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5. U.S. Geological Survey
Water Budgets: Foundations for Effective

Water-Resources and Environmental 

Management

By Richard W. Healy, Thomas C. Winter, James W. LaBaugh, and O. Lehn Franke

Circular 1308

U.S. Department of the Interior

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

DIRK KEMPTHORNE, Secretary

U.S. Geological Survey

Mark D. Myers, Director

U.S. Geological Survey, Reston, Virginia: 2007

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Suggested citation:

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.
iii

Foreword

Water availability is an important concern in the 21st century. Ensuring sustainable water supplies requires an understanding 

of the hydrologic cycle—how water moves through Earth’s atmosphere, 

land surface, and subsurface. Water budgets are tools that water users 

and managers use to quantify the hydrologic cycle. A water budget is 

an accounting of the rates of water movement and the change in water 

storage in all or parts of the atmosphere, land surface, and subsurface. 

Although simple in concept, water budgets may be difficult to accurately 

determine. It is important for the public and decisionmakers to have 

an appreciation of the uncertainties that exist in water budgets and the 

relative importance of those uncertainties in evaluating how much water 

may be available for human and environmental needs.

As part of its mission, the U.S. Geological Survey (USGS) provides 

information that describes the Earth, its resources, and the processes 

that govern the availability and quality of those resources. This 

Circular provides an overview of the hydrologic cycle and a discussion 

of methods for determining water budgets and assessing the uncertainties in those determinations. Examples illustrate the importance of water 

budgets to humans and the environment and demonstrate how water 

budgets can be incorporated into management practices. Through this 

Circular, the USGS seeks to inform the public and decisionmakers about 

a scientific basis for water-resources and environmental management 

and to broaden awareness and understanding of water budgets and the 

hydrologic cycle so as to promote wise use and management of a most 

precious resource—water.

Robert M. Hirsch

Associate Director for Water
iv

Preface

Water is the essence of life. Its availability determines 

where and how animals and plants exist on Earth. Humans 

need water for consumption, for producing food, and for 

manufacturing; we also are attracted to water for its esthetic 

value and for the recreational opportunities it offers. At the 

same time, all other life forms on Earth require water for their 

sustenance. Native plants in grasslands and forests; wheat 

and corn crops in agricultural fields; insects, amphibians, and 

birds in wetlands; fish in streams and lakes; wild mammals 

and reptiles; and domesticated pets and livestock—all depend 

on water.

Competition for water among humans and between 

humans and other life forms is the unavoidable outcome of 

burgeoning populations and a limited resource. Resolution of 

competing needs requires decisions based on science as well 

as societal values. Informed decisions are developed with an 

understanding of the hydrologic cycle—the process by which 

water moves from the atmosphere to land surface as precipitation, infiltrating the subsurface or flowing along land surface 

to the oceans, and eventually returning to the atmosphere by 

evaporation. All water on Earth resides in one of the three 

compartments of the hydrologic cycle: the atmosphere, the 

land surface, and the subsurface. A water budget is an accounting of water stored within and water exchanged among some 

subset of the compartments, such as a watershed, a lake, or 

an aquifer.

Throughout history, humans have managed water for 

their own needs. Ancient Mayan and Egyptian cultures 

prospered on crops produced with intricate irrigation systems. 

Remains of aqueducts built almost two thousand years ago by 

the Roman Empire can still be found throughout Europe. Early 

How long can the water needs of a growing 

urban area be sustained by an aquifer 

that contains a finite amount of water?

What are the ecological effects of withdrawing water from an aquifer that naturally 

discharges to a wetland? Will the withdrawal result in reduced discharge to and 

subsequent drying of the wetland? How will 

plants and animals be affected? 

How will droughts affect agricultural and domestic 

water supplies? Will increased diversions reduce 

storage in surface-water reservoirs to the point 

where recreational uses are limited?

Helen H. Richardson, Denver Post

U.S. Army Corps of Engineers 
v

Can crops be matched to climate so as to 

minimize irrigation requirements?

Can streamflow in arid regions be increased 

by the removal of non-native phreatophytes 

that line channels, thus reducing evapotranspiration? How much water will be used 

by replacement vegetation?

Will dewatering of a surface mine have an effect 

on surface-water expressions many miles away? 

explorers of the American West, such as John Wesley Powell, 

realized that civilization could flourish in this arid region only 

if water could be stored and distributed as needed. Today, 

population centers and agriculture thrive in the West, mainly 

because of the dams and reservoirs constructed on rivers such 

as the Colorado and Columbia. Design and operation of large 

reservoir projects rely on detailed water-budget analyses, 

examination of precipitation and evaporation rates, discharge 

rates of streams, rates of exchange between surface water and 

ground water, and factors such as climate, geology, vegetation, 

and soils that affect those rates. The story of water development in the Western United States is a story that has been 

repeated in various forms all over the Earth.

Reservoirs and ground-water wells are key features of 

the Nation’s water supply infrastructure. They both provide 

great benefits in terms of the reliable delivery of water to 

users. However, it is well-recognized that they can also have 

adverse impacts on aquatic ecosystems. The needs and values 

of society determine whether or not the benefits of these systems outweigh their negative consequences and determine if 

changes in the design or operation of these systems should be 

made. Water needs of ecosystems have become an integral part 

of water management. Operators of reservoirs now take into 

account the health of downstream riparian ecosystems. Managers of aquifers are likely to consider the effects of groundwater withdrawals on the interactions between ground water 

and surface water and the organisms that depend on that 

interaction. These are but a few of the myriad issues that arise 

in balancing the water needs of humans and the environment. 

Water budgets form the foundations of informed management 

strategies for resolving these issues.
vi

Contents

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

Preface ...........................................................................................................................................................iv

Introduction ....................................................................................................................................................1

Hydrologic Cycle............................................................................................................................................3

Storage and Movement of Water Within the Principal Compartments of the Hydrologic Cycle ....8

Water in the Atmosphere ....................................................................................................................8

Water on Land Surface .....................................................................................................................12

Snow and Ice..............................................................................................................................12

Lakes .........................................................................................................................................16

Wetlands .....................................................................................................................................18

Streams........................................................................................................................................19

Water in the Subsurface....................................................................................................................24

Unsaturated Zone ......................................................................................................................24

Saturated Zone...........................................................................................................................28

Exchange of Water Between Compartments of the Hydrologic Cycle ..............................................36

Precipitation.........................................................................................................................................36

Infiltration and Runoff ........................................................................................................................40

Evapotranspiration..............................................................................................................................41

Exchange of Surface Water and Ground Water............................................................................43

Water-Budget Studies.................................................................................................................................46

Water Budget for a Small Watershed: Beaverdam Creek Basin, Maryland ............................46

Soil-Water Budgets for Prairie and Farmed Systems in Wisconsin ..........................................48

Water Budget of Mirror Lake, New Hampshire.............................................................................50

Water Budget at a Waste Disposal Site in Illinois .......................................................................52

Humans and the Hydrologic Cycle............................................................................................................55

Water Storage and Conveyance Structures ..................................................................................55

Land Use...............................................................................................................................................56

Ground-Water Extraction ..................................................................................................................57

Water Budgets and Management of Hydrologic Systems ..................................................................61

Large River System: Colorado River Basin.....................................................................................61

Watersheds and Reservoir Management..............................................................................63

Aquifers in Arizona ....................................................................................................................64

Large Aquifer System: High Plains Aquifer....................................................................................66

Water Budgets and Governmental Units: Lake Seminole............................................................71

Agriculture and Habitat: Upper Klamath Lake ...............................................................................74

Water for Humans and Ecosystems: San Pedro River Ecosystem.............................................77

Urban Water Supply: Chicago ..........................................................................................................81

Concluding Remarks....................................................................................................................................84

References Cited..........................................................................................................................................86
vii

Boxes

A. The Water-Budget Equation ................................................................................................................6

B. Water Budgets are Intimately Linked to Energy and Chemical Budgets ...................................10

C. Chemical, Isotopic, and Energy Tracers Provide Insight into Hydrologic Processes .............14

D. Models—Important Tools in Water-Budget Studies. ....................................................................22

E. Lysimeters—Water-Budget Meters .................................................................................................26

F. Ground-Water Recharge.....................................................................................................................32

G. Estimating Aquifer Hydraulic Conductivity......................................................................................34

H. Uncertainty in Water-Budget Calculations .....................................................................................54

I. Water Use and Availability ..................................................................................................................58

J. Water Budgets of Political Units .......................................................................................................60

Rain is grace;

rain is the sky

condescending to the earth;

without rain, there would be no life.

John Updike (1989)
viii

Conversion Factors and Datums

Multiply By To Obtain

Length

centimeter (cm) 0.3937 inch

millimeter (mm) 0.03937 inch

meter (m) 3.281 foot (ft) 

kilometer (km) 0.6214 mile (mi)

Area

square meter (m2) 0.0002471 acre 

hectare (ha) 2.471 acre

square kilometer (km2) 247.1 acre

square centimeter (cm2) 0.001076 square foot (ft2)

square meter (m2) 10.76 square foot (ft2) 

square centimeter (cm2) 0.1550 square inch (ft2) 

hectare (ha) 0.003861 square mile (mi2) 

square kilometer (km2) 0.3861 square mile (mi2)

Volume

cubic meter (m3) 264.2 gallon (gal) 

cubic centimeter (cm3) 0.06102 cubic inch (in3) 

cubic meter (m3) 0.0008107 acre-foot (acre-ft) 

Flow rate

cubic meter per year (m3/yr) 0.000811 acre-foot per year (acre-ft/yr) 

cubic meter per second (m3/s) 35.31 cubic foot per second (ft3/s)

cubic meter per second (m3/s) 22.83 million gallons per day (Mgal/d) 

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

°F=(1.8×°C)+32

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

°C=(°F–32)/1.8

Vertical coordinate information is referenced to the North American Vertical Datum of 1988 

(NAVD 88).

