increase between 1950 and 1980 as nitrogen inputs from fertilizer and livestock 

increased. Since 1980, the upward trend in nitrate concentration has been much smaller because the upward trend in nitrogen inputs was 

much smaller. This record of nitrate river concentrations and fertilizer use for almost six decades provides a perspective that short-term 

monitoring efforts may miss. For example, if monitoring occurred only over a short period, such as 1990 to 2005, then different conclusions 

would have been reached concerning the relation between nitrate concentration and fertilizer. (Modified from Stets and others, 2015.) 
Connections Between Agriculture and Water Quality 171

Long-term monitoring reveals an early warning 

of future changes in sediment and phosphorus 

loading to the Chesapeake Bay

The Susquehanna River, the largest tributary to the 

Chesapeake Bay, is estimated to have contributed 46 percent 

of the total phosphorus load to the Chesapeake Bay from 

1979 to 2012 (Zhang and others, 2016). The outlet of the 

Conowingo Reservoir, the most downstream and largest 

reservoir on the Susquehanna River, has been monitored for 

suspended sediment, total phosphorus, and other chemicals 

since the 1970s and the primary inflow has been monitored 

since the mid-1980s. These long-term monitoring data allow a 

quantification of how effective the reservoir is at trapping and 

removing sediment and total phosphorus. 

One way of quantifying the removal efficiency of the 

reservoir system is to divide the daily output load by the 

daily input load, adjusted for the water transit time through 

the reservoir. When outputs are less than inputs (that is, 

when the value on the y-axis in figure 9.16 is less than 1), 

the reservoir traps a fraction of the incoming sediment and 

total phosphorus. The Conowingo Reservoir decreased the 

mass of total phosphorus transported to the Chesapeake Bay 

by about 45 percent annually from 1990 to 2000 (Zhang and 

others, 2016).

The storage of phosphorus and sediment by the reservoir 

has improved the water quality of the Susquehanna River 

downstream of the reservoir and in Chesapeake Bay. However, 

the sediment that has collected in the reservoir over the 

last nine decades has also slowly filled up the reservoir. 

Bathymetric studies have shown that the Conowingo Reservoir 

is approaching its maximum storage level. As the reservoir fills 

in, its ability to trap and remove sediment and total phosphorus 

will be limited. In this era when management practices are 

being implemented in agricultural and urban areas to improve 

the water quality of the Chesapeake Bay, this long-existing trap 

for sediment and phosphorus will no longer act as a phosphorus 

and sediment trap. Planning and management of water quality 

of the Chesapeake Bay and other areas depend largely on 

accurate estimates of the loads of sediment and chemicals 

being transported through the system. This long record of 

inputs and outputs to Conowingo Reservoir demonstrates how 

monitoring can provide insights into environmental processes 

and provide an early warning of future changes.

0.0 

0.5 

1.0 

1.5 

2.0 

1985 1990 1995 2000 2005 2010 2015 

Output of total phosphorus 

 Input of total phosphorus 

Year 

EXPLANATION

75th percentile

50th percentile

(median)

25th percentile

Figure 9.16. From 1990 to the early 2000s, the Conowingo Reservoir on the Susquehanna 

River in Pennsylvania and Maryland trapped or removed about 45 percent of the annual total 

phosphorus inputs. As sediment has filled up the reservoir, the amount of phosphorus trapped 

in the reservoir has started to decrease. Since about 2005, there has been an upward trend 

in the ratio of the daily output of total phosphorus to the daily input of total phosphorus. The 

Conowingo Reservoir has, or is near, the end of its lifetime for efficiently removing sediment 

and total phosphorus. For much of the year there will be minimal removal. During extreme 

flows, such as following hurricanes, the reservoir will release stored sediment and total 

phosphorus to the downstream river and to the Chesapeake Bay, due to scour of the bed 

sediments. The asterisk shows the ratio affected by an extreme event. (Modified from Zhang 

and others, 2016.) 
172 Agriculture—A River Runs Through It—The Connections Between Agriculture and Water Quality

Long-term, strategic monitoring of atrazine 

provide insights into annual concentration 

pattern, year-to-year variability, and long-term 

trends

Long-term, strategic monitoring of streamflow and 

pesticide concentrations can aid in our understanding of 

time trends in concentration and environmental processes 

governing their behavior. Documenting the seasonal changes 

in agricultural chemicals in streams over many years has 

improved the understanding of how chemistry, biology, 

hydrology, pesticide use, and agricultural practices govern the 

transport, behavior, and fate of agricultural chemicals in the 

environment. 

The herbicide atrazine has been monitored in the White 

River at Hazleton, Indiana, for more than 20 years (fig. 9.17A). 

These data provide insights into the annual pattern of atrazine 

concentrations in the river, the year-to-year variability, and the 

long-term trends. Atrazine concentrations are highest during 

the “first flush” spring rains after application of the herbicide 

to agricultural fields (generally April–June) (Gilliom and 

others, 2006). The data also indicate that atrazine is present in 

the river at all times, generally in concentrations from 0.1 to 

10 micrograms per liter (μg/L). 

Long-term data can be used to assess trends. Atrazine 

concentrations in the White River decreased by about 

9 percent from 1992 to 2001. No significant trend in atrazine 

concentrations was observed from 2001 to 2010 (Ryberg and 

others, 2014). During this period, many factors influenced 

atrazine use, including a change in the EPA labeled application 

rate, the introduction of new herbicides and genetically 

modified corn, an increase in the area planted in corn due 

to biofuels, and changes in tillage practices. The longterm monitoring data aid the understanding of the multiple 

influences on pesticide concentrations in streams. 

The monitoring data from the White River, and from 

many other sites across the Nation, form the basis of models 

(decision-support systems) used to estimate pesticide 

concentrations and probabilities of exceeding human and 

aquatic life benchmarks (Stone and others, 2013). These 

models can be used to estimate pesticide concentrations in 

unmonitored areas (fig. 9.17B).
Connections Between Agriculture and Water Quality 173

t110566fi0917

OHIO

ILLINOIS INDIANA

MISSOURI KENTUCKY

IOWA

MICHIGAN

TENNESSEE

WEST VIRGINIA

VIRGINIA

WISCONSIN

NORTH CAROLINA

ARKANSAS

PENNSYLVANIA

NEW YORK

MARYLAND

0 100 200 MILES

0 100 200 KILOMETERS

90°00' 87°30' 85°00' 82°30' 80°00'

42°

30'

40°

00'

37°

30'

Base from U.S. Geological Survey National Boundary Dataset

Albers Equal Area Conic, North American Datum of 1983 

Streamflow, in cubic meters per second 

1

10

100

1,000

10,000

1

Atrazine concentration, in micrograms per liter

0.01

0.1

10

100

Streamflow

Atrazine

Streamflow

Atrazine maximum

contaminant level

for drinking water

1992 1994 1996 1998 2000 2002

Year

2004 2006 2008 2010

EXPLANATION

A. White River at Hazelton, Indiana

B. Watershed Regressions for Pesticides (WARP), atrazine, 2012

Probability that annual mean exceeds

benchmark (3 micrograms per liter)

EXPLANATION

Model estimate unavailable

5 to 25 percent

25 to 50 percent

≥ 50 percent

Figure 9.17. (A) A 20-year monitoring record of the herbicide atrazine in the White River, Indiana, shows that 

the atrazine concentrations are highest during the “first flush” spring rains after application of the herbicide to 

agricultural fields (generally April–June) (Gilliom and others, 2006). Atrazine concentrations infrequently exceed 

the Maximum Contaminant Level (MCL) for drinking water (indicated by the red horizontal line). Even though 

the river water at this location is not used as a source of drinking water, it provides a useful concentration 

benchmark. (B) A model simulation based on monitoring data at multiple sites throughout the Midwest and 

geographical information of atrazine use and watershed characteristics provides an estimate of the probability 

that the annual mean atrazine concentration will exceeded its MCL for drinking water for the rivers and streams 

in Illinois, Indiana, and Ohio in 2012 (Stone and Bucknell, 2014). 
174 Agriculture—A River Runs Through It—The Connections Between Agriculture and Water Quality

Water-quality model helps target new waterquality monitoring activities and inform nutrient 

reduction strategies

The Elk River watershed drains south-central Tennessee 

and northern Alabama and flows into an embayment of 

Wheeler Reservoir in Alabama (fig. 9.18). Water-resource 

managers are concerned about eutrophication in the 

embayment. In the past, agricultural activities were thought 

to be the major source of phosphorus to the embayment and 

the cause of the eutrophication. However, simulations from 

the USGS SPARROW (SPAtially Referenced Regressions 

On Watershed Attributes) model of the area indicated that 

the largest source of phosphorus to the embayment was 

from natural phosphate-bearing rocks (83 percent; Garcia 

and others (2011), whereas, agricultural activities (crops 

and animals) contributed about 13 percent, and mined lands 

and wastewater contributed only about 1 percent each. 

Because of the understanding provided from the model, new 

water-quality stream-monitoring sites have been installed 

to provide quantitative information to confirm the estimated 

model simulation results. The implementation of agricultural 

conservation practices would have had limited effect on 

reducing annual inputs of phosphorus to the embayment. 

One of the benefits of a water-quality model is to provide 

information that will help improve the design of monitoring 

programs in the future.

Elk River embayment

TENNESSEE

ALABAMA

Elk River Basin

Wheeler

Reservoir Tennessee River

0 10 20 30 40 MILES

0 10 20 30 40 KILOMETERS

87°30' 87°00' 86°30' 86°00'

35°

30'

35°

00'

34°

30'

Monitoring site, location selected based on 

model results

Model-estimated phosphorus yield transported to stream from 

natural geoloigic materials in each catchment, in kilograms 

per square kilometer

< 100

101 - 200

201 - 300

301 - 400

401 - 500

> 500

EXPLANATION

Figure 9.18. The Elk River watershed, in Tennessee and Alabama, 

is the source of nutrients to the Elk River embayment of Wheeler 

Reservoir, which has undergone eutrophication. Model simulation 

results, based on historical monitoring data, showed that natural 

geologic deposits of phosphate-bearing rocks are the primary 

source of phosphorus. This information has led to a more informed 

monitoring design to enhance water-quality management decisions. 
Connections Between Agriculture and Water Quality 175

Water-quality model provides regional insights 

on nutrient sources to streams

The Chesapeake Bay is a vital ecological and 

economic resource. The National Oceanic and Atmospheric 

Administration reported in 2009 that the commercial seafood 

industry in Maryland and Virginia contributed $3.39 billion in 

sales, $890 million in income, and almost 34,000 jobs to the 

local economy (Chesapeake Bay Foundation, 2016). Excess 

inputs of nitrogen and phosphorus to Chesapeake Bay have 

negatively affected the water quality and the overall health of 

the ecosystem. To improve the aquatic health of Chesapeake 

Bay, a better understanding of nutrient sources and transport 

was needed. 

The USGS SPARROW model provides regional 

insights about which nutrient sources and watershed areas are 

contributing the highest and lowest amounts of nutrients to 

the Chesapeake Bay (fig. 9.19). Model estimates throughout 

80,000 stream reaches in the Chesapeake Bay watershed 

improve the understanding of the areas contributing the most 

nutrients and help water resource managers target the most 

effective nutrient reduction actions. 

A SPARROW model was developed to estimate the 

sources, fate, and transport of nutrients in the Chesapeake 

Bay watershed (fig. 9.19; Ator and others, 2011). The 

estimated source contributions of nitrogen to the Chesapeake 

Bay are 54 percent from the combination of manure and 

fertilizer applications and fixation by crops, 17 percent from 

atmospheric deposition, 16 percent from point sources, 

and 12 percent from urban areas. The estimated source 

contributions of phosphorus to the Chesapeake Bay are 

43 percent from fertilizer and manure applications, 32 percent 

from point sources, 14 percent from natural sources, and 

11 percent from urban sources. 

Crop and animal agricultural in Maryland. Photograph by Bob Nichols, U.S. Department of Agriculture, Natural 

Resources Conservation Service photo gallery, NRCSMD08058, 2008.
176 Agriculture—A River Runs Through It—The Connections Between Agriculture and Water Quality

tac11-0566_fig09-19

Chesapeake Bay

A. Agricultural sources

1,000.1 to 5,000

> 5,000

EXPLANATION

< 50

50.1 to 100

100.1 to 200

200.1 to 300

300.1 to 400

400.1 to 500

500.1 to 750

750.1 to 1,000

Incremental yield of total nitrogen —

in kilograms per square kilometer 

per year

0 100 Miles

0 100 Kilometers

New York N

Pennsylvania

Delaware

Maryland

WDC

West Virginia

Virginia

B. Sources of nitrogen contributed by the 

Chesapeake Bay watershed

C. Sources of phosphorus contributed by the 

Chesapeake Bay watershed

12%

16%

46%

9%

17%

11%

32%

22%

21%

14%

Inputs from

agricultural fertilizers

and legume crops

Inputs from

agricultural

 manure

Inputs from

atmospheric

 deposition

Inputs from 

urban areas 

Inputs from

 wastewater

treatment

 plants 

Inputs from

agricultural fertilizers

Inputs from 

agricultural

 manure 

Inputs from 

natural mineral

deposits

Inputs from

urban areas 

Inputs from 

wastewater 

treatment 

plants

Figure 9.19. Excess inputs of nitrogen and phosphorus to Chesapeake Bay have negatively affected the water quality and the overall 

health of the ecosystem. To improve the aquatic health of Chesapeake Bay, a better understanding of the nutrient sources was needed. 

A complex hydrological model (SPARROW [SPAtially Referenced Regressions On Watershed Attributes]) was developed to estimate 

the spatially variable sources of the nutrients from small catchments across the Chesapeake Bay watershed. (A) Spatial distribution of 

total nitrogen yields across the watershed. The model is based on monitoring observations of the chemicals and streamflows, as well 

as the spatial characteristics of land use, topography, and soils. Model use has greatly increased the understanding of nutrient sources 

to Chesapeake Bay and helped in the development of programs to manage the nutrient inputs. The model also estimates the relative 

magnitude of various sources of (B) nitrogen and (C) phosphorus to Chesapeake Bay. Agricultural activities contribute 54 percent of the 

total nitrogen and 43 percent of the total phosphorus to the Bay (Ator and others, 2011). 
Connections Between Agriculture and Water Quality 177

Understanding the connections among hydrologic settings and chemical behaviors can help set 

realistic expectations for water-quality improvements

Table 9.1. Examples of agricultural management practices 

and other agricultural activities which are used to help 

minimize soil and (or) chemical loss to protect and improve 

water quality.

Understanding the connections between agricultural 

activities, water flowpaths and associated lag times, and 

behavior of specific chemicals and (or) sediment can be 

combined into a framework to guide policy and management 

decisions to reduce current and prevent future impacts on 

water quality (Capel and others, 2018). This framework is 

built on a generalization of stream hydrology—water, whether 

from natural rainfall or irrigation, moves to a stream by a 

combination of slowflow, fastflow, and (or) drainflow, and 

these flowpaths are associated with a range of timescales. 

The framework generalizes the behavior of chemicals and 

sediment—all chemicals are distributed on a continuum 

between those completely associated with water and those 

completely associated with sediment (fig. 8.4). Finally, each 

agricultural activity is assumed to have its expected effect on 

the movement of water and chemicals across the landscape. 

This framework, shown as a decision tree (fig. 9.20), is a 

gross simplification, but it is supported by observations in 

the environment and quantified in mathematical models. 

The decision tree is not intended to be used on a site-specific 

basis. More detailed knowledge of the local environment, 

climate, and agricultural production is needed to make design 

decisions for the implementation of an agricultural activity at a 

particular location.

The expectations of the effectiveness of an agricultural 

activity must consider the local hydrologic flowpaths within 

a specific field and an understanding of the environmental 

behavior of the specific chemical or sediment to be controlled. 