Horizontal coordinate information is referenced to the North American Datum of 1983 (NAD 83).
American Geological Institute, Bruce F. Molnia

I came where the river

Ran over stones

My ears knew

An early joy.

And all the waters

Of all the streams

Sang in my veins

That summer day

Theodore Roethke from “The Lost Son” 

(1948)
Introduction 

Water budgets provide a means for evaluating availability 

and sustainability of a water supply. A water budget simply 

states that the rate of change in water stored in an area, such 

as a watershed, is balanced by the rate at which water flows 

into and out of the area. An understanding of water budgets 

and underlying hydrologic processes provides a foundation for 

effective water-resource and environmental planning and management. Observed changes in water budgets of an area over 

time can be used to assess the effects of climate variability 

and human activities on water resources. Comparison of water 

budgets from different areas allows the effects of factors such 

as geology, soils, vegetation, and land use on the hydrologic 

cycle to be quantified.

Human activities affect the natural hydrologic cycle in 

many ways. Modifications of the land to accommodate agriculture, such as installation of drainage and irrigation systems, 

alter infiltration, runoff, evaporation, and plant transpiration 

rates. Buildings, roads, and parking lots in urban areas tend to 

increase runoff and decrease infiltration. Dams reduce flooding in many areas. Water budgets provide a basis for assessing 

how a natural or human-induced change in one part of the 

hydrologic cycle may affect other aspects of the cycle. 

Water Budgets: Foundations for Effective

Water-Resources and Environmental Management

By Richard W. Healy, Thomas C. Winter, James W. LaBaugh, and O. Lehn Franke

“Only from space can you see that

our planet should not be called Earth,

but rather Water, 

with speck-like islands of dryness

on which people, animals, and birds

surprisingly find a place to live.” 

Oleg Makarov (1988)

National Aeronautics and Space Administration 
This report provides an overview and qualitative 

description of water budgets as foundations for effective 

water-resources and environmental management of freshwater hydrologic systems. Perhaps of most interest to the 

hydrologic community, the concepts presented are also 

relevant to the fields of agriculture, atmospheric studies, 

meteorology, climatology, ecology, limnology, mining, water 

supply, flood control, reservoir management, wetland studies, pollution control, and other areas of science, society, and 

industry. The first part of the report describes water storage 

Natural Resources Conservation Service

Natural Resources Conservation Service

and movement in the atmosphere, on land surface, and in the 

subsurface, as well as water exchange among these compartments. Our ability to measure these phenomena and inherent 

uncertainties in measurement techniques also are discussed. 

The latter part of the report presents a number of case studies that illustrate how water-budget studies are conducted, 

documents how human activities affect water budgets, and 

describes how water budgets are used to address water and 

environmental issues. 

2 Water Budgets: Foundations for Effective Water-Resources and Environmental Management
Figure 1. The hydrologic cycle for 

part of a watershed.

Precipitation

Surface-water inflow,

imported water

(pipelines, canals)

Ground-water inflow

Surface-water outflow,

exported water

(pipelines, canals)

Ground-water outflow

Aquifer

Evapotranspiration

Bedrock

Water table

Unsaturated zone

Hydrologic Cycle

Earth’s water exists on land surface in oceans, ice fields, 

lakes, rivers, streams, and wetlands; it also exists in the subsurface as soil water and ground water and in the atmosphere 

(fig. 1). More than 97 percent of the Earth’s water is in oceans 

(table 1). Of the inland water that resides on and beneath land 

surface, 77 percent is contained in icecaps and glaciers and 

for practical purposes is inaccessible. The remaining inland 

water is stored primarily in the subsurface as ground water. 

Water is constantly moving within the hydrologic cycle, and 

that movement takes place over many pathways (fig. 1). Water 

moves quickly through some pathways; for example, rain 

falling from the atmosphere to a field of corn in summer may 

return to the atmosphere in a matter of hours or days by evaporation. Traveltimes over other pathways are measured in years, 

decades, centuries, or more—ice fields in Greenland contain 

water that fell from the atmosphere thousands of years ago.

“The central concept in the science of hydrology is the so-called hydrologic cycle—a 

convenient term to denote the circulation of the water from the sea, through the 

atmosphere, to the land; and thence, with numerous delays, back to the sea by overland and subterranean routes, and in part, by way of the atmosphere; also, the many 

short circuits of the water that is returned to the atmosphere without reaching the 

sea***. The science of hydrology is especially concerned with the second phase of 

this cycle—that is, with the water in its course from the time it is precipitated upon 

the land until it is discharged into the sea or returned to the atmosphere. It involves 

the measurement of the quantities and rates of movement of water at all times and at 

every stage of its course***.”

O.E. Meinzer (1942, p. 1)

Hydrologic Cycle 3
The atmosphere receives water through evaporation and 

loses it as precipitation, mostly in the form of rain or snow. 

The average residence time for water in the atmosphere is 

about 10 days. A drop of rain can have a multitude of fates, 

depending on where and when it falls. Some rainfall never 

reaches land surface; instead, it evaporates as it falls (a 

phenomenon known as virga) and returns to the atmospheric 

reservoir. A falling raindrop could land on a leaf of a tree, 

from where it might fall to the ground, evaporate, or perhaps 

be imbibed by the plant. Another drop might land directly on 

the ground. That water could puddle in a depression, travel 

over the surface to a lower elevation (runoff), or enter the subsurface (infiltrate). Water in a puddle will likely evaporate or 

infiltrate. Water that runs off may infiltrate at a more favorable 

location or travel to a stream and ultimately be transported to 

an ocean; at any point on this journey, that water can evaporate. The average residence time for water in free-flowing rivers ranges between 16 and 26 days (Vorosmarty and Sahagian, 

2000). Streams that run through reservoirs can have substantially longer residence times. Not all surface water flows to 

oceans. Some lakes and wetlands have no surface drainage. 

They lose water to evaporation and to ground water. Humans 

withdraw water from streams and reservoirs, thus interrupting 

its migration to the ocean. 

Water moves much more slowly in the subsurface than 

in the atmosphere or on land surface. Water that infiltrates the 

subsurface can remain in the unsaturated zone where it will 

most likely be returned to the atmosphere by evaporation or 

plant transpiration; it can discharge to the surface in a channel 

or depression, thus becoming surface flow; or it can traverse 

the unsaturated zone to recharge an underlying aquifer. Most 

water that infiltrates the subsurface is returned to the atmosphere by evaporation from bare soil or by plant transpiration 

(table 2). That returned water typically resides in the subsurface for less than a year. Discharge to land surface of 

unsaturated-zone water, sometimes referred to as interflow, 

may occur days to months after that water has infiltrated, 

depending on the distance between the points of infiltration 

and discharge. Infiltrated water that travels downward past 

the depth of the root zone may eventually reach the saturated 

zone, thus becoming aquifer recharge. Traveltimes of water 

through the entire thickness of the unsaturated zone span a 

very large range: from hours, for thin unsaturated zones in 

humid regions (Freeze and Banner, 1970), to millennia, for 

thick unsaturated zones in arid regions (Phillips, 1994). Water 

that reaches the saturated zone may reside there for days to 

thousands of years (Alley and others, 2005). Under natural 

conditions, ground water discharges to surface-water bodies 

such as streams, wetlands, lakes, or oceans, or it is extracted 

by plants and returned to the atmosphere by transpiration. 

Humans also extract ground water for agricultural, domestic, 

and industrial uses; such water is ultimately reapplied to land 

surface, returned to the subsurface, or discharged to surfacewater bodies.

“It is the sea that whitens the roof.

The sea drifts through the winter air.

It is the sea that the north wind makes.

The sea is in the falling snow.”

Wallace Stevens from

“The Man With the Blue Guitar” (1937)

D.H. Campbell

Water storage

Volume,

in thousands

of km3

Percentage

of total water

Ocean water 1,320,000 97.1

Atmosphere 13 0.001

Water in land areas 37,800 2.8

Freshwater lakes 125 0.009

Saline lakes and

inland seas

104 0.008

Rivers 1.25 0.0001

Icecaps and glaciers 29,200 2.14

Soil root zone 67 0.005

Ground water (to depth 

of 4,000 meters) 8,350 0.61

Table 1. Estimated global water supply (from Nace, 1967).

[km3, cubic kilometers]

4 Water Budgets: Foundations for Effective Water-Resources and Environmental Management
Water-budget component

Annual rate,

in millimeters 

Percentage of

annual

precipitation

Precipitation 834 100

Evapotranspiration 540 65

Total discharge to oceans 294 35

Discharge to oceans from 

surface runoff 204 24

Discharge to oceans from

base flow

90 11

Infiltration of precipitation 630 76

Precipitation in the form of snow can follow several 

courses. In many environments, snow accumulated on land 

surface melts in a few days or less. In other areas, a seasonal 

snowpack exists throughout winter and melts in the spring. 

Still other areas, such as Greenland and Antarctica, have 

snow and ice fields that are thousands of years old. In any of 

these cases, the melting water flows to a surface-water body, 

infiltrates into the subsurface, or is evaporated back into the 

atmosphere. 

It is evident from the preceding discussion that water 

moves within the hydrologic cycle along many complex 

pathways over a wide variety of time scales. The challenge 

for humans is to monitor the hydrologic cycle for some geographic feature of interest, such as a watershed, a reservoir, 

or an aquifer. Such a feature will be referred to as an accounting unit. A water budget states that the difference between 

the rates of water flowing into and out of an accounting unit 

is balanced by a change in water storage: 

Flow In – Flow Out = Change In Storage.

Spring at head of Paris Canyon, Bear Lake County, Idaho 1912.

Rainbow Falls on the Missouri River near Great Falls, Montana, 1904.

Table 2. Water budget for global land mass (from Lvovitch, 

1973). Evapotranspiration is the sum of evaporation and plant 

transpiration.

“And you, vast sea,

sleepless mother, 

Who alone are peace and 

freedom to the river

and the stream, 

Only another winding will 

this stream make,

only another murmur

in this glade, 

And then shall I come to 

you, a boundless drop to a 

boundless ocean.”

Kahlil Gibran (1923)

Simple, yet universal, the water-budget equation is 

applicable over all space and time scales, from studies of 

rapid infiltration in a laboratory soil column to investigations of continental-scale droughts over periods of decades or 

centuries. A 1-m2 soil column in the middle of an agricultural 

field, the entire field itself, or the watershed in which the field 

lies—these are all examples of water-budget accounting units.

Hydrologic Cycle 5
A The Water-Budget Equation

The water-budget equation is simple, universal, and adaptable because it relies on few assumptions on mechanisms of water 

movement and storage. A basic water budget for a small watershed can be expressed as: 

 P + Qin = ET + ∆S + Qout (A1)

where

P is precipitation,

Qin is water flow into the watershed,

ET is evapotranspiration (the sum of evaporation from soils, surface-water bodies, and plants),

∆S is change in water storage,

and

Qout is water flow out of the watershed.

The elements in equation A1 and in all other water-budget equations are referred to as components in this report. Water-budget 

equations can be written in terms of volumes (for a fixed time interval), fluxes (volume per time, such as cubic meters per day or acrefeet per year), or flux densities (volume per unit area of land surface per time, such as millimeters per day). Typically, water budgets are 

tabulated in spreadsheets or tables such as that shown in table A–1, which contains monthly and yearly data for Seabrook, New Jersey, 

from Thornthwaite and Mather (1955). With the approach used by those authors, it is assumed that Qin is zero and Qout is equal to runoff. 

Equation A1 can be refined and customized depending on the goals and scales of a particular study. Precipitation can be written as 

the sum of rain, snow, hail, rime, hoarfrost, fog drip, and irrigation. Water flow into or out of the site could be surface or subsurface flow 

resulting from both natural and human-related causes. Evapotranspiration could be differentiated into evaporation and plant transpiration. Further refinement could be based on the source of the water that is evapotranspired. Evaporation can occur from open water, 

bare soil, or snowpack (sublimation); plants can extract ground water or water from the unsaturated zone. Such refinements must be 

balanced with available measurement techniques, which often are not designed, or lack sufficient resolution, to distinguish among subcomponents. Most methods for measuring evapotranspiration, for example, quantify the flux of water from the land/vegetation surface 

to the atmosphere and do not distinguish between different water sources. Fashioning a viable water-budget approach for estimating 

evapotranspiration or other water-budget components requires analysis of available measurement techniques.

Water storage occurs within all three compartments of the hydrologic cycle. The amount of water stored in the atmosphere is small 

compared to that on land surface and in the subsurface. Surface water is stored in rivers, ponds, wetlands, reservoirs, icepacks, and 

snowpacks. Subsurface storage can be categorized into various subaccounting units, such as the root zone, the unsaturated zone as a 

whole, the saturated zone, or different geologic units. An expanded form, but certainly not an exhaustive refinement, of the water budget 

appropriate for many hydrologic studies can be written as (Scanlon and others, 2002):

 P + Qsw

in + Qgwin = ETsw + ETgw + ETuz + ∆Ssw + ∆Ssnow + ∆Suz + ∆Sgw + Qgwout + RO + Qbf (A2)

where the superscripts refer to surface water (sw), ground water (gw), unsaturated zone (uz); RO is surface runoff; Qgw

out refers to both 

ground-water flow out of the site and any withdrawal by pumping; and Qbf is base flow (ground-water discharge to streams). It is unlikely 

that all elements in equation A2 will be of importance at any one site; some will be of negligible magnitude and can be ignored. Indeed, 

when selecting an accounting unit for developing a water budget, judicious selection of boundaries can greatly facilitate the accounting process. Consider, for example, a small watershed and associated shallow ground-water system. Watershed boundaries are well 

defined: there is no surface flow in, and surface flow out occurs only in a stream channel, where discharge can be readily measured. If 

watershed boundaries correspond to ground-water divides, there is also no subsurface inflow. Suppose all ground water that is not lost 

to ET eventually discharges to the stream; an appropriate water budget for the watershed could be stated as:

 P = ET + ∆S+ RO + Qbf (A3)

If the annual change in storage is small, evapotranspiration can be estimated as the difference between precipitation and streamflow out of the watershed. 

Table A–1. Monthly and yearly water budget, in millimeters, for Seabrook, New Jersey (Thornthwaite and Mather, 1955).

Jan. Feb. Mar. Apr. May June July Aug. Sept. Oct. Nov. Dec. Year

total

Precipitation 87 93 102 88 92 91 112 113 82 85 70 93 1,108

Storage change 0 0 0 0 0 –38 –35 –17 –10 32 51 17 0

Evapotranspiration 1 2 16 46 92 129 147 130 92 53 19 3 730

Runoff 61 76 81 61 31 15 8 4 2 1 1 37 378

6 Water Budgets: Foundations for Effective Water-Resources and Environmental Management
National Aeronautics and Space Administration

Earth’s energy 

budget is directly 

coupled to its water 

budget.

Hydrologic Cycle 7
Storage and Movement of Water 

Within the Principal Compartments of 

the Hydrologic Cycle 

The atmosphere, the land surface, and the subsurface 

are the three compartments that hold the Earth’s water. Each 

compartment acts as a storage reservoir within which water 

moves from its point of entry to the compartment to its point 

of outflow. Water also moves between compartments. A 

water-budget accounting unit may consist of a single part of 

one compartment, such as a lake, or an accounting unit may 

comprise parts of all three compartments, such as a watershed. 

This section discusses storage and movement of water within 

individual compartments. The following section discusses 

exchange of water between compartments. 

Water in the Atmosphere

At any one time, the atmosphere holds only a small 

fraction of the Earth’s water (table 1, fig. 2), the equivalent 

of a layer about 25 mm thick over all of the Earth’s surface. 

Yet this compartment is a vital part of the hydrologic cycle 

in terms of water storage and transport. Water flows to the 

atmosphere in a gaseous form as it evaporates from water, 

plant, and soil surfaces. This water will eventually condense, 

and possibly freeze, and be returned to the Earth’s surface as 

precipitation. Between the times of entry and departure from 

Hurricane Katrina.

National Aeronautics and Space Administration

Figure 2. The atmosphere within the hydrologic cycle. 

Surface-water inflow,

imported water

(pipelines, canals)

Ground-water inflow

Surface-water outflow,

exported water

(pipelines, canals)

Ground-water outflow

Aquifer

Evapotranspiration

Bedrock

Water table

Unsaturated zone

Precipitation

the atmosphere, a water molecule can be transported rapidly 

over long distances. The atmosphere is part of an amazing 

water-distribution system, carrying water from where it is 

plentiful (primarily oceans) and depositing it in regions where 

it is less plentiful (land surfaces). 

Water in the atmosphere is also important in Earth’s 

energy balance and climate. Evaporation and subsequent 

condensation of water require transfers of energy. As water 

moves from the liquid to gaseous state, it absorbs energy; as 

it condenses, that energy is released. 

Thus, the transport of water in the 

atmosphere is accompanied by a large 

transport of energy, effectively distributing energy across the Earth. Atmospheric water also affects radiation 

transfer at land surface. The formation 

of clouds limits the amount of solar 

radiation that reaches land surface. 

Long-wave radiation emitted by the 

Earth is absorbed and reflected back 

by gases, including water vapor, in the 

atmosphere (the greenhouse effect). 

Global climate and water storage in the 

atmosphere are linked. As the Earth’s 

temperature changes, so does the ability 

of the atmosphere to store water. In 

cyclic fashion, changes in the amount 

of water stored in the atmosphere can 

alter the Earth’s energy balance and 

thus affect surface temperatures. 

Movement of water within the 

atmosphere occurs over a range of 

space and time scales. Movement 

occurs both by convection (watervapor transport by moving air masses) 

and molecular diffusion (the natural 

8 Water Budgets: Foundations for Effective Water-Resources and Environmental Management
tendency of water vapor to move from areas of high concentration to areas of low concentration). The lower part of the 

atmosphere, called the atmospheric boundary layer, is the 

part of the atmosphere that is most influenced by the Earth’s 

surface. The layer varies in height between about 500 and 

2,000 m and typically holds about one-half of all atmospheric 

water. It is characterized by turbulent mixing generated as 

warm, moist air pockets move up from the heated surface 

and by frictional drag as the atmosphere moves over the 

Earth’s surface. Horizontal transport rates of water vapor 

Clouds in Hawaii.

within the atmospheric boundary layer can be as high as 50 to 

100 km/day (Oke, 1978).

Atmospheric transport of water is driven by gradients in 

pressure, temperature, and humidity. Predictions of moisture 

storage and movement are integral parts of weather forecasts. 