Few agricultural activities will be effective for all chemicals. 

Although valuable knowledge can be gained from a 

generalized understanding, as presented in this conceptual 

framework, specific knowledge of the hydrology and the 

chemical is required for a specific field or watershed. With this 

specific knowledge, agricultural activities can be designed to 

optimize efficiency on a field-by-field basis. 

 The first question in the decision tree concerns the 

environmental behavior of the chemical of concern: Is the 

chemical of concern primarily sediment associated? The 

second question addresses the presence of permanent or 

semi-permanent agricultural modifications to the landscape 

that are already in place: Is the water moving through 

subsurface drainage? The final question regards the nature of 

the hydrologic setting: Is groundwater an important source 

of flow in the stream? The answers to these three questions 

can guide the selection and expectation of the effectiveness of 

implementing an agricultural activity (table 9.1).

The decision tree is simplified to the non-reactive 

chemical and sediment. The horizontal lines between “yes” 

and “no” on the tree represent three continuums, starting at 

the top, degree of association with sediment (from 100 percent 

water-associated to 100 percent sediment-associated), the 

Trapping practices

Terraces, grassed waterways

Buffer/filter strips

Brims at edge of stream

Cover crops

Srip cropping

Tillage practices

Conservation tillage

No-till tillage

Contoured plowing

Drainage practices

Controlled subsurface drainage

Biofilters on subsurface drains

Removal of subsurface drains

Removal of surface inlets to subsurface drains

Irrigation practices

General decrease in volume and energy of irrigation water

Chemical use practices

Decrease in chemical use

Use of chemicals with short environmental half-lives

Set-aside land for conservation

Conservation reserve programs (Federal and state, for example 

USDA CRP)

Constructed wetlands 

density of subsurface drains (from zero to a closely spaced, 

patterned drain network), and the importance of groundwater 

to total streamflow (from 0 to 100 percent). The decision tree 

defines and traces the two extremes for the degree of sediment 

association by chemicals. Chemicals that are intermediate 

between these two extremes, that is they have a substantial 

fraction associated with both water and sediment phases, will 

follow multiple routes through the decision tree and, thus, 

it may be more difficult to identify realistic expectations 

of the effectiveness of agricultural activities. The rate of 

transformation of chemicals does not affect the use of the 

decision tree, except that chemicals with short lifetimes may 

disappear from the environment as they are transported via 

moving water if the hydrologic transit time is greater than the 

environmental lifetime of the chemical. 

Few agricultural activities will be effective for 

protecting water quality for all chemicals in all hydrological 

settings. Some changes will be effective; some changes will be 

counterproductive (Capel and others, 2018). In some situations 

water quality can be improved and protected by adding 

new, well-selected agricultural activities and (or) landscape 

modifications. In other situations, the removal of the existing 

landscape modifications, such as drainage, may be more 

effective.
178 Agriculture—A River Runs Through It—The Connections Between Agriculture and Water Quality

A Simple Decision Tree Connects Water Flowpaths, Chemical Behavior, and the 

Effectiveness of Agricultural Activities

Is the chemical of concern primarily associated with sediment? 

Yes No

Yes No Yes No

Is the water moving through subsurface drainage?

Not usually a

water quality

concern*

Not usually a

water quality

concern*

Useful activities: increased set-aside land;

decreased drainage, irrigation, and chemical use

Counterproductive: some increases in trapping

practices and conservation tillage

Useful activities: increased set-aside land;

decreased drainage, irrigation, and chemical use

Counterproductive: some increases in trapping

practices and conservation tillage

Useful activities: increased set-aside land;

decreased drainage, irrigation, and chemical use

Counterproductive: none

Is the water moving through groundwater?

Yes No Yes No

Useful activities: increased trapping practices,

conservation tillage, and set-aside land;

decreased irrigation and chemical use

Counterproductive: none

EXPLANATION

Slow flowpaths Fast flowpaths Drain flowpaths

Figure 9.20. The decision tree can assist with identification of which agricultural activity(s) could be effective 

in protecting or improving stream-water quality and which agricultural activity(s) could be counterproductive 

(table 9.1). * denotes that these chemicals do not usually cause a water-quality concern from the part of the water 

moving through subsurface flowpaths (for example, moving through soil to groundwater or to subsurface drains), 

but these chemicals can cause a concern in the part of water moving through flowpaths across the land surface.
Connections Between Agriculture and Water Quality 179

Numerous agricultural activities have been implemented 

over the past few decades to improve and protect water 

quality. Although the effectiveness of many of these 

activities has been quantified at the field scale (Reeder and 

Westermann, 2006), the cumulative effects of these activities 

at a watershed scale have been difficult to quantify because 

of simultaneous, multiple sources (including non-agricultural 

sources) and variability in hydrology and landscapes 

(National Research Council, 1999; Tomer and Locke, 

2011). An innovative approach combining information 

from process-based models from the USDA Agricultural 

Research Service and the Natural Resources Conservation 

Service (Tomer and Locke, 2011; Osmond and others, 2012) 

with a USGS statistical model was used to overcome these 

limitations (Garcia and others, 2016). SPARROW model 

results indicate that soil conservation practices in the Upper 

Mississippi River watershed can reduce nitrogen inputs to 

some streams and rivers by as much as 33 percent (median: 

15; range: 7 to 33 percent), providing new insights on the 

benefits of conservation practices (Garcia and others, 2016). 

Estimated reductions in nitrogen loads are consistent with 

known hydrological and biogeochemical processes. 

Reducing and slowing down runoff and increasing 

infiltration will significantly reduce the amount of nitrogen 

that is eventually transported to streams. Structural and 

erosion control practices, such as conservation tillage, in 

the Upper Mississippi River Basin (fig. 9.21) have been 

shown to decrease runoff and peak flows, thereby increasing 

water infiltration into the soils and the subsurface geology. 

The routing of large quantities of water to the subsurface 

by conservation practices contributes to increased hydraulic 

storage that can lead to an increase in denitrification rates 

and, thus, reductions in nitrogen delivery to streams. The 

effectiveness of conservation practices in reducing nitrogen 

delivery to streams is highly dependent on subsurface 

hydrological and biogeochemical conditions that favor 

the permanent removal of nitrogen through denitrification 

and (or) delay of the delivery of nitrogen to streams in 

discharging groundwater.

tac11-0566fig09-21

Figure 9.21. Innovative modeling approaches provide new 

insights on the benefits of conservation practices in reducing 

nitrogen inputs in the Upper Mississippi River (Garcia and others, 

2016). Model estimates indicate that agricultural conservation 

practices in the upper Mississippi River watershed can reduce 

nitrogen inputs to area streams and rivers by as much as 

33 percent, as compared to the same landscape without soil 

conservation practices. In some areas, slowing the water and 

routing it into the ground can significantly reduce the amount of 

nitrogen that is eventually transported to streams.
180 Agriculture—A River Runs Through It—The Connections Between Agriculture and Water Quality

Final Thoughts

We are entering a time of unprecedented demands upon the Earth and its resources. Some 

6 billion humans inhabit this single planet, all of whom need food to eat, fiber for clothing, 

shelter for protection from the elements, and clean water to drink. The challenges for 

agriculture are great and the stakes are high. Can agriculture on a global basis produce 

enough food, fiber, and fuel to satisfy the needs of humanity, and do it in such a way that it 

is ecologically sustainable, so as not to degrade the quality of our air, water, soil, and other 

natural ecosystems? This, then, is the challenge. Meeting the goal of providing for the 

material needs of humanity will require a cooperative effort among producers, scientists, 

consumers, and policy makers to think seriously, plan carefully, and develop a sound 

strategy to wisely use the Earth’s resources.
References Cited 181

References Cited 

Agency for Toxic Substances and Disease Registry, 2013, 

Ammonia: Agency for Toxic Substances and Disease 

Registry Web site, accessed May 17, 2013, at http://www.

atsdr.cdc.gov/substances/toxsubstance.asp?toxid=2.

Ahren, C.D., 2011, Essentials of meteorology—An invitation 

to the atmosphere, 6th ed.: Independence, Kentucky, 

Brooks/Cole, 528 p., ISBN: 0840049331.

American Academy of Microbiology, 2009, Antibiotic 

resistance—An ecological perspective on an old problem: 

American Academy of Microbiology, 32 p., accessed 

March 17, 2018, at https://www.asm.org/images/stories/

documents/antibioticresistance.pdf.

American Society of Agricultural and Biological Engineers, 

2011, Anhydrous ammonia application technology—2011: 

American Society of Agricultural and Biological Engineers 

Web site, accessed May 17, 2013, at http://www.asabe.org/

awards-landmarks/asabe-historic-landmarks/anhydrousammonia-application-technology-56.aspx. 

Andaluri, G., Suri, R.P.S., and Kumar, K., 2012, Occurrence 

of estrogen hormones in biosolids, animal manure and 

mushroom compost: Environmental Monitoring and 

Assessment, v. 184, no. 2, p. 1,197–1,205.

Anderson, M.T., and Woosley, L.H., Jr., 2005, Water 

availability for the Western United States—Key scientific 

challenges: U.S. Geological Survey Circular 1261, 85 p.

Anderson, R., and Jansen, B., 2010, Wisconsin conservation 

history: U.S. Department of Agriculture, Natural Resources 

Conservation Service, 52 p., accessed March 17, 2018, at 

https://www.nrcs.usda.gov/Internet/FSE_DOCUMENTS/

nrcs142p2_019285.pdf.

Arshad, M.A., Franzluebbers, A.J., and Azooz, R.H., 1999, 

Components of surface soil structure under conventional 

and no-tillage in northwestern Canada: Soil and Tillage 

Research, v. 53, p. 41–47.

Aspelin, A.L., and Grube, A.H., 1999, Pesticides industry 

sales and usage—1996 and 1997 market estimates: U.S. 

Environmental Protection Agency, Pesticide Industry Sales 

and Usage Report 733-R-99-001, 39 p.

Ator, S.W., 2008, Natural and human influences on water 

quality in a shallow regional unconsolidated aquifer, 

Northern Atlantic Coastal Plain: U.S. Geological Survey 

Scientific Investigations Report 2008-5190, 19 p. (Also 

available at http://pubs.usgs.gov/sir/2008/5190/.)

Ator, S.W., Brakebill, J.W., and Blomquist, J.D., 2011, 

Sources, fate, and transport of nitrogen and phosphorus 

in the Chesapeake Bay watershed—An empirical model: 

U.S. Geological Survey Scientific Investigations Report 

2011–5167, 27 p. (Also available at http://pubs.usgs.gov/

sir/2011/5167/.)

Ator, S.W., and Denver, J.M., 2012, Estimating contributions 

of nitrate and herbicides from groundwater to headwater 

streams, Northern Atlantic Coastal Plain, United States: 

Journal of the American Water Resources Association, 

v. 48, no. 6, p. 1,075–1,090, doi: 10.1111/j.1752-

1688.2012.00672.x. 

Ator, S.W., and Denver, J.M., 2015, Understanding nutrients 

in the Chesapeake Bay watershed and implications for 

management and restoration—The Eastern Shore: U.S. 

Geological Survey Circular 1406, 72 p., http://dx.doi.

org/10.3133/cir1406.

Ator, S.W., Denver, J.M., and Brayton, M.J., 2005, Hydrologic 

and geochemical controls on pesticide and nutrient transport 

to two streams on the Delmarva Peninsula: U.S. Geological 

Survey Scientific Investigations Report 2004-5051, 34 p. 

(Available at http://pubs.usgs.gov/sir/2004/5051/.)

Baker, J.L., Helmers, M.J., and Laflen, J.M., 2006, Water 

management practices rain-fed cropland, in Schnepf, M., 

and Cox, C., eds., Environmental Benefits of Conservation 

on Cropland—The status of our knowledge: Ankeny, Iowa, 

Soil and Water Conservation Society, p. 89–130. 

Baker, N.T., and Capel, P.D., 2011, Environmental factors 

that influence the location of crop agriculture in the 

conterminous United States: U.S. Geological Survey 

Scientific Investigations Report 2011-5108, 72 p. (Available 

at http://pubs.usgs.gov/sir/2011/5108/.)

Baker, N.T., Stone, W.W., Frey, J.W., and Wilson, J.T., 2007, 

Water and agricultural-chemical transport in a Midwestern, 

tile-drained, watershed—Implications for conservation 

practices: U.S. Geological Survey Fact Sheet 2007-3084, 

6 p. (Available at http://pubs.usgs.gov/fs/2007/3084/.)

Baker, N.T., Stone, W.W., Wilson, J.T., and Meyer, M.T., 

2006, Occurrence and transport of agricultural chemicals 

in Leary Weber Ditch Basin, Hancock County, Indiana, 

2003-04: U.S. Geological Survey Scientific Investigations 

Report 2006-5251, 44 p. (Available at http://pubs.usgs.gov/

sir/2006/5251/.)

Barber, N.L., 2009, Summary of estimated water use in 

the United States in 2005: U.S. Geological Survey Fact 

Sheet 2009-3098, 2 p. (Available at http://pubs.usgs.gov/

fs/2009/3098/.)
182 Agriculture—A River Runs Through It—The Connections Between Agriculture and Water Quality

Barlow, J.R., and Coupe, R.H., 2012, Groundwater and 

surface-water exchange and resulting nitrate dynamics 

in the Bogue Phalia basin in northwestern Mississippi: 

Journal of Environment Quality, v. 41, no. 1, p. 155–169, 

doi:10.2134/jeq2011.0087. 

Barlow, J.R.B., and Clark, B.R., 2011, Simulation of water-use 

conservation scenarios for the Mississippi Delta using an 

existing regional groundwater flow model: U.S. Geological 

Survey Scientific Investigations Report 2011-5019, 14 p. 

(Also available at http://pubs.usgs.gov/sir/2011/5019/.)

Bartolino, J.R., and Cunningham, W.L., 2003, Ground-water 

depletion across the Nation: U.S. Geological Survey Fact 

Sheet FS-103-03, 4 p. (Available at http://pubs.usgs.gov/fs/

fs-103-03/.)

Belitz, Kenneth, Jurgens, B.C., and Johnson, T.D., 2016, 

Potential corrosivity of untreated groundwater in the United 

States: U.S. Geological Survey Scientific Investigations 

Report 2016-5092, 16 p., http://dx.doi.org/10.3133/

sir20165092.

Bernhardt, E.S., Palmer, M.A., Allan, J.D., Alexander, G., 

Barnas, K., Brooks, S., Carr, J., Clayton, S., Dahm, C., 

Follstad-Shah, J., Galat, D., Gloss, S., Goodwin, P., 

Hart, D., Hassett, B., Jenkinson, R., Katz, S., 

Kondolf, G.M., Lake, P.S., Lave, R., Meyer, J.L., 

O’Donnel, T.K., Pagano, L., Powell, B., and Sudduth, E., 

2005, Synthesizing U.S. river restoration efforts: Science, 

v. 308, no. 5722, p. 636–637, doi: 10.1126/science.1109769. 

Bidleman, T.F., Patton, G.W., Walla, M.D., Hargrave, B.T., 

Vass, W.P., Erickson, P., Fowler, B., Scott, V., and 

Gregor, D.J., 1989, Toxaphene and other organochlorines 

in Arctic Ocean fauna—Evidence for atmospheric delivery: 

Arctic, v. 42, no. 4, p. 307–313.

Bissonnais, L.Y., Bruand, A., and Jamagne, M.O., 1989, 

Laboratory experimental study of soil crusting—Relation 

between aggregate breakdown mechanisms and crust 

structure: Catena, v. 16, p. 377–392.