These forecasts are based on large-scale computer models that 

rely on data collected at National Weather Service surface 

monitoring sites across the United States. These surface sites 

provide point measurements of temperature, pressure, and 

humidity. Radar and satellite imagery provide additional data 

that are integrated over large areas.

Anvil cloud.

Cumulus clouds.

U.S. Department of Energy Atmospheric Radiation Measurement Program U.S. Department of Energy Atmospheric Radiation Measurement Program

Storage and Movement of Water Within the Principal Compartments of the Hydrologic Cycle 9
B Water Budgets are Intimately Linked to Energy and Chemical Budgets

Geothermal plant in California.

Geothermal Education Office, Tiburon, California

Rn

H

G

LE

Net

radiation

Sensible

heat flux

Surface

heat flux

Surface Energy Budget

Rn − G = H + LE

Latent

heat flux

Figure B–1. Schematic of the energy budget at the 

Earth’s surface.

Energy Budget 

The global water budget is intrinsically linked to the global 

energy budget. When water changes among its different phases 

(solid, liquid, and gas) energy is absorbed or released, thus affecting the energy budget. A simple energy budget for the Earth is 

(Sellers, 1965):

 Rn = G + LE + H (B1)

where Rn is net radiation (the sum of incoming solar and longwave radiation minus reflected solar and emitted longwave 

radiation); G is surface-heat flux (that is, the energy used to 

warm soil, or water in the case of a surface-water body); LE is 

latent heat flux (that is, the energy used to evaporate water); 

and H is sensible heat flux, or the energy used to warm air. The 

equation states that available energy at the Earth’s surface 

goes to heating the surface, warming the air, and evaporating water (fig. B–1) . Latent heat flux is the product of latent 

heat of vaporization (λ) and evapotranspiration rate (ET); that 

is, LE = λET. Evapotranspiration provides a direct link between 

the energy-budget and the water-budget equations because it 

appears in both equations. These equations form the basis of 

general circulation computer models that are used to predict 

climate trends. Estimation of ET rates can be addressed from both 

energy-budget and water-budget perspectives.

The movement of heat in ground and surface waters may 

be materially affected by the movement of water. An important component of energy transport is convection, or the movement of heat 

by the movement of water. The transport of energy by surface water is important in studies of powerplant or dam discharges in rivers 

where the health of natural fish populations is affected by heat loads or changing temperatures. Ground-water flow has been shown 

to be an important controlling factor on the occurrence and severity of volcanic eruptions (Matsin, 1991). The interdependence of 

water and energy movement has proved useful for estimating rates of exchange between ground and surface waters (Lapham, 1989; 

Stonestrom and Constantz, 2003). 

10 Water Budgets: Foundations for Effective Water-Resources and Environmental Management
Chemical Budget

Chemical fluxes are important to our environment. 

For example, fluxes and storage of carbon in the ocean, on 

land, within inland waters, and in the atmosphere have vital 

implications for ecosystems and climate. Water movement 

within and among the atmosphere, surface, and subsurface is an important mechanism for transport of chemicals 

through the environment. The water budget provides a foundation for understanding chemical fluxes and balances. As 

water contacts rocks, sediment, and organic materials, its 

chemistry is altered by reactions such as dissolution, precipitation, ion exchange, and oxidation/reduction. Groundand surface-water flows sustain many wetlands, lakes, and 

ponds. In addition to supplying water, these inflows also 

provide nutrients and chemicals that support biogeochemical processes within these bodies.

Chemicals are transported to the atmosphere naturally (by diffusion and wind advection, and through plants, 

fires, and volcanic activity) and as a result of human activities (combustion of fossil fuels, application of agricultural 

chemicals, and production of chemical compounds). Some 

chemicals become dissolved in atmospheric water and fall 

back to Earth in precipitation. Sulfate-bearing precipitation has been implicated as a major cause for acidification 

of some lakes in the Adirondack Mountains of New York 

(Driscoll and others, 2003).

Surface waters are reservoirs and conveyance 

mechanisms for chemicals and sediment. Sediment and 

contaminants can be washed off of streets and fields 

during rainfalls and be carried through storm drains to 

streams. It is estimated that, in one year, the Mississippi 

River discharged 900,000 tons of nitrate and 35,000 tons of 

orthophosphate to the Gulf of Mexico (Antweiler and others, 

1995). Severe rainfalls can lead to flooding, which can 

greatly enhance the transport capabilities of surface water. 

Floods are capable of transporting not only sediment and 

chemicals but also pathogens, animals, cars, and even houses.

Water moves more slowly through the subsurface than it does through surface-water bodies or the atmosphere. Hence, removal 

of subsurface contaminant plumes may take much longer than cleanup of surface plumes. Long residence times in the subsurface allow 

more time for reactions to occur and, in some instances, may promote natural remediation of contaminants by indigenous microbes 

(Lahvis and others, 1999).

Floodwaters can transport debris.

The Mississippi discharging water and sediment to the Gulf 

of Mexico.

National Aeronautics and Space Administration

Box B Water Budgets are Intimately Linked to Energy and Chemical Budgets 11

Water vapor rises from 

hot springs in Yellowstone 

National Park.
Snowpits are dug to determine water content and chemistry

 of snowpacks.

Ice fields in Antarctica, such as the Ross Ice Shelf, store 

about 70 percent of Earth’s freshwater.

National Aeronautics and Space Administration

Water on Land Surface 

Freshwater is present on the Earth’s land surface in solid 

and liquid forms. Solid forms include snow and ice; liquid 

water is stored in lakes, surface-water reservoirs, some wetlands, and streams. 

Snow and Ice

The largest amount of freshwater on Earth 

(29.2 million km3) is stored in glaciers and polar ice (Nace, 

1967). Most of this ice is present in Antarctica and Greenland 

and is largely inaccessible to humans. Solid water present as 

glaciers and snow in more temperate regions may be available for humans (fig. 3). Here, snow and ice serve as seasonal 

storage receptacles that contribute to water supplies upon 

melting. Melt from the annual snowpack, especially that captured in reservoirs, is the primary source of water for humans 

and aquatic ecosystems in many parts of the world. Glaciers 

represent a more permanent form of water storage. Residence 

time of water stored in glaciers can be decades to centuries. 

Meltwater from glaciers can sustain streamflows throughout 

the year.

12 Water Budgets: Foundations for Effective Water-Resources and Environmental Management
Atmospheric water, primarily in the form of snow, is the 

source of water to glaciers and snowfields. Water moves from 

these bodies to the atmosphere (as ablation), to the subsurface 

(as infiltration), and to streams. Measurement of water storage 

in seasonal snowpacks generally is done by conducting snow 

surveys, where snow depth and the water content of snow are 

determined in designated areas or along snow courses that 

transect an area. Measurement of changes in water stored in 

glaciers has historically been difficult because high mountain 

terrain is often inaccessible. Storage changes were determined 

Toboggan Glacier, Alaska, photographed by S. Paige on June 29, 1909 (left), and by Bruce F. Molnia on September 4, 2000 (right).

Precipitation

Surface-water inflow,

imported water

(pipelines, canals)

Ground-water inflow

Surface-water outflow,

exported water

(pipelines, canals)

Ground-water outflow

Aquifer

Evapotranspiration

Bedrock

Water table

Unsaturated zone

Figure 3. Snow and ice 

within the hydrologic cycle.

by repeated detailed surveys of the ice surface topography. In 

recent years, remote sensing from aircraft or satellite, used in 

conjunction with high-resolution digital-elevation models, has 

greatly enhanced the accuracy of these measurements.

Accurate determinations of water budgets of glaciers are 

rare. Only a few studies of glaciers have resulted in detailed, 

long-term monitoring of their water budgets (Mayo and others, 

2004). However, in a general way, comparative photographs 

(a form of remote sensing) of glaciers show that many glaciers 

have been shrinking over the last few decades. 

Storage and Movement of Water Within the Principal Compartments of the Hydrologic Cycle 13
C Chemical, Isotopic, and Energy Tracers Provide Insight

 into Hydrologic Processes

Direct physical measurements of water-budget components may at times be inconvenient, problematic, or impractical. In such 

cases, indirect methods may provide estimates of water-budget components or act to reduce the uncertainty associated with those 

estimates. Chemical, isotopic, and energy (heat) tracers are commonly used to provide insight into processes such as ground-water 

recharge, ground-water discharge to lakes and wetlands, and base flow. A tracer is simply a chemical or isotope (or property, in the 

case of heat) that is transported by water. Analysis of spatial or temporal patterns of tracer concentrations can be used to identify 

trends in water movement and therefore can provide insight for shaping conceptual models of water budgets.The ideal hydrologic 

tracer is one that moves with water, is conservative (that is, not altered by reactions or other processes in water, porous media, or 

atmosphere), and is easily and accurately detected. Tracers can be categorized as environmental, historical, and applied. Environmental 

tracers are those that occur naturally in the environment. Isotopes of oxygen and hydrogen have been used for decades to distinguish 

sources of water and to examine water balances (Gat and Gonfiantini, 1981). These isotopes are well suited as tracers because they are 

part of the water molecule itself. Carbon isotopes, chloride, sulfate, and nitrate are other useful environmental tracers. Historical tracers 

are those that were released to the environment continuously or or at specific times during the past. Radionuclides (including tritium, 3H, 

14 Water Budgets: Foundations for Effective Water-Resources and Environmental Management

Tracers are used to 

determine the age of 

subsurface water (that is, 

the time since that water 

last had contact with the 

atmosphere), velocities 

and traveltimes for ground 

and surface waters, and 

travel paths of water in 

the subsurface.

Applying dye tracer to land surface at research site in Minnesota.

Tracers are used in streams to measure traveltimes.

Dye tracer is visible in trench excavated beneath 

application area.
Table C–1. Examples of tracers used in water-budget studies.

Use Naturally occurring 

in the environment

Historical — Added to the 

environment from human 

activity in the past

Applied — Added to 

the environment in the 

present

Example study

Ground-water age — 

Time since recharge 

water became isolated 

from the atmosphere

35S, 14C, 3

H/3He, 39Ar, 36Cl, 32Si

3

H, 36Cl, 85K, 

chlorofluorocarbons, 

herbicides, caffeine, 

pharmaceuticals

Plummer and others (2001)

Temperature of recharge N2/Ar solubility Plummer (1993)

Tracing ground-water 

flow paths 18O, 2

H, 13C, 87Sr 

Chlorofluorocarbons,

herbicides, caffeine, 

pharmaceuticals

Cl, Br, dyes Renken and others (2005)

Exchange of surface

water and ground water

18O, 2

H, 3H, 14C, 222Rn Cl, Br, dyes Katz and others (1997)

Surface-water discharge 

and traveltime Cl, Br, dyes Kimball and others (2004)

600

400

200

0

1940 1960 1980

0

100

200

2000

36Cl

3

H CFC–12

CFC–11

YEAR

CONCENTRATION, IN PARTS PER

TRILLION BY VOLUME (CFC–11

and CFC–12) OR TRITIUM UNITS (3H)

36Cl FALLOUT, IN ATOMS PER

SQUARE METER PER YEAR

and chlorine-36, 36Cl) released to the atmosphere from testing 

of nuclear bombs in the 1950s and 1960s fall into this class 

(fig. C–1). Chlorofluorocarbons (CFCs) and sulfur hexafluoride 

were released to the atmosphere by industrial processes 

over the last 50 years and are common hydrologic tracers 

(http://water.usgs.gov/lab/). For example, Katz and others 

(1995) used concentrations of CFCs to estimate the ages of 

ground water near Lake Barco in Florida (fig. C–2). Applied 

tracers include those introduced intentionally (for example, 

chloride, bromide, and dyes) and those inadvertently introduced to the environment, such as through a chemical spill. 

Applied tracers commonly are used to determine velocities 

of streamflow and ground-water flow, to identify subsurface 

flow paths, and to quantify exchange rates between surface 

and ground waters. Properties and uses of common hydrologic tracers are given in table C–1.

VERTICAL SCALE GREATLY EXAGGERATED

–50

SEA

LEVEL

SEA

LEVEL

50

100

150

FEET

–50

50

100

150

FEET

0 500 FEET

Water table

Lake Barco

1986

1964

1986

1987

1987

1981

1978

1973

1967

1963

1962

1959

1958

Bedrock

Surficial sand

WELL LOCATION–Number is the year

 that ground water at that location

 was recharged

ORGANIC-RICH SEDIMENTS

EXPLANATION

Figure C–1. Atmospheric concentrations for historical 

tracers, including 3H, 36Cl, CFC–11, and CFC–12 (after Scanlon 

and others, 2002).

Figure C–2. Lake Barco, in northern Florida, is a 

flow-through lake with respect to ground water. The 

dates when water in different parts of the groundwater system was recharged indicate how long it 

takes water to move from the lake or the water table 

to a given depth (after Katz and others, 1995).

Box C Chemical, Isotopic, and Energy Tracers Provide Insight into Hydrologic Processes 15
Surface-water inflow,

imported water

(pipelines, canals)

Ground-water inflow

Surface-water outflow,

exported water

(pipelines, canals)

Ground-water outflow

Aquifer

Evapotranspiration

Bedrock

Water table

Unsaturated zone

Precipitation

Figure 4. Lakes, wetlands, and streams within the hydrologic cycle.

Lakes 

Lakes are the fourth largest reserve of water in the global 

water budget. The volume of water in natural lakes is estimated to be about 229,000 km3

 (table 1; fig. 4). Of this total 

volume, 125,000 km3 are in freshwater lakes and 104,000 km3

are in saline lakes; the Caspian Sea alone contains about 

95 percent of the total volume of water in saline lakes. For 

this report, surface-water reservoirs are considered to be 

lakes. Lvovitch (1973) estimated the total volume of water in 

reservoirs to be about 5,000 km3. The largest reservoir in the 

United States, Lake Mead, contains about 38 km3 of water at 

full pool elevation; Lake Powell contains about 33 km3 at full 

pool elevation.

Lakes interact with the atmosphere, the subsurface, and 

other surface-water features. They gain water from precipitation, streamflow, and ground water and lose water by 

evaporation, surface outflow, and seepage to ground water. 

However, all these interactions do not occur for every lake. 

Some topographically high lakes have no stream or groundwater inflows, gaining water only from precipitation. At the 

other topographic extreme, some lakes, called terminal lakes, 

receive water from precipitation, streams, and ground-water 

inflow and lose water only by evaporation. 

The volume of water in a lake may be determined by 

preparing a bathymetric map of the lake bottom and by 

calculating the volume of water present at a given lake stage 

(lake level). Lake stage is measured by reading a staff gage, 

or it can be continuously monitored by a recording gage. A 

stage-volume relation is then established that can be used 

to determine the volume of water at any given stage. This 

approach can produce accurate results if the bathymetry is 

well defined.

Residence times of water in lakes span a wide range. 

Residence time is calculated by dividing the volume of a lake 

by the rate of outflow. For very large lakes, like Lake Superior, 

residence time is nearly 200 years. Lake Powell, much smaller 

but still a large surface-water reservoir, has a residence time 

of about 2.3 years. Lakes with no stream outlet, like many in 

glacial terrain, can have residence times of several years to a 

decade, and small lakes with outlet streams commonly have 

residence times of days to weeks (Winter, 2003)

16 Water Budgets: Foundations for Effective Water-Resources and Environmental Management
Crater Lake, Oregon.

The Great Lakes.

Lake country in northern Wisconsin.

National Aeronautics and Space Administration

Storage and Movement of Water Within the Principal Compartments of the Hydrologic Cycle 17
“Wetlands are lands where 

saturation with water is the dominant factor determining the nature 

of soil development and the types 

of plant and animal communities 

living in the soil and on the surface. The single feature that most 

wetlands share is soil or substrate 

that is at least periodically saturated 

with or covered by water.” 

Cowardin and others (1979, p. 3)

Wetlands

Wetlands in depressions generally contain standing water 

and in many respects are much like lakes. Many types of 

wetlands do not contain standing water, however, or contain 

it for only brief periods each year. Such wetlands consist 

mainly of saturated soils. Most wetlands receive surfacewater inflow at some time of the year, some are fed by both 

surface and ground water, and others are supported solely by 

ground-water flow. Like lakes, some wetlands located high in 

the landscape gain water only from precipitation; others, low 

in the landscape, like terminal lakes, lose water only to the 

atmosphere. A major difference between wetlands and lakes is 

that wetlands lose water to the atmosphere largely by transpiration from plants, whereas lakes lose water to the atmosphere 

mostly by evaporation.

Determining the volume of water in a wetland and the 

change in that volume over time is more difficult than it is for 

lakes because, other than the open-water portion, water is present in wetland soils. Measurement of water storage in soils is 

addressed in the section “Unsaturated Zone.”

U.S. Fish and Wildlife Service

Lower Klamath National Wildlife Refuge, California.

Depressional wetlands, North Dakota.

Red Rock Lakes National Wildlife Refuge, Montana.

18 Water Budgets: Foundations for Effective Water-Resources and Environmental Management
Streams

The volume of water in the Earth’s streams at any given 

time (about 1,250 km3 according to Nace, 1967) represents 

only a small part of the total volume stored on land surface.

Streams, then, are generally not important in terms of global 

water storage. Streams function mainly to transport water, 

conveying it from higher to lower altitudes on the land surface 

and, in most cases, ultimately to the oceans. Streams also 

facilitate water exchange between the surface and the subsurface, and to a lesser extent between the surface and the 

atmosphere. 

Sources of water in streams can be surface-water bodies, 

surface runoff of precipitation (as well as direct precipitation on a stream), interflow (shallow subsurface flow usually 

associated with hillslopes), and base flow (ground-water 

discharge). Along their course, streams can lose water to other 

surface-water bodies, to the subsurface, and to the atmosphere 

(by evaporation). Streams range in size from small rivulets 

in headwater areas that flow only after precipitation events to 

large rivers, such as the Mississippi and the Amazon. Magnitudes of velocities in streams are variable; 30 cm/s may 

be typical, whereas 300 cm/s is quite high. Because they are 

confined to channels on the Earth’s surface, streams are visible 

and relatively accessible for measurement of discharge and are 

therefore the part of the hydrologic cycle that can be measured 

most accurately.

“A river seems a magic thing. A magic, moving, living part of the very 

earth itself—for it is from the soil, both from its depth and from its 

surface, that a river has its beginning.” 

Laura Gilpin (1949)

Storage and Movement of Water Within the Principal Compartments of the Hydrologic Cycle 19
Large streams in the United States tend to show seasonal 

trends (fig. 5A); highest discharges generally occur in spring, 

a time when snow melts, soils thaw, and soil moisture contents 

are high. Small streams are usually more dynamic than large 

streams and they show rapid rises and falls in response to 

storms (fig. 5B). The source of water in a stream also influences discharge patterns. Streams dominated by snowmelt or 

base flow follow a more predictable pattern than those dominated by surface runoff.