Bohan, D.A., Boffey, C.W.H., Brooks, D.R., Clark, S.J., 

Dewar, A.M., Firbank, L.G., Haughton, A.J., Hawes, C., 

Heard, M.S., May, M.J., Osborne, J.L., Perry, J.N., 

Rothery, P., Roy, D.B., Scott, R.J., Squire, G.R., 

Woiwod, I.P., and Champion, G.T., 2005, Effects on weed 

and invertebrate abundance and diversity of herbicide 

management in genetically modified herbicide-tolerant 

winter-sown oilseed rape: Proceedings of the Royal Society 

Biological Sciences, v. 272, no. 1562, p. 463–474.

Borlaug, N., 1972, The green revolution, peace, and humanity, 

in Haberman, F.W., ed., Nobel Lectures, Peace 1951–1970: 

Amsterdam, Elsevier Publishing Company, accessed 

May 17, 2013, at http://www.nobelprize.org/nobel_prizes/

peace/laureates/1970/borlaug-lecture.html.

Bos, M.G., 1994, Subsurface drainage systems, in Ritzema, 

H.P. ed., Drainage principles and applications: Wageningen, 

The Netherlands, International Institute for Land 

Reclamation and Improvement, Publication 16, p. 725–798.

Boxall, A.B.A., Kolpin, D.W., Halling-Sorensen, B., 

and Tolls, J., 2003, Are veterinary medicines causing 

environmental risks?: Environmental Science and 

Technology, v. 37, no. 15, p. 286A–294A.

Bracmort, K.S., Arabi, M., Frankenberger, J.R., Engel, B.A., 

2006, Modeling long-term water quality impact of structural 

BMPs: Transactions of the American Society of Agricultural 

and Biological Engineers, v. 49, no. 2, p. 367–374.

Bracmort, K.S., Engel, B.A., and Frankenberger, J.R., 2004, 

Evaluation of the current condition of structural best 

management practices 20 years after installation—Black 

Creek watershed, Indiana: Journal of Soil and Water 

Conservation, v. 59, no. 5, p. 191–196.

Bullock, D.G., 1992, Crop rotation: Critical Reviews in Plant 

Sciences, v. 11, no. 4, p. 309–326.

Bureau of Reclamation, 2011, Brief history of the Bureau of 

Reclamation: Bureau of Reclamation History Program, 

12 p., accessed March 17, 2018, at https://www.usbr.gov/

history/briefhis.pdf. 

Burkholder, J., Libra, B., Weyer, P., Heathcote, S., 

Kolpin, D.W., Thorne, P.S., and Wichman, M., 2007, 

Impacts of waste from concentrated animal feeding 

operations on water quality: Environmental Health 

Perspectives, v. 115, no. 2, p. 308–312.

Burow, K.R., Shelton, J.L., and Dubrovsky, N.M., 2008, 

Regional nitrate and pesticide trends in ground water 

in the eastern San Joaquin Valley, California: Journal 

of Environmental Quality, v. 37, no. 5, Supplement, 

p. S249–S263, doi: 10.2134/jeq2007.0061.

Camargo, J.A., and Alonso, A., 2006, Ecological and 

toxicological effects of inorganic nitrogen pollution in 

aquatic ecosystems—A global assessment: Environment 

International, v. 32, no. 6, p. 831–849.

Capel, P.D., and Hopple, J.A., 2018, Data and citations 

describing the connections between agriculture and water 

quality in the United States: U.S. Geological Survey data 

release, https://doi.org/10.5066/F7639MZX.
References Cited 183

Capel, P.D., Wolock, D.M., Coupe, R.H., and Roth, J.L., 

2018, A conceptual framework for effectively anticipating 

water-quality changes resulting from changes in agricultural 

activities: U.S. Geological Survey Scientific Investigations 

Report 2017–5095, 35 p., https://doi.org/10.3133/

sir20175095. 

Capel, P.D., Velkoverh, J.L., and Hopple, J.A., 2017, Selected 

chemicals, solids, and microorganisms that are used in 

agriculture or produced as by-products of agriculture in 

the United States as of 2009: U.S. Geological Survey data 

release, accessed April 4, 2017, at https://doi.org/10.5066/

F7348HGF.

Capper, J.L., Cady, R.A., and Bauman, D.E., 2009, The 

environmental impact of dairy production—1944 

compared with 2007: Journal of Animal Science, v. 87, 

p. 2,160–2,167, doi:10.2527/jas.2009-1781.

Carson, Rachel, 1962, Silent spring: Boston, Mass., Houghton 

Mifflin Company, ISBN 0-618-24906-0, 368 p. 

Caskey, B.J., and Frey, J.W., 2009, Biological-community 

composition in small streams and its relations to habitat, 

nutrients, and land use in agriculturally dominated 

landscapes in Indiana and Ohio, 2004, and implications 

for assessing nutrient conditions in Midwest streams: 

U.S. Geological Survey Scientific Investigations Report 

2009-5055, 21 p., plus CD-Rom containing four apps. 

(Available at http://pubs.usgs.gov/sir/2009/5055/.)

Caskey, B.J., Frey, J.W., and Lowe, B.S., 2007, Relations of 

principal components analysis site scores to algal biomass, 

habitat, basin-characteristics, nutrient, and biologicalcommunity data in the Whitewater River and East Fork 

White River Basins, Indiana, 2002: U.S. Geological Survey 

Scientific Investigations Report 2007-5229, 31 p. (Available 

at http://pubs.usgs.gov/sir/2007/5229/.)

Caskey, B.J., Frey, J.W., and Selvaratnam, S., 2010, 

Breakpoint analysis and assessment of selected stressor 

variables on benthic macroinvertebrates and fish 

communities in Indiana streams—Implications for 

developing nutrient criteria: U.S. Geological Survey 

Scientific Investigations Report 2010-5026, 35 p. (Available 

at http://pubs.usgs.gov/sir/2010/5026/.)

Chai, G., 2010, Sales of antibacterial drugs in kilograms: 

Food and Drug Administration, p. 8, accessed May 17, 

2013, at http://www.fda.gov/downloads/Drugs/DrugSafety/

InformationbyDrugClass/UCM261174.pdf.

Chang, F.-C., Simcik, M.F., and Capel, P.D., 2011, 

Occurrence and fate of the herbicide glyphosate and its 

degradate aminomethylphosphonic acid in the atmosphere: 

Environmental Toxicology and Chemistry, v. 30, no. 3, 

p. 548–555, doi: 10.1002/etc.431.

Chesapeake Bay Foundation, 2016, The economic importance 

of the Bay: Chesapeake Bay Foundation, Web site accessed 

March 7, 2016, at http://www.cbf.org/about-the-bay/issues/

cost-of-clean-water/economic-importance-of-the-bay. 

Chesapeake Bay Program, 2013, Agriculture: Chesapeake Bay 

Program Web site, accessed May 17, 2013, at http://www.

chesapeakebay.net/issues/issue/agriculture#inline.

Committee on the Impact of Biotechnology on Farm-Level 

Economics and Sustainability, National Research Council, 

2010, Impact of genetically engineered crops on farm 

sustainability in the United States: The National Academies 

Press, 270 p., accessed May 17, 2013, at http://www.nap.

edu/catalog.php?record_id=12804. 

Conkin, P.K., 2008, A revolution down on the farm—The 

transformation of American agriculture since 1929: 

Lexington, The University Press of Kentucky, 223 p., 

ISBN 9780813125190.

Conley, D.J., Carstensen, J., Vaquer-Sunyer, R., and Duarte, 

C.M., 2009, Ecosystems thresholds with hypoxia: 

Hydrobiologia, v. 629, no. 1, p. 21–29, doi:10.1007/s10750-

009-9764-2.

Cordell, D., Drangert, J.-O., and White, S., 2009, The story 

of phosphorus—Global food security and food for thought: 

Global Environmental Change, v. 19, no. 2, p. 292–305.

Coupe, R.H., Barlow, J.R.B., and Capel, P.D., 2012, 

Complexity of human and ecosystem interaction in an 

agricultural landscape: Environmental Development, v. 4, 

p. 88–104.

Coupe, R.H., and Capel, P.D., 2015, Trends in pesticide use 

on corn, cotton and soybean since the introduction of major 

genetically-modified crops in the United States: Pesticide 

Management Science, 10 p., doi: 10.1002/ps.4082.

Coupe, R.H., Kalkhoff, S.J., Capel, P.D., and Gregoire, 

C., 2011, Fate and transport of glyphosate and 

aminomethylphosphonic acid in surface waters of 

agricultural basins: Pest Management Science, v. 68, no. 1, 

p. 16–30.

Craig, J.K., and Crowder, L.B., 2005, Hypoxia-induced 

habitat shifts and energetic consequences in Atlantic croaker 

and brown shrimp on the Gulf of Mexico shelf: Marine 

Ecology Progress Series, v. 294, p. 79–94, doi: 10.3354/

meps294079.

Crosswhite, W.M. and Sandretto, C.L., 1991, Trends in 

resource protection policies in agriculture, in Agricultural 

resources—Cropland, water, and conservation situation 

and outlook report: Washington, D.C., U.S. Department of 

Agriculture, Economic Research Service, no. AR–23.
184 Agriculture—A River Runs Through It—The Connections Between Agriculture and Water Quality

Crowder, D.W., Northfield, T.D., Strand, M.R., and 

Snyder, W.E., 2010, Organic agriculture promotes evenness 

and natural pest control: Nature, v. 466, p. 109–112.

Cruse, R.M., and Herndl, C.G., 2009, Balancing corn stover 

harvest for biofuels with soil and water conservation: 

Journal of Soil and Water Conservation, v. 64, no. 4, 

p. 286–291.

Dahl, T.E., 1990, Wetlands—Losses in the United States 

1780’s to 1980’s: U.S. Fish and Wildlife Service, 13 p., 

accessed March 17, 2018, at https://www.fws.gov/wetlands/

Documents/Wetlands-Losses-in-the-United-States-1780s-to1980s.pdf.

Dahl, T.E., 2006, Status and trends of wetlands in the 

conterminous United States 1998 to 2004: U.S. 

Fish and Wildlife Service, 116 p., accessed May 17, 

2013, at http://permanent.access.gpo.gov/lps71140/

StatusTrendsWetlandsConterminousUS1998to2004.pdf.

Dahl, T.E., and Allord, G.J., 1996, History of wetlands 

in the conterminous United States, in Fretwell, J.D., 

Williams, J.S., and Redman, P.J., compilers, National water 

summary on wetland resources: U.S. Geological Survey 

Water Supply Paper 2425, p. 19–26. (Available at http://

water.usgs.gov/nwsum/WSP2425/.)

Dahl, T.E., and Johnson, C.E., 1991, Wetlands—Status and 

trends in the conterminous United States, mid-1970’s to 

mid-1980’s: U.S. Fish and Wildlife Service, 28 p., accessed 

https://www.fws.gov/wetlands/Documents/WetlandsLosses-in-the-United-States-1780s-to-1980s.pdf

Debrewer, L.M., Ator, S.W., and Denver, J.M., 2007, Factors 

affecting spatial and temporal variability in nutrient and 

pesticide concentrations on the surficial aquifer on the 

Delmarva Peninsula: U.S. Geological Survey Scientific 

Investigations Report 2005-5257, 43 p. (Available at http://

pubs.usgs.gov/sir/2005/5257/.)

Denver, J.M., Tesoriero, A.J., and Barbaro, J.R., 2010, Trends 

and transformation of nutrients and pesticides in a Coastal 

Plain Aquifer System: Journal of Environmental Quality, 

v. 39, p. 154–167.

DeSimone, L.A., Hamilton, P.A., and Gilliom, R.J., 2009, 

Quality of water from domestic wells in principal aquifers 

of the United States, 1991–2004—Overview of major 

findings: U.S. Geological Survey Circular 1332, 48 p.

Dimitri, C., Effland, A., and Conklin, N.C., 2005, The 20th 

century transformation of U.S. Agriculture and Farm Policy: 

U.S. Department of Agriculture, Economic Information 

Bulletin, no. 3, 14 p., accessed May 17, 2013, at http://purl.

umn.edu/59390.

Domagalski, J.L., and Johnson, H.M., 2011, Subsurface 

transport of orthophosphate in five agricultural watersheds, 

USA: Journal of Hydrology, v. 409, no. 1–2, p. 157–171, 

accessed May 17, 2013, at http://www.sciencedirect.com/

science/article/pii/S0022169411005403.

Dougherty, T.C., Hall, A.W., and Wallingford, H.R., 1995, 

Environmental impact assessment of irrigation and drainage 

projects: Irrigation and Drainage Paper 53, ISBN 92-5-

103731 -0, accessed May 17, 2013, at http://www.fao.org/

docrep/V8350E/V8350E00.htm.

Drache, H.H., 2001, The impact of John Deere’s plow.: 

Illinois History Teacher, v. 8, no. 1, p. 2–13, accessed March 

17, 2018, at http://www.lib.niu.edu/2001/iht810102.html.

Drinkwater, L.E., Wagoner, P., and Sarrantonio, M., 1998, 

Legume-based cropping systems have reduced carbon and 

nitrogen losses: Nature, v. 396, p. 262–265.

Dubrovsky, N.M., Burow, K.R., Clark, G.M., Gronberg, J.M., 

Hamilton P.A., Hitt, K.J., Mueller, D.K., Munn, M.D., 

Nolan, B.T., Puckett, L.J., Rupert, M.G., Short, T.M., 

Spahr, N.E., Sprague, L.A., and Wilber, W.G., 2010, The 

quality of our Nation’s waters—Nutrients in the Nation’s 

streams and groundwater, 1992–2004: U.S. Geological 

Survey Circular 1350, 174 p. (Available at http://pubs.usgs.

gov/circ/1350/.)

Ebeling, W., 1979, The Fruited Plain—The Story of American 

Agriculture: Berkeley, Calif., University of California Press, 

433 p., ISBN 0-520-03751-0.

Eberts, S.M., Thomas, M.A., and Jagucki, M.L., 2013, The 

quality of our Nation’s waters—Factors affecting publicsupply-well vulnerability to contamination—Understanding 

observed water quality and anticipating future water quality: 

U.S. Geological Survey Circular 1385, 120 p. (Available 

online at https://pubs.usgs.gov/circ/1385/.)

Eisenhauer, D.E., Bjornberg, D., and Westermann, D., 

2006, Water management practices, irrigated cropland, in

Schnepf, M., and Cox, C., eds., Environmental benefits of 

conservation on cropland—The status of our knowledge: 

Ankeny, Iowa, Soil and Water Conservation Society, 

p. 133–144. ISBN 978-0-9769432-3-5.

Eldakak, M., Milad, S.I., Nawar, A.I., and Rohila, J.S., 2013, 

Proteomics—A biotechnology tool for crop improvement: 

Frontiers in Plant Science, v. 4, p. 1–12, doi: 10.3389/

fpls.2013.00035.
References Cited 185

Essaid, H.I., Baker, N.T., and McCarthy, K.A., 

2016, Contrasting nitrogen fate in watersheds using 

agricultural and water quality information: Journal of 

Environmental Quality, v. 45, no. 5, p. 1,161–1,626. 

doi:10.2134/jeq2016.02.0071.

Evans, R.O., Skaggs, Wayne, R., and Gilliam, J.W., 1995, 

Controlled versus conventional drainage effects on water 

quality: Journal of Irrigation and Drainage Engineering, 

v. 121, no. 4, p. 271–275.

Evans, Robert, and Sneed, R.F., 1996, Selection and management 

of efficient center-pivot and linear move irrigation systems: 

North Carolina Cooperative Extension Service Report EBAE91-151, accessed March 17, 2018, at https://content.ces.ncsu.

edu/selection-and-management-of-efficient-hand-move-solidset-and-permanent-irrigation-system.

Faunt, C.C., ed., 2009, Groundwater availability of the 

Central Valley aquifer, California: U.S. Geological Survey 

Professional Paper 1766, 225 p. (Available at http://pubs.

usgs.gov/pp/1766/.)

Feyereisen, G.W., Francesconi, W., Smith, D.R., 

Papiernik, S.K., Krueger, E.S., and Wente, C.D., 2015, 

Effect of replacing surface inlets with blind or gravel inlets 

on sediment and phosphorus subsurface drainage losses: 

Journal of Environmental Quality, v. 44, no. 2, p. 594–604, 

doi:10.2134/jeq2014.05.0219.

Fischer, G., van Velthuizen, H.T., Shah, M.M., and 

Nachtergaele, F.O., 2002, Global agro-ecological 

assessment for agriculture in the 21st century—

Methodology and results: International Institute for Applied 

Systems Analysis Report RR-02-002, accessed March 17, 

2018, at http://pure.iiasa.ac.at/id/eprint/6667/.

Flanagan, D.C., Ascough, J.C., II, Nicks, A.D., Nearing, 

M.A., and Laflen, J.M., 1995, Overview of the WEPP 

erosion prediction model, in Flanagan, D.C., ed., Water 

Erosion Prediction Project: West Lafayette, Indiana, U.S. 

Department of Agriculture, Hillslope Profile and Watershed 

Model Documentation, Agricultural Research Service, 

p. 12.

Flickinger, A., 2010, Iowa’s forests today: Iowa Department 

of Natural Resources, 329 p., accessed March 17, 2018, at 

www.iowadnr.gov/portals/idnr/uploads/forestry/ift_intro.pdf.

Foley, J.A., Ramankutty, N., Brauman, K.A., Cassidy, E.S., 

Gerber, J.S., Johnston, M., Mueller, N.D., O’Connell, 

C., Ray, D.K., West, P.C., Balzer, C., Bennett, E.M., 

Carpenter, S.R., Hill, J., Monfreda, C., Polasky, S., 

Rockstrom, J., Sheehan, J., Siebert, S., Tilman, D., and 

Zaks, D.P.M, 2011, Solutions for a cultivated planet: 

Nature, v. 478, p. 337–342.

Folk, R.L., 1980, Petrology of sedimentary rocks: Austin, 

Tex., Hemphill Publishing Company, 184 p., ISBN-13: 

9780914696148.

Frankforter, J.D., Weyers, H.S., Bales, J.D., Moran, P.W., 

and Calhoun, D.L., 2009, The relative influence of nutrients 

and habitat on stream metabolism in agricultural streams: 

Environmental Monitoring and Assessment, v. 168, 

p. 461–479.

Frey, J.W., Bell, A.H., Hambrook Berkman, J.A., and 

Lorenz, D.L., 2011, Assessment of nutrient enrichment by 

use of algal-, invertebrate-, and fish-community attributes 

in wadeable streams in ecoregions surrounding the Great 

Lakes: U.S. Geological Survey Scientific Investigations 

Report 2011-5009, 49 p. (Available at http://pubs.usgs.gov/

sir/2011/5009/.)

Galloway, D.L., Jones, D.R., and Ingebritsen, S.E., 2000, Land 

subsidence in the United States: U.S. Geological Survey 

Fact Sheet 165-00, 4 p. (Available at http://pubs.er.usgs.gov/

publication/fs16500.)

Galloway, J.N., and Cowling, E.B., 2002, Reactive nitrogen 

and the world—200 years of change: Ambio, v. 31, no. 2, 

p. 64–71.

García, A.M., Alexander, R.B., Arnold, J.G., Norfleet, L., 

White, M.J., Robertson, D.M., and Schwarz, G., 2016, 

Regional effects of agricultural conservation practices on 

nutrient transport in the Upper Mississippi River Basin: 

Environmental Science and Technology, v. 50, no. 13, 

p. 6,991–7,000, doi: 10.1021/acs.est.5b03543.

García, A.M., Hoos, A.B., and Terziotti, S., 2011, A regional 

modeling framework of phosphorus sources and transport 

in streams of the southeastern United States: Journal of 

the American Water Resources Association, v. 47, no. 5, 

p. 991–1,010, doi: 10.1111/j.1752-1688.2010.00517.x.

Gardner, B.L., 2002, American agriculture in the twentieth 

century: Cambridge, Mass., Harvard University Press, 395 p.

Gilliam, J.W., and Skaggs, R.W., 1986, Controlled agricultural 

drainage to maintain water quality: Journal of Irrigation and 

Drainage Engineering, v. 112, no. 3, p. 254–263.

Gilliom, R.J., Barbash, J.E., Crawford, C.G., Hamilton, P.A., 

Martin, J.D., Nakagaki, N., Nowell, L.H., Scott, J.C., 

Stackelberg, P.E., Thelin, G.P., and Wolock, D.M., 2006, 

Pesticides in the Nation’s streams and groundwater, 

1992–2001: U.S. Geological Survey Circular 1291, 172 p. 

(Available at http://pubs.usgs.gov/circ/2005/1291/.)
186 Agriculture—A River Runs Through It—The Connections Between Agriculture and Water Quality

Gilliom, R.J., and Thelin, G.P., 1997, Classification and 

mapping of agricultural land for National Water-Quality 

Assessment: U.S. Geological Survey Circular 1131, 70 p. 

(Available at http://pubs.usgs.gov/circ/1131/.)

Gollehon, N. and Quinby, W., 2006, Water and wetland 

resources, in Wiebe, K.D., and Gollehon, N.R., eds., 

Agricultural resources and environmental indicators: U.S. 

Department of Agriculture, Economic Information Bulletin 

16, p. 24–32, accessed May 17, 2013, at http://purl.umn.

edu/7207.

Grey, K.M., Capel, P.D., Baker, N.T., and Thelin, G.P., 

2012, Selected historic agricultural data important to 

environmental quality in the United States: U.S. Geological 

Survey Data Series Report 689, 2 p. plus tables. (Available 

at http://pubs.usgs.gov/ds/689/.)

Grube, A., Donaldson, D., Kiely, T., and Wu, L., 2011, 

Pesticide industry sales and usage—2006 and 2007 market 

estimates: Washington, D.C., U.S. Environmental Protection 

Agency, 33 p., accessed March 17, 2018, at https://www.

epa.gov/sites/production/files/2015-10/documents/market_

estimates2007.pdf.

Gurdak, J.J., McMahon, P.B., Dennehy, K.F., and Qi, S.L., 

2009, Water quality in the High Plains aquifer, Colorado, 

Kansas, Nebraska, New Mexico, Oklahoma, South Dakota, 

Texas, and Wyoming, 1999–2004: U.S. Geological Survey 

Circular 1337, 63 p. (Available at http://pubs.usgs.gov/

circ/1337/.)

Gurdak, J.J., and Qi, S.L., 2006, Vulnerability of recently 

recharged ground water in the High Plains aquifer to 

nitrate contamination: U.S. Geological Survey Scientific 

Investigations Report 2006-5050, 39 p. (Available at http://

pubs.usgs.gov/sir/2006/5050/.)

Hammer, D.A., and Bastian, R.K., 1989, Wetlands 

ecosystems—Natural water purifiers, in Hammer, D.A., ed., 

Constructed wetlands for wastewater treatment—Municipal, 

industrial, agricultural: Chelsea, Mich., Lewis Publishers, 

Inc., p. 5–20.

Hancock, T.C., and Brayton, M.J., 2006, Environmental 

setting of the Morgan Creek Basin, Maryland, 2002–04: 

U.S. Geological Survey Open-File Report 2006-1151, 27 p. 

(Available at http://pubs.usgs.gov/of/2006/1151/.)

Hanrahan, C.E., and Canada, C., 2011, International food 

aid—U.S. and other donor contributions: Congressional 

Research Service RS21279, 13 p., accessed May 17, 2013, 

at http://www.fas.org/sgp/crs/misc/RS21279.pdf.

Hanselman, T.A., Graetz, D.A., and Wilkie, A.C., 2003, 

Critical review manure-borne estrogens as potential 

environmental contaminants—A review: Environmental 

Science and Technology, v. 37, no. 24, p. 5,471–5,478.

Harper, D., and Hartke, E., 2009, Agricultural best 

management practices application to remediate nitrate 

contamination in a major outwash aquifer in Jackson 

County, Indiana: Indiana Geological Survey, Occasional 

Paper 70, accessed May 17, 2013, at http://igs.indiana.edu/

Groundwater/JacksonCounty.cfm?sublogo=CGDA.

Healy, R.W., Winter, T.C., LaBaugh, J.W., and Franke, O.L., 

2007, Water budgets—Foundations for effective waterresources and environmental management: U.S. Geological 

Survey Circular 1308, 90 p. (Available at http://pubs.usgs.

gov/circ/2007/1308/.)

Heap, I., 2016, The International survey of herbicide resistant 

weeds: Weed Science Web site, accessed August 22, 2016,

at http://www.weedscience.org.

Helms, D., 1992, The development of the land capability 

classification, in Readings in the history of the Soil 

Conservation Service: Washington, D.C., U.S. Department 

of Agriculture, p. 60–73, accessed May 17, 2013, at http://

www.nrcs.usda.gov/wps/portal/nrcs/detail/national/about/

history/?&cid=nrcs143_021436.

Herring, J.R., and Fantel, R.J., 1993, Phosphate rock demand 

into the next century—Impact on world food supply: 

Natural Resources Research, v. 2, no. 3, p. 226–246.

Hurt, D.R., 2002, American agriculture—A brief history 

(rev. ed.): Purdue University Press, 424 p. ISBN 978-

1557532817.

Hutchison, W.D., Burkness, E.C., Mitchell, P.D., Moon, 

R.D., Leslie, T.W., Fleischer, S.J., Abrahamson, M., 

Hamilton, K.L., Steffey, K.L., Gray, M.E., Hellmich, R.L., 

Kaster, L.V., Hunt, T.E., Wright, R.J., Pecinovsky, K., 

Rabaey, T.L., Flood, B.R., and Raun, E.S., 2010, Area 

wide suppression of European corn borer with Bt maize 

reaps savings to non-Bt maize growers: Science, v. 330, 

p. 222–225.

Hutson, S.S., Barber, N.L., Kenny, J.F., Linsey, K.S., 

Lumia, D.S., and Maupin, M.A., 2004, Estimated use of 

water in the United States in 2000: U.S. Geological Survey 

Circular 1268, 46 p. (Available at http://pubs.usgs.gov/

circ/2004/circ1268/.)

Jayne, S.O., 1907, Irrigation in the Yakima Valley, 

Washington: U.S. Department of Agriculture, Office of 

Experiment Stations Bulletin 188, 89 p.
References Cited 187

Jurgens, B.C., Fram, M.S., Belitz, Kenneth, Burow, K.R., 

and Landon, M.K., 2010, Effects of groundwater 

development on uranium—Central Valley, California, USA: 

Groundwater, v. 48, no. 6, p. 913–928. (Available at http://

ca.water.usgs.gov/pubs/2010/JurgensEtAl2010.pdf.)

Kalkhoff, S.J., Vecchia, A.V., Capel, P.D., and Meyer, M.T., 

2012, Eleven-year trend in acetanilide pesticide degradates 

in the Iowa River, Iowa: Journal of Environmental Quality, 

v. 41, no. 5, p. 1,566–1,579. 

Kenny, J.F., Barber, N.L., Hutson, S.S., Linsey, K.S., 

Lovelace, J.K., and Maupin, M.A., 2009, Estimated use of 

water in the United States in 2005: U.S. Geological Survey 

Circular 1344, 52 p. (Available at http://pubs.usgs.gov/

circ/1344/.)

Khachatourians, G.G., 1998, Agricultural use of antibiotics 

and the evolution and transfer of antibiotic-resistant 

bacteria: Canadian Medical Association Journal, v. 159, 

no. 9, p. 1,129–1,136.

Kladivko, E.J., Frankenberger, J.R., Jaynes, D.B., Meek, D.W., 

Jenkinson, B.J., and Fausey, N.R., 2004, Nitrate leaching to 

subsurface drains as affected by drain spacing and changes 

in crop production system: Journal of Environmental 

Quality, v. 33, p. 1,803–1,813.

Kling, C., 2010, Economic incentives to improve water quality 

in agricultural landscapes—Using abatement actions and 

evolutionary algorithms to support policy design: American 

Journal of Agricultural Economics, v. 93, p. 297–309.

Kollmuss, A., Zink, H., and Polycarp, C., 2008, Making 

sense of the voluntary carbon market—A comparison 

of carbon offset standards: Stockholm Environment 

Institute and Tricorona, 105 p., accessed May 17, 2013, 

at http://sei-us.org/Publications_PDF/SEI-WWFComparisonCarbonOffset-08.pdf.

Kolpin, D.W., Hoerger, C.C., Meyer, M.T., Wettstein, F.E., 

Hubbard, L.E., and Bucheli, T.D., 2010, Phytoestrogens 

and mycotoxins in Iowa streams—An examination of 

underinvestigated compounds in agricultural basins: Journal 

of Environmental Quality, v. 39, p. 2,089–2,099.

Kumar, A., Kanwar, R.S., and Hallberg, G.R., 1997, 

Separating preferential and matrix flows using subsurface 

tile flow data: Journal of Environmental Science and 

Health—Part A Toxic/Hazardous Substances and 

Environmental Engineering, v. 32, no. 6, p. 1,711–1,729.

Landers, T.F., Cohen, B., Wittum, T.T., and Larson, E.L., 2012, 

A review of antibiotic use in food animals—Perspective, 

policy, and potential: Public Health Reports, v. 127, no. 1, 

p. 4–22.

Landon, M.K., Green, C.T., Belitz, K., Singleton, M.J., 

and Esser, B.K., 2011, Relations of hydrogeologic 

factors, groundwater reduction-oxidation conditions, and 

temporal and spatial distributions of nitrate, central-eastside 

San Joaquin Valley, California, USA: Hydrogeology 

Journal, v. 19, no. 6, p. 1,203–1,224, doi 10.1007/s10040-

011-0750-1.

Lathrop, T.R., 2006, Environmental setting of the Sugar 

Creek and Leary Weber Ditch Basins, Indiana, 2002–2004: 

U.S. Geological Survey Scientific Investigations Report 

2006-5170, 27 p. (Available at http://pubs.usgs.gov/

sir/2006/5170/.)

Love, D.C., Davis, M.F., Bassett, A., Gunther, A., and 

Nachman, K.E., 2011, Dose imprecision and resistance—

Free-choice medicated feeds in industrial food animal 

production in the United States: Environmental Health 

Perspectives, v. 119, no. 3, p. 279–283.

Lowrance, R., Isenhart, T.M., Gburek, W.J., Shields, F.D., 

Wiginton, P.J., and Dabney, S.M., 2006, Landscape 

management, in Schnepf, M., and Cox, C., eds., 

Environmental benefits of conservation on cropland—

The status of our knowledge: Ankeny, Iowa, Soil and Water 

Conservation Society, p. 269–317., ISBN 978-0-9769432-3-5.

MacDonald, J.M., Ribaudo, M.O., Livingston, M.J., 

Beckman, J., and Huang, W., 2009, Manure use for fertilizer 

and for energy—Report to Congress: U.S. Department of 

Agriculture, Economic Research Service Administrative 

Publication No. (AP-037), 53 p., acessed March 17, 

2018, at https://www.ers.usda.gov/publications/pubdetails/?pubid=42740.

Mackun, P., and Wilson, S., 2011, Population distribution and 

change—2000 to 2010: U.S. Department of Commerce, U.S. 

Census Bureau C2010BR-01, accessed May 17, 2013, at 

http://www.census.gov/prod/cen2010/briefs/c2010br-01.pdf.

Mäder, P., Fliessbach, A., Dubois, D., Gunst, L., Fried, P., and 

Niggli, U., 2002, Soil fertility and biodiversity in organic 

farming: Science, v. 296, p. 1,694–1,697.