For major streams, the U.S. Geological Survey maintains a network of thousands of stream gages across the 

United States (http://water.usgs.gov). Stream level (stage) is 

Oct Jan Apr

2000 2001

DAILY DISCHARGE, IN CUBIC FEET PER SECOND DAILY DISCHARGE, IN CUBIC FEET PER SECOND

2002 2003

July Oct Jan Apr July Oct Jan Apr July Oct

Oct Jan Apr

2000 2001 2002 2003

July Oct Jan Apr July Oct Jan Apr July Oct

60

100

0

500

1,000

1,500

1,000

10,000

20,000

A

B

Figure 5. Streamflow hydrographs for two gaging sites: (A) Kishwaukee 

River near Perryville, Illinois (U.S. Geological Survey station number 05440000; 

drainage area 1,099 square miles) and (B) South Branch Kishwaukee River at 

DeKalb, Illinois (U.S. Geological Survey station number 05439000; drainage area 

77.7 square miles). Red indicates estimated values.

20 Water Budgets: Foundations for Effective Water-Resources and Environmental Management

Stream network in arid region, Organ Pipe National Monument, 

Arizona.

American Geological Institute, Michael Collier
monitored continuously at these sites, and a stage/discharge 

relation is developed using periodic discharge measurements. 

Discharge is the product of stream velocity and cross-sectional 

area integrated over that area. Velocity has historically been 

measured manually at many locations along a cross section by 

using a current meter. Recently, acoustic velocity meters have 

reduced the need for manual measurements (Yorke and Oberg, 

2002). By establishing good stage/discharge relations, stream 

USGS gaging station.

Flumes can be used to measure discharge in small streams.

Discharge is determined with measurements of stream 

depth and velocity.

discharge can be determined from measurements of stage. 

Typical errors in stream discharge measurements are 10 percent (Rantz and others, 1982). 

For small streams, more accurate measurements of 

discharge can be obtained by installing a flume or a weir and 

a stage recorder in the channel. Flumes and weirs are carefully calibrated in hydraulic laboratories, so measurements of 

discharge commonly have errors of about 5 percent. 

Storage and Movement of Water Within the Principal Compartments of the Hydrologic Cycle 21
D Models—Important Tools in Water-Budget Studies

Plant canopy

interception 

Precipitation

Solar

radiation

Air temperature

Surface runoff

to stream

Soil-Zone Reservoir Impervious-Zone Reservoir

Snowpack

Transpiration

Evaporation

and

Transpiration Evaporation

Rain Rain

Snowmelt

Evaporation Sublimation

Throughfall

Recharge zone

Lower zone

Subsurface

Reservoir

Subsurface recharge

Ground-water recharge

Ground-water

recharge

Ground-Water

Reservoir Ground-water flow to stream

Ground-water

sink

Interflow or

subsurface

flow to stream

Figure D–1. Shematic diagram showing various reservoirs and processes 

that are considered in a watershed model 

(R.S. Regan, written commun., 2007).

Hydrologic computer-simulation models 

contribute substantially to our understanding of 

the hydrology of watersheds, rivers, and aquifers. They are integral tools for managing water 

resources in many areas. Using calculations 

that are too cumbersome to be performed by 

hand, these models allow detailed investigation 

of complex hydrologic processes and provide 

predictions of responses within a specific 

water-budget accounting unit to external or 

internal stresses. Most hydrologic computersimulation models are derived from some 

variant of equation A2 and thus are truly waterbudget models. As water-budget equations vary 

greatly in complexity, so do the models that are 

based on them. A simple model may provide a 

quick view of the water budget for an accounting unit but is unlikely to provide insight into the 

processes that drive water movement within 

that unit. A more complex model may provide 

that insight but at substantially greater expense. 

Watershed models are perhaps the most 

complete form of a water-budget model. They predict stream discharge within a basin in response to 

precipitation and snowmelt, usually accounting for 

processes such as evapotranspiration, groundwater/surface-water exchange, and surface-water 

routing (fig. D–1). Watershed models are widely 

used for watershed management and planning. For 

example, they can be used to predict the effects of 

land-use changes (such as urban development) on 

streamflow (fig. D–2).

Figure D–2. Steuer and Hunt (2001) used a watershed model to simulate water fluxes in the Pheasant Branch Creek watershed 

near Middleton, Wisconsin, for the period 1993 to 1998. The model was subsequently used to predict the effects of urban 

development in the watershed.

Ground water to regional system Base flow

EXPLANATION

Interflow Overland flow

Evapotranspiration Interception

1993 1994 1995 1996 1997 1998

INCHES

–5

5

15

25

35

45

55

Change in local ground-water storage

Average annual basin water budget, in inches,

for water years 1993–98

Precipitation

Evapotranspiration

Overland flow

Interflow

Ground water to regional system (not captured by stream)

35.2 23.9

2.8

0.4

Base flow

1.5

6.0

Budget not balanced because of change in ground-water storage.

22 Water Budgets: Foundations for Effective Water-Resources and Environmental Management
30

25

20

20

20

15

10

5

15

25

5

5

25

20

20 15

5 5

10

15

10

20

15

30

15

10 5 5

98 ̊

98 ̊

97 ̊

97 ̊

98 ̊

97 ̊

97 ̊

41 ̊ 41 ̊ 41 ̊ 41 ̊

40 ̊ 40 ̊ 98 ̊40 ̊ 40 ̊ NEBRASKA

NEBRASKA

KANSAS

NEBRASKA

KANSAS

0

0 20 KILOMETERS

20 MILES

20

EXPLANATION

Line of predicted equal

 water-level decline, in feet

20 Line of measured equal

 water-level decline, in feet

Little Blue River

Big Blue River

Little Blue River

Big Blue River

A B

MAP AREA

Figure D–3. Contours of model predicted (A) and measured (B) 

ground-water levels for the Blue River Basin, Nebraska (Alley and 

Emery, 1986).

89 ̊32'30"

89 ̊35'

43 ̊07'30"

43 ̊05' 

Pheasant Branch Basin

Ground-water-flow models predict how water levels 

in an aquifer will be affected by changes in withdrawals 

or in recharge rates (fig. D–3). They are used in studies 

of ground-water supply and ground-water contaminant 

transport. Most of these models simulate flow only in 

the saturated zone (that is, the region beneath the water 

table). Other more complex models simulate water movement within both the unsaturated and saturated zones.

Streamflow routing models predict stream discharge and velocity. Managers use these models to 

estimate where, when, and at what stage flood waves will 

crest, allowing them to adjust release rates from reservoirs to mitigate adverse effects of flooding. 

General circulation models forecast weather and 

climate trends at the continental scale over periods of 

days to centuries. 

Soil–vegetation–atmospheric transport models

are used to study the movement of water from the 

atmosphere to the soil through plants and back into the 

atmosphere. 

Coupled models combine water-budget models with 

mass or energy transport models and are useful for simulating contaminant transport in surface or ground water. 

Statistical techniques (such as regression, 

nonparametric statistics, and geostatistics), while not 

water-budget models, are important in many water-budget studies. They can be used for quantifying uncertainty in simulation results, 

determining which types of data can improve simulation results, and interpolating and integrating point measurements (from a rain 

gage, for example) over entire watersheds or basins. 

Figure D–2. Steuer and Hunt (2001) used a watershed 

model to simulate water fluxes in the Pheasant Branch 

Creek watershed near Middleton, Wisconsin, for the 

period 1993 to 1998. The model was subsequently used 

to predict the effects of urban development in the 

watershed.—Continued

Box D Models—Important Tools in Water-Budget Studies 23
Water in the Subsurface

The Earth’s subsurface consists of solid rock, mineral 

grains, organic matter, and varying amounts of water and 

other liquids and gases that occupy open spaces or voids. The 

subsurface serves as the major reservoir of extractable freshwater, accounting for more than 95 percent of worldwide storage. On the annual global scale, change in storage of water 

in the subsurface is negligible. At smaller scales, changes in 

subsurface storage can be substantial and significant. Groundwater levels in the San Joaquin Valley of California declined 

as much as 100 m between 1920 and 1970 as 

a result of pumping for irrigation. In addition 

to a reduction in the amount of water stored 

in the subsurface, the declining water levels 

resulted in land-surface subsidence of more 

than 9 m in some areas (Galloway and others, 

1999).

A principal difficulty in quantifying the 

movement and storage of water in the subsurface is the natural variability in the physical 

and hydrologic properties of earth materials at 

all spatial scales. For convenience, discussion 

of subsurface hydrology is divided into the 

unsaturated zone (where open spaces or voids 

in the earth materials are partly filled with 

water and partly filled with air) and the saturated zone (where voids in the earth materials 

are completely filled with water). 

Unsaturated Zone

The unsaturated zone, sometimes 

referred to as the vadose zone or zone of aeration, encompasses the earth materials that lie 

between the land surface and the water table 

(fig. 6). The thickness of this zone varies spatially and temporally and may range from 0 to more than 1,000 m. In general, 

thicker unsaturated zones are found in more arid regions. 

No known estimates exist for the amount of water stored in 

unsaturated zones at the global or continental scales. The 

importance of the unsaturated zone as a storage reservoir is 

often overlooked because the water held there generally is not 

extractable for human use. The unsaturated zone, however, is 

the primary source of water for vegetation and therefore plays 

a critical role in the hydrologic cycle. An estimated 76 percent 

of precipitation infiltrates the subsurface (table 2). Because 

water moves through the unsaturated zone at a relatively slow 

rate, plants are able to extract that water over extended periods 

of time. About 85 percent of the water that infiltrates the soil 

surface returns to the atmosphere either by evaporation from 

soil or by plant transpiration. 