Madramootoo, C.A., Johnston, W.R., and Willardson, L.S., 

eds., 1997, Management of agricultural drainage water 

quality: Food and Agriculture Organization of the United 

Nations International Commission on Irrigation and 

Drainage, Water Report 13, accessed May 17, 2013, at 

http://www.fao.org/docrep/W7224E/W7224E00.htm.

Majewski, M.S., Coupe, R.H., Foreman, W.T., and 

Capel, P.D., 2014, Pesticides in Mississippi air and rain—A 

comparison between 1995 and 2007: Environmental 

Toxicology and Chemistry. v. 33, n. 6, p. 1,283–1,293, doi: 

10.1002/etc.2550.
188 Agriculture—A River Runs Through It—The Connections Between Agriculture and Water Quality

Martínez-Carballo, Elena, González-Barreiro, Carmen, Scharf, 

Sigrid, and Gans, Oliver, 2007, Environmental monitoring 

study of selected veterinary antibiotics in animal manure 

and soils in Austria: Environmental Pollution, v. 148, no. 2, 

p. 570–579, doi: 10.1016/j.envpol.2006.11.035.

McCarthy, K.A., and Johnson, H.M., 2009, Effect of 

agricultural practices on hydrology and water chemistry in a 

small irrigated catchment, Yakima River Basin, Washington: 

U.S. Geological Survey Scientific Investigations Report 

2009-5030, 22 p. (Available at http://pubs.usgs.gov/

sir/2009/5030/.) 

McGuire, V.L., 2009, Changes in water levels and storage 

in the High Plains aquifer, predevelopment to 2007: U.S. 

Geological Survey Fact Sheet 2009-3005, 2 p. (Available at 

http://pubs.usgs.gov/fs/2009/3005/.)

McGuire, V.L., 2011, Water-level changes in the High Plains 

aquifer, predevelopment to 2009, 2007–08, and 2008–09, 

and change in water in storage, predevelopment to 2009: 

U.S. Geological Survey Scientific Investigations Report 

2011-5089, 13 p. (Available at http://pubs.usgs.gov/

sir/2011/5089/.)

McGuire, V.L., Ryter, D.W., and Flynn, A.S., 2012, Altitude, 

age, and quality of groundwater, Papio-Missouri River 

Natural Resources District, eastern Nebraska, 1992 to 

2009: U.S. Geological Survey Scientific Investigations 

Report 2012–5036, 68 p. (Available at http://pubs.usgs.gov/

sir/2011/5089/.)

McMahon, P.B., Böhlke, J.K., Kauffman, L.J., Kipp, 

K.L., Landon, M.K., Crandall, C.A., Burow, K.R., and 

Brown, C.J., 2008, Source and transport controls on the 

movement of nitrate to public supply wells in selected 

principal aquifers of the United States: Water Resources 

Research, 44, W04401, doi:10.1029/2007WR006252.

McMahon, P.B., Burow, K.R., Kauffman, L.J., Eberts, S.M., 

Bohlke, J.K., and Gurdak, J.J., 2008, Simulated response 

of water quality in public supply wells to land use 

change: Water Resources Research, v. 44, W00A06, 

doi:10.1029/2007WR006731.

McMahon, P.B., Dennehy, K.F., Bruce, B.W., Böhlke, J.K., 

Michel, R.L., Gurdak, J.J., and Hurlbut, D.A., 2006, 

Storage and transit time of chemicals in thick unsaturated 

zones under rangeland and irrigated cropland, High 

Plains, USA: Water Resources Research, 42, W03413, 

doi:10.1029/2005WR004417.

McMahon, P.B., Dennehy, K.F., Bruce, B.W., Gurdak, J.J., and 

Qi, S.L., 2007, Water-quality assessment of the High Plains 

aquifer, 1999–2004: U.S. Geological Survey Professional 

Paper 1749, 136 p. (Available at http://pubs.usgs.gov/

pp/1749/.)

Merced Irrigation District, 2013, Merced Irrigation District—

Water and power: Merced Irrigation District Web site, 

accessed January 14, 2013, at http://www.mercedid.org.

Merten, G.H., Welch, H.L., and Tomer, M.D., 2016, Effects 

of hydrology, watershed size, and agricultural practices on 

sediment yields in two river basins in Iowa and Mississippi: 

Journal of Soil and Water Conservation, v. 71, no. 3, 

p. 267–278, doi: 10.2489/jswc.71.3.267.

Mitchell, R., Vogel, K.P., and Uden, D.R., 2014, The 

feasibility of switchgrass for biofuel production, Biofuels, 

v. 3, no. 1, p. 47–59. doi: 10.4155/bfs.11.153.

Monsanto Company, 2013, Company history: Monsanto 

Company Web site, accessed May 17, 2013, at http://www.

monsanto.com/whoweare/Pages/monsanto-history.aspx.

Morgan, D.S., Dinicola, R.S., and Bartolino, J.R., 2008, 

Ground-water availability assessment for the Columbia 

Plateau Regional Aquifer System, Washington, Oregon, and 

Idaho: U.S. Geological Survey Fact Sheet 2008-3086, 2 p. 

(Available at http://pubs.usgs.gov/fs/2008/3086/.)

National Oceanic and Atmospheric Administration, 2015, 

2015 Gulf of Mexico dead zone ‘above average’: National 

Oceanic and Atmospheric Administration Web site, 

accessed July 22, 2016, at http://www.noaanews.noaa.gov/

stories2015/080415-gulf-of-mexico-dead-zone-aboveaverage.html.

National Research Council, 1999, New strategies for 

America’s watersheds: Washington, D.C., National 

Academies Press, 328 p., doi: 10.17226/6020, accessed 

July 10, 2016, at http://www.nap.edu/catalog/6020/newstrategies-for-americas-watersheds.

National Research Council, 2010, The impact of genetically 

engineered crops on farm sustainability in the United States: 

Washington, D.C., National Academies Press, 250 p., doi: 

10.17226/12804, accessed May 17, 2013, at http://www.

nap.edu/catalog.php?record_id=12804.

Neitsch, S.L., Arnold, J.G., Kiniry, J.R., Williams, J.R., 

and King, K.W., 2002, Soil and water assessment tool 

theoretical documentation, ver. 2000: College Station, Tex., 

Texas Water Resources Institute, TWRI Report TR-191, 

506 p., accessed May 17, 2013, at http://swat.tamu.edu/

media/1290/swat2000theory.pdf. 
References Cited 189

Nickerson, C., Ebel, R., Borchers, A., and Carriazo, F., 

2011, Major uses of land in the United States, 2007: 

U.S. Department of Agriculture, Economic Information 

Bulletin 89, 67 p.

Nolan, B.T., and Hitt, K.J., 2006, Vulnerability of shallow 

groundwater and drinking-water wells to nitrate in the 

United States: Environmental Science and Technology, 

v. 40, no. 24, p. 7,834–7,840.

Nowak, D.J., and Walton, J.T., 2005, Projected urban growth 

(2000–2050) and its estimated impact on the U.S. Forest 

Resource: Journal of Forestry, v. 103, no. 8, p. 383–389.

Osmond, D.L., Meals, D.W., Hoag, D.L., and Arabi, M., 

eds., 2012, How to build better agricultural conservation 

programs to protect water quality—The National Institute 

of Food and Agriculture Conservation Effects Assessment 

Project Experience: Ankeny, Iowa, Soil and Water 

Conservation Society, 387 p.

Parisi, C., Vigani, M., and Rodríguez-Cerezo, E., 2015, 

Agricultural nanotechnologies—What are the current 

possibilities?: Nano Today, v. 10, no. 2, p. 124–127, 

doi:10.1016/j.nantod.2014.09.009.

Pavelis, G.A., 1987, Farm drainage in the United States—

History, status, and prospects: U.S. Department of 

Agriculture, Economic Research Service, Miscellaneous 

Publication Number 1455, 170 p. 

Payne, K.L., Johnson, H.M., and Black, R.W., 2007, 

Environmental setting of the Granger Drain and DR2 

basins, Washington, 2003–04: U.S. Geological Survey 

Scientific Investigations Report 2007-5102, 26 p. (Available 

at http://pubs.usgs.gov/sir/2007/5102/.)

Petrolia, D.R., and Gowda, P.H., 2006, Missing the boat—

Midwest farm drainage and Gulf of Mexico hypoxia: 

Review of Agricultural Economics, v. 28, no. 2, p. 240–253.

Pimentel, D., Hepperly, P., Hanson, J., Douds, D., and 

Seidel, R., 2005, Environmental, energetic, and economic 

comparisons of organic and conventional farming systems: 

BioScience, v. 55, no. 7, p. 573.

Pimentel, D., and Patzek, T.W., 2005, Ethanol production 

using corn, switchgrass, and wood; biodiesel production 

using soybean and sunflower: Natural Resources Research, 

v. 14, no. 1, p. 65–76.

Puckett, L.J., Tesoriero, A.J., and Dubrovsky, N.M., 2011, 

Nitrogen contamination of surficial aquifers—A growing 

legacy: Environmental Science and Technology, v. 45, no. 3, 

p. 839–844.

Rakocinski, C.F., Brown, S.S., Gaston, G.R., Heard, R.W., 

Walker, W.W., and Summers, J.K., 1997, Macrobenthic 

responses to natural and contaminant-related gradients in 

northern Gulf of Mexico estuaries: Ecological Applications, 

v. 7, no. 4, p. 1,278–1,298.

Rabalais, N.N, Turner, R.E., and Wiseman, W.J., Jr., 2002, 

Gulf of Mexico Hypoxia, a.k.a. “The Dead Zone”: Annual 

Review of Ecology and Systematics, v. 33, p. 235–263.

Ravishankara, A.R., Daniel, J.S., and Portmann, R.W., 2009, 

Nitrous oxide (N2

O)—The dominant ozone-depleting 

substance emitted in the 21st century: Science, v. 326, 

p. 123–125.

Raymond, P.A., Oh, N.-H., Turner, R.E., and Broussard, W., 

2008, Anthropogenically enhanced fluxes of water 

and carbon from the Mississippi River: Nature, v. 451, 

p. 449–452.

Reeder, R., and Westermann, D., 2006, Soil management 

practices, in Schnepf, M., and Cox, C., eds., Environmental 

benefits of conservation on cropland—The status of our 

knowledge: Ankeny, Iowa, Soil and Water Conservation 

Society, p. 1–88., ISBN 978-0-9769432-3-5.

Reilly, T.E., Dennehy, K.F., Alley, W.M., and 

Cunningham, W.L., 2008, Ground-water availability in the 

United States: U.S. Geological Survey Circular 1323, 70 p. 

(Available at http://pubs.usgs.gov/circ/1323/.)

Robertson, D.M., and Saad, D.A., 2013, SPARROW Models 

used to understand nutrient sources in the Mississippi/

Atchafalaya River Basin: Journal of Environmental Quality, 

v. 42, no. 5, p. 1,422–1,440, doi:10.2134/jeq2013.02.0066.

Rodrigues, P., Eckersall, D., and de Almeida, A., eds., 2012, 

Farm animal proteomics: Wageningen, The Netherlands, 

Wageningen Academic Publishers, 210 p., doi 10.3920/978-

90-8686-751-6.

Rose, C.E., Coupe, R.C., Capel, P.D., and Webb, R.M.T., 

2018, Holistic assessment of occurrence and fate of 

metolachlor within environmental compartments of 

agricultural watersheds: Science of the Total Environment, 

v. 612, p. 708–719. doi: 10.1016/j.scitotenv.2017.08.154.

Rosenzweig, C.E., Tubiello, F., Goldberg, R., Mills, E., and 

Bloomfield, J., 2002, Increased crop damage in the U.S. 

from excess precipitation under climate change: Global 

Environmental Change, v. 12, no. 3, p. 197–202. doi: 

10.1016/S0959-3780(02)00008-0.
190 Agriculture—A River Runs Through It—The Connections Between Agriculture and Water Quality

Rosi-Marshall, E.J., Tank, J.L., Royer, T.V., Whiles, 

M.R., Evans-White, M., Chambers, C., Griffiths, N.A., 

Pokelsek, J., and Stephen, M.L., 2007, Toxins in transgenic 

crop byproducts may affect headwater stream ecosystems: 

Proceedings of the National Academy of Sciences of the 

United States of America, v. 104, no. 41, p. 16,204–16,208.

Roth, J.L, 2010, Water, sediment, and nutrient transport from a 

drained topographical depression within an agricultural field 

in North-Central Iowa: University of Minnesota, Master’s 

thesis, 94 p., accessed March 10, 2015, at http://purl.umn.

edu/102695.

Roth, J.L., and Capel, P.D., 2012a, Changes in the water 

budget of and sediment yield from an agricultural field as 

a function of land and management characteristics—A unit 

field modeling approach: U.S. Geological Survey Scientific 

Investigation Report 2012-5203, 42 p. (Available at http://

pubs.usgs.gov/sir/2012/5203/.)

Roth, J.L., and Capel, P.D., 2012b, The hydrology of a drained 

topographical depression within an agricultural field in 

North-Central Iowa: Transactions of the American Society 

of Agricultural and Biological Engineers, v. 55, no. 5, 

p. 1,801–1,814.

Russo, R.C., 1985, Ammonia, nitrite and nitrate, in Rand, 

G.M., and Petrocelli, S.R., eds., Fundamentals of aquatic 

toxicology—Methods and applications: Washington, D.C., 

Hemisphere Publishing, p. 455–471.

Ryberg, K.R., Vecchia, A.V., Gilliom, R.J., and Martin, 

J.D., 2014, Pesticide trends in major rivers of the United 

States, 1992–2010: U.S. Geological Survey Scientific 

Investigations Report 2014–5135, 63 p., http://dx.doi.

org/10.3133/sir20145135. 

Scanlon, B.R., Reedy, R.C., Stonestrom, D.A., Prudic, D.E., 

and Dennehy, K.F., 2005, Impact of land use and land 

cover change on groundwater recharge and quality in the 

southwestern U.S.: Global Change Biology, v. 11, no. 10, 

p. 1,577–1,593.

Schaible, G.D., and Aillery, M.P., 2012, Water conservation 

in irrigated agriculture—Trends and challenges in the face 

of emerging demands: U.S. Department of Agriculture, 

Economic Research Service, Economic Information 

Bulletin No. 99, 60 p.

Schilling, K.E., Jha, M.K., Zhang, Y.-K., Gassman, P.W., 

and Wolter, C.F., 2008, Impact of land use and land 

cover change on the water balance of a large agricultural 

watershed—Historical effects and future directions: Water 

Resources Research, v. 44, p. 1–12.

Schuck, E.C., Frasiera, W.M., Webb, R.S., Ellingson, L.J., 

and Umberger, W.J., 2005, Adoption of more technically 

efficient irrigation systems as a drought response: 

International Journal of Water Resources Development, 

v. 21, no. 4, p. 651–662, doi :10.1080/07900620500363321.

Scott, N., and Chen, H., 2012, Nanoscale science and 

engineering for agriculture and food systems: Industrial 

Biotechnology, v. 8, no. 6, p. 340–343, doi: 10.1089/

ind.2012.1549.

Seiber, J.N., Wilson, B.W., and McChesney, M.M., 1993, 

Air and fog deposition residues of four organophosphorus 

insecticides used on dormant orchards in the San Joaquin 

Valley, California: Environmental Science and Technology 

v. 27, p. 2,236–2,243.

Senthil-Kumar, M., and Mysore, K.S., 2010, RNAi in plants—

Recent developments and applications in agriculture, 

chap. 7 of Catalano, A.J., ed., Gene silencing—Theory, 

techniques and applications: Nova Science Publishers, Inc., 

p. 183–199, ISBN: 1-61728-276-8.