Water storage within the unsaturated zone is determined 

by measuring moisture content at different depths between the 

land surface and the water table. Repeated measurements over 

time can be used to infer rates of storage change. Moisture 

content can be measured directly by collecting samples in the 

field and weighing the sample before and after oven drying. 

Indirect techniques, which are more conducive to automatic 

recording, take advantage of electrical or physical properties 

of the sediment-water continuum (for example, time domain 

reflectometry and neutron moderation). 

Infiltrated water moves predominantly in a downward 

direction through the unsaturated zone toward the water table. 

Water also can move upward (in response to evaporative 

demand) or laterally (in the case of impeding layers of soil). 

Rates of water movement are notoriously difficult to measure 

American Geological Institute, Bruce F. Molnia

Heterogeneity of subsurface sediments complicates study of 

water movement in the subsurface.

Surface-water inflow,

imported water

(pipelines, canals)

Ground-water inflow

Ground-water outflow

Aquifer

Evapotranspiration

Bedrock

Water table

Surface-water outflow,

exported water

(pipelines, canals)

Unsaturated zone

Precipitation

Figure 6 . The unsaturated zone within the hydrologic cycle.

24 Water Budgets: Foundations for Effective Water-Resources and Environmental Management
directly because of problematic measurement techniques and 

the variable nature of the fluxes. Lysimeters (see Box E—

Lysimeters: Water-Budget Meters) can provide accurate, albeit 

expensive, measurements of these fluxes. More commonly, 

flux rates are inferred by using indirect approaches such as the 

Darcy approach or unsaturated-zone water-budget methods. 

The Darcy approach requires measuring depth profiles of pressure head (sometimes referred to as matric potential or soilwater tension, measured with tensiometers, heat-dissipation 

or electrical conductivity probes, or thermocouple psychrometers) and unsaturated hydraulic conductivity. Unsaturatedzone water-budget methods are based on measurement of 

changes in water storage in the unsaturated zone over time (for 

example, the zero-flux plane method) or analysis of fluctuations in water-table elevations (Scanlon and others, 2002).

Moisture content profiles within the unsaturated zone 

typically display seasonal trends (fig. 7). Largest fluctuations 

occur near land surface; the magnitude of the annual fluctuaWater in the unsaturated zone sustains most vegetation.

Installing moisture content sensors through the wall of a trench; 

the trench was later backfilled.

0.20 0.25 0.30 0.35 0.40

MOISTURE CONTENT, DIMENSIONLESS

0

2

4

6

8

DEPTH, IN METERS 

Winter

Spring

Summer

Fall

Figure 7. Hypothetical moisture-content profiles at four 

different times of the year. As depth increases, the variation in 

moisture content decreases. 

The unsaturated zone at Yucca Mountain, Nevada is as thick as 500 

meters.

tions decreases with depth. At some depth, moisture contents 

may show no measurable change throughout the year. This 

does not mean that there is no flow occurring at these sites; 

rather, this implies a constant flux of water (usually small in 

magnitude). 

Residence times of water within the unsaturated zone 

depend upon factors such as climate, geology and soils, depth 

to water table, and vegetation. In most areas, the residence 

time of water in the root zone ranges from days to months 

(although some water is maintained in small pores over much 

longer periods; this is referred to as immobile water, and its 

presence has been identified through tracer tests). For the 

region below the root zone, residence times can be estimated 

as the amount of water stored there divided by the estimated 

flux through that region. In humid areas with thin unsaturated zones, residence times are usually a year or less. In arid 

regions, residence times may be millennia.

Forest Service

Storage and Movement of Water Within the Principal Compartments of the Hydrologic Cycle 25
E Lysimeters—Water-Budget Meters

The GSF National Research Center for Environment and Health of the Institute for Soil Ecology, Neuherberg, Germany, operates 

32 lysimeters for studies of water budgets, ground-water recharge, and nutrient uptake by plants. (http://www0.gsf.de/eus/index_e.html, 

accessed on February 26, 2007) 

GSF – National Research Center for Environment and Health

Lysimeters are instruments specifically designed for measuring one or more components of the water budget, such as evapotranspiration or ground-water recharge. Most lysimeters consist of containers filled with soil, hydrologically isolated from the surrounding 

undisturbed environment but intended to mimic the hydrologic behavior of that environment. Lysimeters vary in design from simple 

collection vessels with a surface area on the order of 100 cm2 to units constructed on sensitive weighing balances with surface areas of 

several square meters (Young and others, 1996). Some instruments are capable of resolving fluxes of less than 1 mm/d. When properly 

constructed and maintained, lysimeters provide perhaps the most sophisticated approach for studying water budgets at a small scale. 

Assuming that there is no surface or subsurface flow to it, the water budget for a lysimeter is:

 ∆S = P – ET – RO – D (E1)

where

∆S is change in storage within the lysimeter and is determined on a weight basis, 

P is precipitation and irrigation, 

ET is evapotranspiration, 

RO is runoff, 

and 

D is drainage out the bottom of the lysimeter.

Installations with weighing lysimeters typically are also equipped with precipitation gages and runoff collectors. In addition, most 

lysimeters permit measurement and collection of drainage, either by having a free-draining base or by having a porous plate base 

across which a tension can be imposed by means of a vacuum or wick system. With independent measurements of P, RO, and D, the 

lysimeter provides a direct measurement of ET: 

 ET = P – ∆S– RO – D (E2)

During periods when precipitation, runoff, and drainage are all zero, changes in weight of the lysimeter are due solely to evapotranspiration.

26 Water Budgets: Foundations for Effective Water-Resources and Environmental Management
January

February

March

April

May

June

July

August

September

October

NovemberDecember

January

February

March

April

May

June

July

August

September

October

NovemberDecember

120

100

80

60

40

20

1978

Precipitation

Lysimeter drainage

1979

0

WATER DEPTH, IN MILLIMETERS

Figure E–1. Monthly rainfall and drainage from lysimeter at Fleam Dyke (Kitching and Shearer, 1982).

Installing pan lysimeter through trench wall with a 

hydraulic jack.

Pan lysimeter.

Large drainage lysimeters are expensive to construct and often problematic to maintain. As such, they are rarely used in hydrologic studies. Figure E–1 shows measurements of rainfall and drainage at one such lysimeter at Fleam Dyke, England (Kitching and 

Shearer, 1982). Small, simple lysimeters are easier to install and maintain and are practical for evaluation of spatial variability of 

evaporation and irrigation, for example. With any lysimeter, careful design and installation are required to avoid altering the natural 

hydrologic conditions of the system under study. 

Box E Lysimeters—Water-Budget Meters 27
Precipitation

Surface-water inflow,

imported water

(pipelines, canals)

Ground-water outflow

Evapotranspiration

Bedrock

Water table

Surface-water outflow,

exported water

(pipelines, canals)

Ground-water inflow Unsaturated zone

Aquifer

Figure 8. The saturated zone 

within the hydrologic cycle.

Inflow to the saturated zone, often referred to as groundwater recharge, occurs when water from precipitation (and 

perhaps irrigation) percolates downward through the unsaturated zone or when water moves from surface-water bodies 

to the water table (see Box F—Ground-Water Recharge). 

Outflow from the saturated zone occurs naturally to surfacewater bodies (for example, through seeps or springs) and 

to the atmosphere by evapotranspiration. In humid regions, 

ground-water discharge to streams is typically the dominant 

outflow mechanism and can account for more than 90 percent 

of annual flow in some streams. In arid regions, there may 

be essentially no ground-water discharge to streams but high 

rates of ground-water evapotranspiration. In some regions, 

human extraction of ground water for domestic, agricultural, 

and industrial uses constitutes the major portion of outflow.

The subsurface is composed of geologic materials of 

varying chemical and physical properties. Conceptualization 

of the geologic features and how they affect ground-water 

flow (fig. 9) is a difficult but fundamental part of groundwater investigations. Insight on boundaries of ground-water 

flow systems, rates of water movement, amounts of water in 

storage, and rates and locations of recharge and discharge 

must be inferred from sources such as geologic maps, geophysical tests, ground-water levels, physical and chemical 

properties of water and rock, spring and streamflow records, 

and ground-water-flow models. 

An aquifer is a body of earth material that contains 

sufficient permeable material to yield significant quantities 

of water to wells. “Significant quantities” is a relative term: 

pumping rates of 2 m3

/min or greater are considered large 

rates; 0.04 m3

/min is considered a small rate even though it is 

more than sufficient to supply the needs of most households. 

Saturated Zone

Ground water, water stored within 

the saturated zone, constitutes the largest reservoir of extractable freshwater on 

Earth (table 1, fig. 8). More than 1.5 billion 

people worldwide, including about 50 percent of the population of the United States, 

rely on ground water for their drinking 

water. The importance of ground water is 

sometimes overlooked simply because the 

subsurface is hidden from our view. There 

are no windows through which we can 

view the vastness and complexities of the 

saturated zone.

The saturated zone is bounded above 

by the water table or by the fixed interfaces 

at the bottom of surface-water bodies. The 

lower boundary of the saturated zone is 

difficult to define. There is a tendency for 

pores in earth materials to become smaller 

and fewer with depth, thus limiting the 

availability of the stored water to humans. 

Saline ground water underlies fresh ground 

water in most areas.

Artesian well in Boyd County, Nebraska, circa 1900.