Shanley, J.B., Kendall, C., Smith, T.E., Wolock, D.M., and 

McDonnell, J.J., 2002, Controls on old and new water 

contributions to stream flow at some nested catchments 

in Vermont, USA: Hydrological Processes, v. 16, no. 3, 

p. 589–609.

Sharpley, A.N., Withers, P.J.A., Abdalla, C., and Dodd, A., 

2005, Strategies for the sustainable management of 

phosphorus, in Sims, J.T., and Sharpley, A.N., eds., 

Phosphorus—Agriculture and the Environment: Madison, 

Wis., American Society of Agronomy Monograph 46, 

p. 1,069–1,101. 

Sherman, J.H., Munyikwa, T., Chan, S.Y., Petrick, J.S., 

Witwer, K.W., and Choudhuri, S., 2015, RNAi technologies 

in agricultural biotechnology—The toxicology Forum 

40th Annual Summer Meeting: Regulatory Toxicology 

and Pharmacology, v. 73, no. 2, p. 671–680, doi:10.1016/j.

yrtph.2015.09.001.

Sloto, R.A., and Crouse, M.Y., 1996, HYSEP—A computer 

program for streamflow hydrograph separation and analysis: 

U.S. Geological Survey Water-Resources Investigations 

Report 96-4040, 46 p. 

Smith, D., and Joern, B., 2012, Dietary phytase to reduce 

phosphorus losses from animal manure: SERA-17 Report, 

2 p., accessed March 17, 2018, at http://www.cost869.

alterra.nl/SERA17_BMP/BMP_dietary_phytase.pdf.
References Cited 191

Smith, E.A., 2012, Spatial and temporal variability in a 

subsurface-drained landscape in north-central Iowa: St. 

Paul, Minn., University of Minnesota, PhD Dissertation, 

Water Resources Science, 254 p., accessed March 17, 2018, 

at http://hdl.handle.net/11299/122877.

Snow, A.A., Andow, D.A., Gepts, P., Hallerman, E.M., 

Power, A., Tiedje, J.M., and Wolfenbarger, L.L., 2005, 

Genetically engineered organisms and the environment—

Current status and recommendations: Ecological 

Applications, v. 15, no. 2, p. 377–404.

Soil Science Society of America, 2012, Glossary of soil 

science terms: Soil Science Society of America database, 

accessed December 6, 2012, at https://www.soils.org/

publications/soils-glossary. 

Spielmaker, Debra, 2012, Growing a Nation—The story of 

American agriculture—Historical timeline: Utah State 

University Web site, accessed May 17, 2013, at http://www.

agclassroom.org/gan/timeline/index.htm.

Stanton, J.S., and Fahlquist, Lynne, 2006, Ground-water 

quality beneath irrigated cropland of the northern and 

southern High Plains aquifer, Nebraska and Texas, 2003–

04: U.S. Geological Survey Scientific Investigations Report 

2006-5196, 39 p., apps. on compact disk. (Available at 

http://pubs.usgs.gov/sir/2006/5196/.)

Stanton, J.S., Peterson, S.M., and Fienen, M.N., 2010, 

Simulation of groundwater flow and effects of groundwater 

irrigation on stream base flow in the Elkhorn and Loup 

River Basins, Nebraska, 1895–2055—Phase Two: U.S. 

Geological Survey Scientific Investigations Report 2010-

5149, 78 p., with apps. (Available at http://pubs.usgs.gov/

sir/2010/5149/.)

Steele, G.V., Johnson, H.M., Sandstrom, M.W., Capel, P.D., 

and Barbash, J.E., 2008, Occurrence and fate of pesticides 

in four contrasting agricultural settings in the United 

States: Journal of Environmental Quality, v. 37, no. 3, 

p. 1,116–1,132.

Steinwand, A.L., and Fenton, T.E., 1995, Landscape evolution 

and shallow groundwater hydrology of a till landscape 

in central Iowa: Soil Science Society of America, v. 59, 

p. 1,370–1,377.

Stets, E.G., Kelly, V.J., and Crawford, C.G., 2015, Regional 

and temporal differences in riverine nitrate trends discerned 

from long-term water quality monitoring data: Journal of 

the American Water Resources Association, v. 51, no. 5, 

p. 1,394–1,407, doi: 10.1111/1752-1688.12321.

Stone, W.W., and Bucknell, M.S., 2014, Watershed 

Regressions for Pesticides (WARP): U.S. Geological Survey 

Web page, http://dx.doi.org/10.5066/F7R20ZD3, accessed 

October 17, 2016, at https://cida.usgs.gov/warp/.

Stone, W.W., Crawford, C.G., and Gilliom, R.J., 2013, 

Watershed Regressions for Pesticides (WARP) models for 

predicting stream concentrations of multiple pesticides: 

Journal of Environmental Quality, v. 42, p. 1,838–1,851, 

doi:10.2134/jeq2013.05.0179.

Stone, W.W., and Wilson, J.T., 2006, Preferential flow 

estimates to an agricultural tile drain with implications for 

glyphosate transport: Journal of Environmental Quality, 

v. 35, no. 5, p. 1,825–1,835, doi: 10.2134/jeq2006.0068.

Sullivan, D.J., Vecchia, A.V., Lorenz, D.L., Gilliom, R.J., 

and Martin, J.D., 2009, Trends in pesticide concentrations 

in corn-belt streams, 1996–2006: U.S. Geological Survey 

Scientific Investigations Report 2009-5132, 75 p. (Available 

at http://pubs.usgs.gov/sir/2009/5132/.)

Tabashnik, B.E., Gassmann, A.J., Crowder, D.W., and 

Carriére, Y., 2008, Insect resistance to Bt crops—Evidence 

versus theory: Nature Biotechnology, v. 26, no. 2, 

p. 199–202.

Terziotti, Silvia, Capel, P.D., Tesoriero, A.J., Hopple, J.A., 

and Kronholm, S.C., 2017, Estimates of nitrate loads and 

yields from groundwater to streams in the Chesapeake Bay 

watershed based on land use and geology: U.S. Geological 

Survey Scientific Investigations Report 2017–5160, 20 p., 

https://doi.org/10.3133/sir20175160.

Tesoriero, A.J., Duff, J.H., Saad, D.A., Spahr, N.E., and 

Wolock, D.M., 2013, Vulnerability of streams to legacy 

nitrate sources: Environmental Science and Technology, 

v. 47, p. 3,623–3,629, doi: 10.1021/es305026x.

Tilman, D., 1998, The greening of the green revolution: 

Nature, v. 396, p. 211–212.

Tomer, M.D., and Locke, M.A., 2011, The challenge of 

documenting water quality benefits of conservation 

practices—A review of USDA-ARS’s conservation effects 

assessment project watershed studies: Water Science 

and Technology, v. 6, no. 1, p. 300–310, doi: 10.2166/

wst.2011.555.

Tomer, M.D., Wilson, C.G., Moorman, T.B., Cole, K.J., 

Heer, D., and Isenhart, T.M., 2010, Source-pathway 

separation of multiple contaminants during a rainfall-runoff 

event in an artificially drained agricultural watershed: 

Journal of Environmental Quality, v. 39, p. 882–895. 
192 Agriculture—A River Runs Through It—The Connections Between Agriculture and Water Quality

Tweeten, L.G., and Thompson, S.R., 2008, Long-term global 

agricultural output supply-demand balance and real farm 

and food prices: Ohio State University, Department of 

Agricultural, Environmental, and Development Economics 

Working Paper, AEDE-WP 0044-0833 p., accessed May 17, 

2013, at http://purl.umn.edu/46009.

United Nations, 2009, World population to exceed 9 billion 

by 2050: United Nations Press Release, accessed May 17, 

2013, at http://www.un.org/esa/population/publications/

wpp2008/pressrelease.pdf.

U.S. Census Bureau, 2013, 2012 National population 

projections, summary tables: U.S. Census Bureau database, 

accessed March 17, 2018, at https://www.census.gov/data/

tables/2012/demo/popproj/2012-summary-tables.html.

U.S. Department of Agriculture, 2009, 2007 Census of 

agriculture, United States summary and State data: U.S. 

Department of Agriculture Report AC-07-A-51, 739 p., 

accessed May 17, 2013, at http://www.agcensus.usda.gov/

Publications/2007/Full_Report/.

U.S. Department of Agriculture, 2010, 2007 Census of 

agriculture, farm and ranch irrigation survey (2008): U.S. 

Department of Agriculture Report AC-07-SS1, v. 3, no. 1, 

268 p., accessed May 17, 2013, at http://www.agcensus.

usda.gov/Publications/2007/Online_Highlights/Farm_and_

Ranch_Irrigation_Survey/fris08.pdf.

U.S. Department of Agriculture, 2011, Agriculture Secretary 

Vilsack announces funding to expand and improve 

broadband services in rural areas: News Release No. 

0485.11, accessed September 19, 2012, at http://www.usda.

gov/wps/portal/usda/usdahome?contentid=2011/11/0485.

xml&navid=NEWS_RELEASE&navtype=RT&p

arentnav=LATEST_RELEASES&edeployment_

action=retrievecontent.

U.S. Department of Agriculture, 2012, Environmental quality 

incentives program: U.S. Department of Agriculture Web 

site, accessed May 17, 2013, at http://www.nrcs.usda.gov/

wps/portal/nrcs/main/national/programs/financial/eqip/.

U.S. Department of Agriculture, 2013, United States 2012 census 

of agriculture: U.S. Department of Agriculture Form Number 

12-A101, accessed May 17, 2013, at http://www.agcensus.

usda.gov/Help/Report_Form_and_Instructions/2012_Report_

Form/Region1_12a101_121311.pdf.

U.S. Department of Agriculture, 2016, 2007 Census Ag Atlas 

Maps, acres used for organic production: U.S. Department 

of Agriculture Map No. 07-M097, Web site, accessed 

August 22, 2016, at https://www.agcensus.usda.gov/

Publications/2007/Online_Highlights/Ag_Atlas_Maps/

Farms/.

U.S. Department of Agriculture, Agricultural Marketing 

Service, 2012a, National Organic Program: U.S. 

Department of Agriculture Web site, accessed May 17, 

2013, at http://www.ams.usda.gov/AMSv1.0/nop.

U.S. Department of Agriculture, Agricultural Marketing 

Service, 2012b, National Organic Program—

National list of allowed and prohibited substances: 

U.S. Department of Agriculture Web site, accessed 

May 17, 2013, at http://www.ams.usda.gov/AMSv1.0/

NOPPetitionedSubstancesDatabase.

U.S. Department of Agriculture, Agricultural Research 

Service, 2008, ARS timeline—144 years of ag research—

History of research at the U.S. Department of Agriculture 

and Agricultural Research Service: U.S. Department of 

Agriculture Web site, accessed May 17, 2013, at http://

www.ars.usda.gov/is/timeline/.

U.S. Department of Agriculture, Agricultural Research 

Service, 2010, Revised universal soil loss equation 

(RUSLE)—Welcome to RUSLE 1 and RUSLE 2: U.S. 

Department of Agriculture Web site, accessed May 17, 

2013, at http://www.ars.usda.gov/Research/docs.

htm?docid=5971.

U.S. Department of Agriculture, Economic Research Service, 

2010a, Organic production: U.S. Department of Agriculture 

Web site, accessed May 17, 2013, at http://www.ers.usda.

gov/data-products/organic-production.aspx.

U.S. Department of Agriculture, Economic Research Service, 

2010b, Fertilizer use and price: U.S. Department of 

Agriculture Web site, accessed May 17, 2013, at http://www.

ers.usda.gov/data-products/fertilizer-use-and-price.aspx.

U.S. Department of Agriculture, Economic Research Service, 

2017, Recent trends in GE adoption: U.S. Department of 

Agriculture Web site, accessed November 9, 2017, at https://

www.ers.usda.gov/data-products/adoption-of-geneticallyengineered-crops-in-the-us/recent-trends-in-ge-adoption/.

U.S. Department of Agriculture, Economic Research Service, 

2015, Irrigation and water use: U.S. Department of 

Agriculture Web site, accessed August 17, 2016, at http://

www.ers.usda.gov/topics/farm-practices-management/

irrigation-water-use/background.aspx. 

U.S. Department of Agriculture, Economic Research Service, 

2106a, Organic market overview: U.S. Department of 

Agriculture Web site, accessed August 17, 2016, at http://

www.ers.usda.gov/topics/natural-resources-environment/

organic-agriculture/organic-market-overview.aspx. 
References Cited 193

U.S. Department of Agriculture, Economic Research Service, 

2016b, Organic production, overview, table 2—U.S. 

certified organic farmland acreage, livestock numbers, 

and farm operations: U.S. Department of Agriculture 

Web site, accessed August 17, 2016, at http://www.ers.

usda.gov/datafiles/Organic_Production/National_Tables_/

Farmlandlivestockandfarm.xls.

U.S. Department of Agriculture, Economic Research Service, 

2016c, Feed grains—Yearbook tables: U.S. Department of 

Agriculture Web site, accessed August 17, 2016, at http://

www.ers.usda.gov/data-products/feed-grains-database/feedgrains-yearbook-tables.aspx.

U.S. Department of Agriculture, Economic Research Service, 

2016d, Animal products: U.S. Department of Agriculture 

Web site, accessed August 17, 2016, at http://www.ers.usda.

gov/topics/animal-products.aspx.

U.S. Department of Agriculture, Farm Service Agency, 

2012, Conservation programs—Conservation reserve 

enhancement program: U.S. Department of Agriculture Web 

site, accessed May 17, 2013, at https://www.fsa.usda.gov/

FSA/webapp?area=home&subject=copr&topic=cep.

U.S. Department of Agriculture, Farm Service Agency, 

2013, Conservation reserve program (CRP) evolving: U.S. 

Department of Agriculture News Release, June 19, 2012, 

accessed May 17, 2013, at http://www.fsa.usda.gov/FSA/

newsReleases?mystate=ne&area=stnewsroom&subject=stnr

&topic=landing&newstype=stnewsrel&type=detail&item=s

tnr_ne_20120619_rel_244.html.

U.S. Department of Agriculture, National Agricultural Library, 

2012a, Water-quality information center: U.S. Department 

of Agriculture Web site, accessed May 17, 2013, at http://

wqic.nal.usda.gov.

U.S. Department of Agriculture, National Agricultural Library, 

2012b, Water quality: U.S. Department of Agriculture Web 

site, accessed May 17, 2013, at http://wqic.nal.usda.gov/

water-quality.

U.S. Department of Agriculture, National Agricultural Library, 

2015, Sustainable agriculture—Definitions and terms: U.S. 

Department of Agriculture Web site, accessed May 17, 

2013, at http://afsic.nal.usda.gov/sustainable-agriculturedefinitions-and-terms-1.

U.S. Department of Agriculture, National Institute of Food 

and Agriculture, 2018, Organic agriculture program: U.S. 

Department of Agriculture Web site, accessed March 18, 

2018, at https://nifa.usda.gov/program/organic-agricultureprogram.

U.S. Department of Agriculture, Natural Resources 

Conservation Service, 2009, Wildlife habitat incentives 

program: U.S. Department of Agriculture Web site, accessed 

May 17, 2013, at http://www.nrcs.usda.gov/wps/portal/nrcs/

main/national/programs/financial/whip/.

U.S. Department of Agriculture, Natural Resources 

Conservation Service, 2010, Tillage equipment pocket 

identification guide: U.S. Department of Agriculture pocket 

guide, 78 p., accessed March 18, 2018, at https://www.nrcs.

usda.gov/Internet/FSE_DOCUMENTS/nrcs142p2_007135.

pdf.

U.S. Department of Agriculture, Natural Resources 

Conservation Service, 2011, National conservation 

practice standards: U.S. Department of Agriculture Web 

site, accessed May 17, 2013, at http://www.nrcs.usda.

gov/wps/portal/nrcs/detail/national/technical/?cid=NRCS

DEV11_001020.

U.S. Department of Agriculture, Natural Resources 

Conservation Service, 2012a, Conservation effects 

assessment project (CEAP): U.S. Department of Agriculture 

Web site, accessed May 17, 2013, at http://www.nrcs.usda.

gov/wps/portal/nrcs/main/national/technical/nra/ceap.

U.S. Department of Agriculture, Natural Resources 

Conservation Service, 2012b, Conservation reserve 

program: U.S. Department of Agriculture Web site, accessed 

May 17, 2013, at http://www.nrcs.usda.gov/wps/portal/nrcs/

detail/national/technical/nra/ceap/?cid=stelprdb1041269.

U.S. Department of Agriculture, Natural Resources 

Conservation Service, 2012c, Wetlands reserve program: 

U.S. Department of Agriculture Web site, accessed May 17, 

2013, at http://www.nrcs.usda.gov/wps/portal/nrcs/main/

national/programs/easements/wetlands/.

U.S. Department of Agriculture, National Soil Erosion 

Research Laboratory, 2010, WEPP downloads, version 

2010.1: U.S. Department of Agriculture Web site, accessed 

May 17, 2013, at http://www.ars.usda.gov/Research/docs.

htm?docid=18084. 

U.S. Department of Commerce, 1981, Drainage of agricultural 

lands: U.S. Department of Commerce, Census of 

Agriculture, 1978, v. 5, 98 p.

U.S. Energy Information Administration, 2014, Monthly 

energy review, December 2014: U.S. Energy Information 

Administration report DOE/EIA-0035(2014/12), 201 p., 

accessed March 10, 2015, at http://www.eia.gov/

totalenergy/data/monthly/archive/00351412.pdf.
194 Agriculture—A River Runs Through It—The Connections Between Agriculture and Water Quality

U.S. Environmental Protection Agency, 2009, National 

primary drinking water regulations: U.S. Environmental 

Protection Agency publication, 6 p., accessed May 17, 

2013, at http://water.epa.gov/drink/contaminants/upload/

mcl-2.pdf.

U.S. Environmental Protection Agency, 2011, Animal feeding 

operations overview: U.S. Environmental Protection 

Agency Web site, accessed March 18, 2018, at https://www.

epa.gov/npdes/animal-feeding-operations-afos.

U.S. Environmental Protection Agency, 2012a, Overview of 

greenhouse gases: U.S. Environmental Protection Agency 

Web site accessed March 7, 2016, at http://www3.epa.gov/

climatechange/ghgemissions/gases/n2o.html. 

U.S. Environmental Protection Agency, 2012b, Wetlands: U.S. 

Environmental Protection Agency Web site, accessed May 17, 

2013, at http://water.epa.gov/type/wetlands/index.cfm.

U.S. Environmental Protection Agency, 2012c, What is a 

CAFO?: U.S. Environmental Protection Agency Web site, 

accessed May 17, 2013, at http://www.epa.gov/region7/

water/cafo/index.htm.

U.S. Environmental Protection Agency, 2012d, DDT—A brief 

history and status: U.S. Environmental Protection Agency 

Web site, accessed May 17, 2013, at http://www.epa.gov/

pesticides/factsheets/chemicals/ddt-brief-history-status.htm. 

U.S. Environmental Protection Agency, 2012e, Agriculture: 

U.S. Environmental Protection Agency Web site, accessed 

May 17, 2013, at http://water.epa.gov/polwaste/nps/

agriculture.cfm.

U.S. Environmental Protection Agency, 2012f, Ammonia: 

U.S. Environmental Protection Agency Web site, accessed 

March 18, 2018, at https://www.epa.gov/wqc/aquatic-lifecriteria-ammonia.

U.S. Environmental Protection Agency, 2012g, Relation 

between nitrate in water wells and potential sources in the 

Lower Yakima Valley, Washington: U.S. Environmental 

Protection Agency report EPA-910-R-12-003, accessed 

March 18, 2018, at https://www.epa.gov/sites/production/

files/2017-12/documents/lower-yakima-valley-groundwaterreport-2013.pdf.

U.S. Environmental Protection Agency, 2012h, Watershed 

assessment, tracking and environmental results: U.S. 

Environmental Protection Agency Web site, accessed 

May 17, 2013, at http://www.epa.gov/waters/.

U.S. Environmental Protection Agency, 2012i, Sustainability: 

U.S. Environmental Protection Agency Web site, accessed 

May 17, 2013, at http://www.epa.gov/agriculture/tsus.html.

U.S. Environmental Protection Agency, 2015, Mississippi 

River/Gulf of Mexico Watershed Nutrient Task Force, 2015 

Report to Congress, August 2015: U.S. Environmental 

Protection Agency, 93 p., Web site, accessed July 10, 2016, 

at https://www.epa.gov/sites/production/files/2015-10/

documents/htf_report_to_congress_final_-_10.1.15.pdf.

U.S. Food and Drug Administration, 2002, The use of 

steroid hormones for growth promotion in food-producing 

animals: U.S. Food and Drug Administration Veterinarian 

Newsletter, accessed May 17, 2013, at http://www.fda.gov/

AnimalVeterinary/NewsEvents/FDAVeterinarianNewsletter/

ucm110712.htm.

U.S. Food and Drug Administration, 2010, 2009 Summary 

report on antimicrobials sold or distributed for use in foodproducing animals: U.S. Food and Drug Administration 

Web site, accessed May 17, 2013, at http://www.fda.gov/

AnimalVeterinary/NewsEvents/CVMUpdates/ucm236143.htm.

U.S. Food and Drug Administration, 2011, Bovine 

Somatotropin (BST): U.S. Food and Drug Administration 

Web site, accessed May 17, 2013, at http://www.fda.gov/

AnimalVeterinary/SafetyHealth/ProductSafetyInformation/

ucm055435.htm.

U.S. Food and Drug Administration, 2012, Animals with 

intentionally altered genomic DNA: U.S. Food and Drug 

Administration Web site, Animals with Intentionally Altered 

Genomic DNA.

U.S. Geological Survey, 2006, National Atlas of the United 

States—Major dams of the United States: U.S. Geological 

Survey Web site accessed March 18, 2018, at https://

nationalmap.gov/small_scale/mld/dams00x.html.

U.S. Geological Survey, 2012, National Water-Quality 

Assessment (NAWQA) Program: U.S. Geological Survey 

Web site, accessed May 17, 2013, at http://water.usgs.gov/

nawqa.

Vaccari, D.A., 2009, Phosphorus—A looming crisis: Scientific 

American Magazine, June 3, 2009, p. 54–59.

van der Schans, M.L., Harter, T., Leijnse, Anton, Mathews, 

M.C., and Meyer, R.D., 2009, Characterizing sources of 

nitrate leaching from an irrigated dairy farm in Merced 

County, California: Journal of Contaminant Hydrology, 

v. 110, no. 1, p. 9–21, doi: 10.1016/j.jconhyd.2009.06.002. 

Voss, F.D., Curran, C.A., and Mastin, M.C., 2008, Modeling 

water temperature in the Yakima River, Washington, 

from Roza Diversion Dam to Prosser Dam, 2005–06: 

U.S. Geological Survey Scientific Investigations Report 

2008-5070, 43 p. [Also available at http://pubs.usgs.gov/

sir/2008/5070/.]
References Cited 195

Wahl, K.L., and Wahl, T.L., 1995, Determining the flow 

of Comal Springs at New Braunfels, Texas, in Jennings, 

M.E., Martin, Q.W., and Knowles, T.R., eds., Texas Water 

’95—Proceedings of Component Conference of the First 

International Conference on Water Resources Engineering, 

San Antonio, Texas, August 16–17, 1995: San Antonio, Texas, 

American Society of Civil Engineers, p. 77–86.

Welch, H.E., Muir, C.G., Billeck, B.N., Lockhart, 

W.L., Brunskill, G.J., Kling, H.J., Olson, M.P., and 

Lemoine, R.M., 1991, Brown snow—A long-range 

transport event in the Canadian Arctic: Environmental 

Science and Technology, v. 25, no. 2, p. 280–286. 

Welch, H.L., Green, C.T., and Coupe, R.H., 2011, The fate and 

transport of nitrate in shallow groundwater in northwestern 

Mississippi, USA: Hydrogeology Journal, v. 19, no. 6, 

p. 1,239–1,252.

Welch, H.L., Green, C.T., Rebich, R.A., Barlow, J.R.B., and 

Hicks, M.B., 2010, Unintended consequences of biofuels 

production—The effects of large-scale crop conversion on 

water quality and quantity: U.S. Geological Survey OpenFile Report 2010–1229, 6 p. [Also available at http://pubs.

usgs.gov/of/2010/1229/.]

White, William, 2008, Economic history of tractors in 

the United States, in Whaples, Robert, ed.: EH.Net 

Encyclopedia Web site, accessed May 17, 2013, at https://

eh.net/encyclopedia/economic-history-of-tractors-in-theunited-states/.

Wiersema, Robyn, Burns, M.A., and Hershberger, Doug, 

2012, Glyphosate pathway map: Eawag Aquatic Research 

Web page, accessed May 17, 2013, at http://umbbd.ethz.ch/

gly/gly_map.html.

Wilton, Tom, 2002, Stream fish kill follow-up assessment—

Fish community sampling results: Iowa Department 

of Natural Resources, 21 p., accessed December 3, 

2012, at http://www.iowadnr.gov/Portals/idnr/uploads/

watermonitoring/wqassessments/Stream_Fishkill_

Followup.pdf.

Winchester, P.D., Huskins, J., and Ying, J., 2009, 

Agrichemicals in surface water and birth defects in the 

United States: Acta Paediatrica, v. 98, p. 664–669.

Windsor, H.H., ed., 1922, Big ditcher quickly turns swamps 

into farms: Popular Mechanics Magazine, v. 37, no. 3, 

p. 344–345.

Winter, T.C., Harvey, J.W., Franke, O.L., and Alley, W.M., 

1998, Ground water and surface water a single resource: 

U.S. Geological Survey Circular 1139, 79 p. [Also available 

at http://pubs.usgs.gov/circ/circ1139/.]

Wise, D.R., and Johnson, H.M., 2011, Surface-water nutrient 

conditions and sources in the United States Pacific 

Northwest: Journal of the American Water Resources 

Association, v. 47, no. 5, p. 1,110–1,135, doi.org/10.1111/

j.1752-1688.2011.00580.x.

Wise, D.R., Zuroske, M.L., Carpenter, K.D., and Kiesling, 

R.L., 2009, Assessment of eutrophication in the Lower 

Yakima River Basin, Washington, 2004–07: U.S. Geological 

Survey Scientific Investigations Report 2009-5078, 108 p. 

[Also available at http://pubs.usgs.gov/sir/2009/5078/.]

Zeichner, O., 1939, The transition from slave to free 

agricultural labor in the southern States: Agricultural 

History, v. 13, no. 1, p. 22–32. [Available at http://www.

jstor.org/stable/3739389.]

Zelt, R.B., and Munn, M.D., 2009, Riparian and associated 

habitat characteristics related to nutrient concentrations 

and biological responses of small streams in selected 

agricultural areas, United States, 2003–04: U.S. Geological 

Survey Scientific Investigations Report 2009-5224, 78 p. 

[Also available at http://pubs.usgs.gov/sir/2009/5224/.]

Zhang, Q., Hirsch, R.M., and Ball, W.P., 2016, Longterm changes in sediment and nutrient delivery from 

Conowingo Dam to Chesapeake Bay—Effects of reservoir 

sedimentation: Environmental Science and Technology, 

v. 50, no. 4, p. 1,877–1,886, doi: 10.1021/acs.est.5b04073.

Zhang, Y.-K., and Schilling, K.E., 2006, Increasing streamflow 

and baseflow in Mississippi River since the 1940s—Effect 

of land use change: Journal of Hydrology, v. 324, no. 1–4, 

p. 412–422.
196 Agriculture—A River Runs Through It—The Connections Between Agriculture and Water Quality

Glossary of Terms 

aerobic Conditions or processes requiring the 

presence of air or free oxygen.

anaerobic Conditions or processes occurring in 

the absence of air or free oxygen.

antimicrobial Substance produced by or 

a semisynthetic substance derived from a 

microorganism and in dilute solution, able to 

inhibit or kill another microorganism.

artificial drainage Engineered removal of 

surface and subsurface water from an area.

baseflow Sustained flow of a stream in the 

absence of direct runoff. Baseflow includes 

natural and human-induced streamflows. Natural 

baseflow is sustained largely by groundwater 

discharge.

basin geographic area drained by a single 

major stream; consists of a drainage system 

comprised of streams and often natural or manmade lakes.

best management practices (BMPs) Practical, 

structural, or nonstructural methods that prevent 

or reduce the movement of sediment, nutrients, 

pesticides, and other pollutants from the land to 

surface or groundwater, or that otherwise protect 

water quality from potential adverse effects 

of agricultural activities. These practices are 

developed to achieve a balance between waterquality protection and the production of crops 

within natural and economic limitations.

buffer strip Strips of grass or other erosionresisting vegetation between or below cultivated 

strips or fields.

channel Natural or artificial watercourse with 

a definite bed and banks to confine and conduct 

flowing water; a ditch or channel excavated for 

the flow of water. River, creek, run, branch, and 

tributary are some of the terms used to describe 

natural channels, which may be single or braided. 

Canal, aqueduct, and floodway are some of the 

terms used to describe artificial (man-made) 

channels.

constructed wetlands Wetlands constructed 

either as part of a wetland banking, wetland 

clumping (aggregation), or wetland mitigation 

program, or to achieve the goals of some other 

environmental preservation or restoration program. 

cultivar Variety of a plant developed for 

desirable characteristics and maintained under 

cultivation.

ecosystem Complex of interacting plants 

and animals with their physical surroundings. 

Ecosystems are isolated from each other 

by boundaries that confine and restrict the 

movement of energy and matter; for example, an 

ecosystem could be recognized at a watershed 

scale by designating an area of common 

drainage.

evapotranspiration Loss of water to the 

atmosphere from the soil and surface-water 

bodies by evaporation and by transpiration 

through plants. 

floodplain Strip of relatively flat and normally 

dry land alongside a stream, river, or lake that is 

covered by water during a flood.

flowpath (pathway) Course a water molecule or 

chemical follows in a given location.

hypoxic Condition in which natural waters 

have a low concentration of dissolved oxygen.

intrabasin transfers Water movement within 

the same water basin. 

lag time Time during which some action is 

awaited.

micropores Fine soil pores, typically a fraction 

of a millimeter in diameter. They are responsible 

for the water holding capacity of soil.

moldboard plow Curved iron plate attached 

above a plowshare used to lift and turn the soil in 

conventional tillage.

overland flow Flow of rainwater or snowmelt 

over the land surface toward stream channels.

pathogen Microorganism (for example, virus, 

bacterium, prion, fungus) that causes disease in 

animals and (or) plants.

pH Measure of acidity and alkalinity of a solution.

photosynthesis Process in green plants and 

certain other organisms by which carbohydrates 

are synthesized from carbon dioxide and water 

using light as an energy source. Most forms of 

photosynthesis release oxygen as a byproduct. 

Chlorophyll typically acts as the catalyst in this 

process.
Glossary of Terms 197

recharge Water added to an aquifer. For 

instance, rainfall that seeps into the ground.

reservoir Artificially created lake.

riparian Transition area between a stream and 

the nearby, upland terrestrial ecosystem. Zones 

are identified by soil characteristics and (or) plant 

communities and include the wet areas in and 

near streams, ponds, lakes, springs, and other 

surface waters.

root zone Upper layer of the soil to the depth 

interwoven by plant roots.

runoff Precipitation discharged into stream 

channels from an area. The water that flows off 

the surface of the land without infiltrating into 

the soil is called surface runoff.

saturated soil Soil that has absorbed, to the 

maximum extent possible, water from rainfall or 

snowmelt. Any further precipitation on saturated 

soils will result in surface runoff with downgradient effects on flooding and erosion. 

stream corridor (stream valley) Complex and 

valuable ecosystem that includes the land, plants, 

animals, and network of streams within it.

streamflow Volume of water passing through a 

channel during a specified time.

subsurface drainage Process of directing 

excess water away from the root zones of plants 

by natural or artificial means, such as by using a 

system of pipes and drains placed below ground 

surface level.

surface inlet Vertical pipe with one opening at 

or above land surface and the other connected to 

a horizontal drain used to remove ponded water 

from the land surface.

tailwater Water from irrigation that reaches the 

lower end of a field. Tailwater is not necessarily 

lost; it can be collected and reused on the same or 

adjacent fields.

tillage Plowing of the soil, seedbed preparation, 

and cultivation practices.

tolerant A plant capable of resisting the adverse 

effects of a particular substance (for example, an 

herbicide or an antibiotic) or an environmental 

factor (for example, drought, freezing, high 

temperature, or high salinity).

transformation product Chemical compound 

formed from the reaction of a chemical or a 

chemical compound (or another transformation 

product) after its release into the environment.

transpiration Process by which water 

vapor escapes from a living plant, principally 

through the leaves, and enters the atmosphere. 

Transpiration, combined with evaporation from 

the soil, is referred to as evapotranspiration. 

Transpiration, combined with evaporation from 

the soil, is referred to as evapotranspiration.

uplands Ground above a floodplain; that 

zone sufficiently above and (or) away from 

transported waters as to be dependent upon local 

precipitation for its water supplies; land which is 

neither a wetland nor covered with water.

water budget Accounting of the inflows to, the 

outflows from, and the storage changes of water 

in a hydrologic unit or system.

water table Upper surface of the saturated zone 

in soil (or rock) that is equivalent to the water 

level in a well in an unconfined aquifer.

wetland Area that is periodically inundated 

or saturated by surface or groundwater on an 

annual or seasonal basis, that displays hydric 

soils (formed under saturation conditions), and 

that typically supports or is capable of supporting 

hydrophytic vegetation.

yield (crop) Measure of cereal yield per unit area.
198 Agriculture—A River Runs Through It—The Connections Between Agriculture and Water Quality

Glossary of Farm Implements

This glossary includes definitions of some common types of tillage, cultivation, and chemical 

application implements used in modern, mechanized agriculture. The variety of specific agricultural 

implements currently available is large and, to some extent, manufacturer specific and, new types of 

implements are continually introduced. 

The implements mentioned are basic implements traditionally used in the production of the 

Nation's major crops (corn, soybean, and wheat); however, these implements often are used in other 

cropping systems as well. Besides the obvious exclusion of implements used in specialty crops, such 

as fruit and vegetable production, this glossary does not attempt to include the implements used in 

relatively new methods of production of the aforementioned major crops, such as ridge-till agriculture.

Tillage, as it relates to soil preparation, can be classified in three ways: primary, secondary, and 

cultivating. These classifications are related to production goals, and are based on the intensity and 

timing of tillage. Another way to classify tillage practices is by the amount of “residue” left on the soil 

surface, using categories such as “conventional” and “conservation” tillage. In many cases, detailed 

implement settings can determine whether a tillage implement will perform satisfactorily to meet a 

production goal (for example, helping dry out the soil sufficiently for planting activities) while also 

conforming to conservation goals (for example, maintaining a status of conservation tillage). Tillage 

implements also can be used in combination, resulting in another category of tillage called “combination tillage.” Combination tillage typically involves the use of two or more dissimilar tillage technologies (for example, chisels with discs in combination primary tillage, or packer rollers with spring teeth 

in combination secondary tillage) to achieve either primary or secondary tillage goals. 

Primary Tillage Implements Primary tillage 

involves displacing and shattering soil, reducing 

soil strength, and mixing plant materials, air, and 

soil amendments in the zone of tillage depth. 

Primary tillage generally leaves a rough soil 

surface, which is smoothed using secondary 

tillage techniques that break up the large soil 

clods left behind by primary tillage. As compared 

with secondary tillage implements, primary 

tillage implements operate at deeper depths and 

act more aggressively on the soil. 

Secondary and Cultivating Tillage 

Implements Secondary tillage involves the 

further mixing and pulverization of soil in the 

zone of tillage depth. Specifically, secondary 

tillage is used to: smooth rough surfaces and 

break up large soil clods left behind by primary 

tillage; mix in soil amendments such as nutrients 

and pesticides; close soil air pockets; level 

and firm soil for seedbed preparation; and 

eradicate weeds. Secondary tillage implements 

have shallower operating depths than primary 

tillage implements and leave the soil in a 

finer, more uniform state. Cultivating tillage 

is, essentially, focused secondary tillage that 

occurs post-planting. The goals are to aid the 

crop by loosening the soil and by mechanically 

eradicating weeds. Cultivating tillage implements 

are designed for in-row cultivation (for example, 

for row-crops such as corn) as well as for other 

production methods (for example, for broadcastseeding systems such as wheat); implements also 

are designed for either pre-emergent or postemergent cultivation.

Chemical Application Implements Depending 

on the types of chemicals being applied (for 

example, herbicides, insecticides, fungicides, or 

nutrients), there are various ways of classifying 

different chemical-application techniques. 

Chemicals can be applied directly to the foliage 

of a crop, applied to the soil surface, or applied 

deeper in the soil (incorporation). Chemicals 

are applied as granules, solids, dissolved in 

water, or as emulsifications in water. Manure 

can be applied in liquid or solid form. Another 

option for application of soluble chemicals is 

chemigation, which involves the addition of 

chemicals to irrigation water. 
Glossary of Farm Elements 199

Table G.1. Implements for primary tillage.

Implement name Description Implement depiction

Used in:

Conventional 

tillage

Conservation 

tillage

Chisel

The chisel plow is one of the most commonly used 

implements today, and has been in widespread use 

since the 1950s. Chisels consist of individual curved 

shanks with interchangeable sweep, chisel, spike, 

winged, or shovel tips that penetrate and shatter the 

soil without inverting it or burying surface residue. 

Twisted shanks may be used to provide some degree 

of soil inversion and mixing. The chisel was originally 

developed to help control wind erosion after the 

Dust Bowl, so it is often used in conservation-tillage 

systems. Chiseling is generally done in the fall, and is 

often accompanied by one or more secondary tillage 

operations in the spring. Operating depth is between 

15 and 30 centimeters.

Yes Yes

Heavy disk

Heavy disks (or Heavy offset disks) consist of circular, 

concave cutting blades (disks) that are drawn 

through the soil at an angle. The goal is to pulverize 

and partially or completely invert the soil, burying 

surface residue. Disking leaves large soil clods and 

appreciable surface roughness, but soil generally is 

left bare and susceptible to erosion. Disking typically 

is done on relatively well drained soils, either fall 

or spring, and is often accompanied by one or more 

secondary tillage operations in the spring. Gangs of 

disks are “offset” for primary tillage, making a “V” 

shape; serrated disks are also used. Operating depth 

can be up to 23 centimeters.

Yes No

Moldboard plow

The moldboard plow is designed to partially or 

completely invert the soil, burying surface residue. It 

is used often in poorly drained soils. The plow leaves 

large soil clods and significant surface roughness, but 

the soil is left susceptible to erosion. Plowing is done 

in either fall or spring, and is often accompanied by 

one or more secondary tillage operations in the spring. 

Operating depth can be up to 35 centimeters. This 

once ubiquitous implement in historical agriculture is 

still in use today, but its use has waned with the advent 

of conservation-till and no-till practices. 

Yes No

Subsoiler

Subsoilers (rippers) consist of heavy duty shanks 

designed to break up and loosen soil deep into the 

profile, particularly in soils where compacted layers (or 

“pans”) are formed by machinery traffic. In conservation 

tillage, subsoiling practices are often used in lieu of 

conventional tillage practices that would otherwise 

break compaction. Subsoilers also are used to increase 

infiltration into the soil. Subsoiling is done in either fall 

or spring, and requires relatively dry soil conditions. 

Operating depth is between 30 and 56 centimeters.

Yes Yes
200 Agriculture—A River Runs Through It—The Connections Between Agriculture and Water Quality

Table G.2. Implements for secondary and cultivating tillage.

Implement name Description Implement depiction

Used in:

Conventional 

tillage

Conservation 

tillage

Disk harrow

Disk harrows utilize circular, concave cutting blades 

(disks), smaller than those of the disk plow, and 

are used in fall or spring. Like other harrows, disk 

harrows break up larger soil clods. The intensity of 

disk harrowing depends greatly on the size, shape, and 

orientation of its disks, and the speed of travel. Disk 

harrows are commonly used on light- to mediumtextured, well-drained soils. Gangs of disks generally 

are “tandem” for secondary tillage, making an “X” 

shape.

Photo goes here Yes No

Harrow

Harrows (drag harrows) are designed to break up soil 

clods and smooth the soil surface for planting by 

dragging the implement across the field surface. There 

are three harrow technologies distinct from the disk 

harrow: chain harrows, spring tine harrows, and 

roller harrows. Chain harrows consist of a connected 

network of heavy wire (much like a heavy chain-link 

fence, but with pointed teeth). Tine harrows consist 

of various rows of tines, of which there are many 

varieties (for example, spring tooth, wire-tooth, spiketooth). Roller harrows use cultivating teeth between 

two in-line gangs of ridged rollers.

Photo goes here Yes No

Field cultivator

Field cultivators (or simply cultivators) generally utilize 

shanks with attachments (like the chisel plow), but 

are operated at a shallow depth (2–5 centimeters) to 

uproot or bury weeds, and for seedbed preparation. 

Depending on the implement design, settings and 

operation, cultivation may or may not leave sufficient 

residue on the soil surface for conservation tillage 

practices. Field cultivators differ from row-crop 

cultivators in that they affect the entire field surface 

and perform some secondary tillage functions in 

addition to cultivation. 

Photo goes here Yes Yes

Roller

There are various types of rollers, but there generally 

are two technologies: packer rollers (or simply 

packers) and basket rollers (or crumblers). Packer 

rollers consist of one or two in-line gangs of rollers, 

which are made of lugged or ridged wheels of varying 

design, and are designed to crush soil clods and 

compact the soil. Basket rollers consist of cylindrical, 

reel-type assemblies (baskets) made of wire rods, bars 

or blades, and are used to break soil clods and to mix 

and level the soil surface.

Photo goes here Yes No
Glossary of Farm Elements 201

Table G.3. Implements for chemical application.

Implement name Description Implement depiction

Anhydrous ammonia

applicator

Anhydrous ammonia is an efficient and widely used method of 

nitrogen application. This method is used particularly often in 

row-crop agriculture. Because anhydrous ammonia becomes a gas 

when exposed to air, it must be injected into the soil, and is stored 

in tanks that are pulled behind the injecting implement. Application 

generally is of low disturbance to the soil surface. 

Photo goes here

Fertilizer spreader

This type of implement generally is used to spread soil amendments 

over the field surface, including fertilizers (for example, granular 

urea) and other amendments such as lime, gypsum, and compost. 

Special spreaders are used for manure. For some amendments, such 

as urea, application is followed with incorporation by either tillage 

or irrigation.

Photo goes here

Manure injector

As the name suggests, these implements apply manure in an 

“injected” fashion. Manure injectors can be of the umbilical cord 

type, where the manure supply is stationary, or of the tank type, 

where the manure supply is pulled behind in a tank. They also can 

be of high or low disturbance to the soil, dependent on implement 

design and settings, which affect the resulting residue levels. 

Photo goes here

Manure spreader

There are a variety of manure spreader designs, because these 

implements can spread liquid, slurry, or solid manure. Like manure 

injectors, spreaders can be of the umbilical cord or tank type. 

Manure is applied in a “broadcast” fashion, and is sometimes 

incorporated by tillage after application.

Photo goes here

Boom sprayer

Boom sprayers (field sprayers) are used to apply chemicals, 

commonly pesticides, to the field surface or to plant foliage. They 

can be calibrated for broadcast or banding depending on crop 

and chemical. Sprayers can be self-propelled or towed behind 

tractors, and can be used with a row-crop sprayer (which has tall, 

narrow tires to spray in standing crops) or a floater sprayer (which 

has 3-wide, floater tires to minimize crop damage by distributing 

machine weight).

Photo goes here

Aerial applicator

Also known as a Crop Duster or Spray Plane. Aerial application 

involves the use of an airplane or helicopter to apply chemicals to 

a field. Aerial applicators are often used to apply pesticides, but 

can also be used to apply fertilizers—known as aerial topdressing. 

Aerial application is fast and does not compact the land, but 

generally is more expensive than other methods.

Photo goes here

Chemigation

Chemigation (fertigation when referring to nutrients) involves the 

use of irrigation water as a solvent for soluble fertilizers and 

pesticides. The chemicals are then transported to the foliage or soil 

directly through irrigation, both by above-ground irrigation and by 

subirrigation

Photo goes here

Acknowledgments

We wish to say thank you to the many individuals and organizations who contributed to this report. These include 

USGS personnel across the Nation who collected, analyzed, and interpreted data. In addition, numerous USGS 

partners in the governmental and non-governmental sectors helped guide the scientific efforts and ensure that USGS 

information meets the needs of local, State, tribal, regional, and National stakeholders. Finally, we would like to thank 

those who contributed sections of the text and reviewed parts of the report.

Contributors to sections of this report:

Jeannie R. Barlow, “Strategic, long-term monitoring, in concert with hydrologic modeling, provides a basis for sound 

 management decisions”

Jack E. Barbash, “Organic Agriculture”

Scott C. Kronholm, “Manure”, figure 2.4, figure 8.5, figure 8.9

Michael S. Majewski, “Important Agricultural Chemicals”

Gustavo H. Merten, “Effects of Tillage on Runoff and Recharge”

Jason L. Roth, “Quantifying Water and Sediment Budgets”, figure 6.3, figure 9.15

Michael T. Talbot, “A Year with Corn in Central Indiana,” “Nutrient Management Plans,” 

 “Glossary of Farm Implements”

Jason R. Vogel, parts of Chapters 6 and 7

Heather L. Welch, “Nitrogen and phosphorus, used in fertilizers, are transformed into different chemical forms, but 

 they do not disappear. These elements move throughout the environment”

External reviewers for sections of the report:

• William A. Arnold, University of Minnesota

• Mark V. Boggess, USDA/ARS

• Leslie A. Everett, University of Minnesota

• Norman R. Fausey, USDA/ARS

• Satish C. Gupta, University of Minnesota

• David C. Love, Johns Hopkins University

• Roberta A. Parry, USEPA

• Mark D. Tomer, USDA/ARS

U.S. Geological Survey reviewers:

• Victoria Glenn Christensen

• Lynne S. Fahlquist

• Jack Fisher

• James M. Gerhart

• Christopher T. Green

• Angel Martin, Jr.

• Jeff D. Stoner

• Patricia L. Toccalino

• David M. Wolock

Technical Editor:

• Debra A. Grillo

Technical Illustrations:

• Jacqueline Olson

• Luis Menoyo

Layout:

• Sharon L. Wahlstrom

Graphic Design:

• William D. Gibbs

For more information concerning the research in this report, contact:

 National Water-Quality Program

 U.S. Geological Survey

 413 National Center

 12201 Sunrise Valley Drive

 Reston, VA 20192

or visit our Web site at: https://water.usgs.gov/nawqa/. 

Publishing support provided by the 

U.S. Geological Survey 

Scientific Publishing Network, 

Tacoma Publishing Service Center

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Capel and others— Agriculture—A River Runs Through It—The Connections Between Agriculture and Water Quality—Circular 1433

ISSN 2330-5703 (online)

https://doi.org/10.3133/cir1433