28 Water Budgets: Foundations for Effective Water-Resources and Environmental Management
High hydraulic conductivity

Moderate hydraulic conductivity

Low hydraulic conductivity

Direction of ground-water flow

EXPLANATION

Bedrock

2 2

1

1

3

4

3

Figure 9. Ground-water flow systems in complex 

geological terrain. Ground water in the uppermost 

part of the ground-water system flows from 

surface-water body to surface-water body in the 

high hydraulic conductivity zone (characteristic of 

glacial outwash) because the water table slopes 

uniformly from one to the other (1). However, 

where the hydraulic conductivity of the deposits 

is lower (characteristic of glacial till), water-table 

mounds beneath the land-surface highs cause 

the local flow systems to discharge to contiguous 

surface-water bodies, such that there is no 

flow from one surface-water body to the next 

lower surface-water body (2). In both settings, 

intermediate-scale ground-water flow systems 

pass at depth beneath the local flow systems (3), 

and a regional ground-water flow system passes 

at depth beneath both local and intermediate flow 

systems (4).

18 A

20

22

24

26

Oct.

1986

Oct.

1988

Oct.

1990

Oct.

1992

Oct.

1994

Oct.

1996

WATER LEVEL, IN FEET BELOW LAND SURFACE

50

70

90

110

130

150

1925 1935 1945 1955 1965 1975 1985 1995

WATER LEVEL, IN FEET BELOW LAND SURFACE

Missing

record

Well Sh:P-76 at Memphis

Land-surface altitude: 287 feet

Well depth: 488 feet

B

Figure 10. (A) Hydrograph of daily water-level measurements 

over a 10-year period for a well in Vanderburgh County, Indiana; 

and (B) water-level trends in a long-term observation well in 

Memphis, Tennessee (Taylor and Alley, 2001). 

Aquifers in direct connection to the atmosphere are unconfined or water-table aquifers. Confined aquifers are separated 

from the atmosphere by a confining unit, which consists of 

materials with a hydraulic conductivity much lower than that 

of the aquifer. Hydraulic conductivity is a measure of a geologic material’s ability to transmit water (see Box G—Estimating Aquifer Hydraulic Conductivity).

Wells are important in ground-water studies. They provide direct access to the subsurface environment and make it 

possible to measure ground-water levels, to obtain water samples for chemical analysis, to conduct aquifer tests to estimate 

aquifer properties, and to apply geophysical techniques to 

estimate physical and chemical properties of earth materials. 

Long-term measurements of ground-water levels provide data 

for evaluating trends over time, calibrating ground-water-flow 

models, and assessing resource-management schemes. 

Systematic water-level measurements in networks of monitoring wells across the country are conducted by a wide array 

of organizations. Measurements are made electronically or 

manually at frequencies ranging from hourly to annually.

Water levels in many aquifers fluctuate seasonally in 

response to recharge and discharge patterns. Levels tend to 

rise from fall through early spring, when precipitation rates 

usually exceed evapotranspiration rates, and decline in late 

spring and summer when evapotranspiration rates are high 

(fig. 10A). The magnitude of fluctuations varies from aquifer 

to aquifer and year to year depending on geology, water use, 

and climate. In addition to seasonal patterns, long-term trends 

occur in some aquifers as a result of changes in climate (for 

example, droughts) or stresses imposed by humans. Figure 10B shows water levels for a period of almost 70 years 

from a well located in Memphis, Tennessee. Water levels 

declined by about 70 ft between 1928 and 1975 as the rate 

of pumping from the aquifer increased. After 1975, pumping 

rates stabilized and the long-term decline abated (Taylor and 

Alley, 2001). 

Storage and Movement of Water Within the Principal Compartments of the Hydrologic Cycle 29
EXPLANATION

152.31

Location of well and

 altitude of water

 table above sea

 level, in feet

Water-table contour—

 Shows altitude of

 water table. Contour

 interval 10 feet. Datum

 is sea level

Ground-water flow

 line

140

138.47 152.31

131.42

126.78

132.21

137.90

121.34 128.37

138.47 152.31

131.42

145.03 145.03

126.78

132.21

137.90

121.34 128.37

150

140

130

120

138.47 152.31

131.42

145.03

126.78

132.21

137.90

121.34 128.37

150

140

130

120

A B

C

Figure 11. By using known altitudes of the water table at 

individual wells (A), contour maps of the water-table surface can 

be drawn (B), and directions of ground-water flow along the water 

table can be determined (C) because flow usually is approximately 

perpendicular to the contours (Winter and others, 1998).

Well installation requires special equipment.

Water-level contour maps of aquifers provide a means by 

which ground-water movement and storage can be assessed. 

These maps can be constructed from water-level measurements obtained from multiple wells at about the same time 

(fig. 11). Alternatively, maps can be based on water levels 

generated by ground-water-flow models. In general, ground 

water moves from areas of higher water-level altitudes to 

areas of lower water-level altitudes. As shown in figure 11C, 

lines drawn perpendicular to water-level contours indicate 

direction of ground-water flow. 

The amount of water stored in the subsurface changes as 

pores (voids between soil grains) drain or fill and as water and 

the geologic material compress or expand. Change in aquifer 

storage between any two points in time is calculated as the 

product of the difference in water levels at the two times and a 

storage coefficient, Sc. For unconfined aquifers, gravity drainage and filling of pores is the dominant mechanism for storage 

change, and the storage coefficient, called specific yield (Sy), 

has values that range from about 0.02 for fine-grained sediments to 0.35 for very coarse grained sediments. Storage 

30 Water Budgets: Foundations for Effective Water-Resources and Environmental Management
PUMPED WELL

RECHARGE AREA

Stream

 DISCHARGE AREA

Days

Years

Years

Days

Centuries

Millennia

Confining bed

Confining bed

Confined

aquifer

Unconfined

aquifer

Confined

aquifer

Water table

Figure 12. Ground-water flow paths vary greatly in length, depth, 

and traveltime from points of recharge to points of discharge in 

the ground-water system.

Measuring water levels in an observation well in 

Colorado.

changes for confined aquifers are dominated by water and 

sediment compression and expansion, and storage coefficient 

values are much less, typically in the range of 10–5 to 10–4. 

Values of storage coefficient are best determined with aquifer 

tests that integrate over a fairly large area. Laboratory and 

empirical methods for determining specific yield (Healy and 

Cook, 2002) are easier to apply but test only a very small 

sample of an aquifer. 

Ground-water flow simulations and chemical, isotopic, 

and energy tracers are useful tools for identifying groundwater flow paths and estimating traveltimes (fig. 12). Magnitudes of ground-water velocities vary widely. A value of 

1 meter per day or greater is considered high. A value of 

1 meter per decade is considered low but not unusual for a 

confining unit. Thus, even for water-table aquifers, the time 

needed for small parcels of ground water to traverse the aquifer along the longest flow paths from point of recharge at the 

water table to point of discharge can be decades or longer.

Storage and Movement of Water Within the Principal Compartments of the Hydrologic Cycle 31
F Ground-Water Recharge

Water

Recharge―in centimeters 

 per year

 Greater than 30

 25.01 to 30

 20.01 to 25

 15.01 to 20

 10.01 to 15

 5.01 to 10

 0 to 5

 Unclassifiable

EXPLANATION

0 20 40 60 80 M ILES

0 20 40 60 80 KILOMETERS

94o 

92o 

90o 

96o 

48o 

46o 

44o 

NORTH DAKOTA

SOUTH DAKOTA

CANADAWISCONSIN

MINNESOTA

IOWA

Lake Superior

Mississippi

River

Minnesota River

 Ground-water recharge is an important component in aquifer water budgets. Information on recharge rates is useful in assessing the sustainable yield of aquifers, but recharge rates are difficult to quantify accurately because they vary widely in space and time. 

How, where, and when ground-water recharge occurs depends on factors such as climate, geology, soils, land-use practices, and depth 

to the water table. Annual rates of recharge in Minnesota (fig. F–1) tend to increase from west to east, similar to the trend in annual 

precipitation. In humid areas, recharge generally occurs as the widespread movement of water from land surface to the water table as 

a result of precipitation infiltrating and percolating through the unsaturated zone. This type of recharge generally occurs in winter and 

spring when evapotranspiration rates are low (fig. F–2A). In arid regions, focused recharge, or water that percolates down to the water 

Figure F–1. Estimated annual rates of recharge in Minnesota (Lorenz and Delin, 2007). 

32 Water Budgets: Foundations for Effective Water-Resources and Environmental Management
Figure F–2. Water budgets for (A) an unirrigated 

agricultural field in central Indiana, where most 

recharge occurs in winter and spring, and (B) an 

irrigated agricultural field in central California, 

where most recharge occurs during the growing 

season (after Fisher and Healy, 2007). Water input 

refers to the sum of precipitation and irrigation. 

Local depressions in land surface may be areas of enhanced recharge.

Natural recharge to aquifers can be augmented by induced 

recharge through spreading basins such as these in the 

Avra Valley of Arizona.

Central Arizona Project 

table from streams, is usually the dominant mechanism of natural recharge. 

If those streams drain mountainous areas, then their flow may be mostly 

snowmelt, and recharge would be expected in spring. Irrigation also can be a 

source of ground-water recharge during the growing season (fig. F–2B). 

The water budget of an aquifer provides some insight into what 

becomes of water that enters an aquifer as recharge (R): 

 R = ∆Sgw + Qbf + ETgw + ∆Qgw (F1)

Recharge arriving at the water table augments ground-water storage (∆Sgw), 

discharges to the surface as base flow (Qbf), is extracted by plant transpiration (ETgw), or moves out of the accounting unit as ground-water flow 

(∆Qgw). Often, one of these processes dominates the others. So, for example, 

measurements of base flow or changes in storage are sometimes used to 

infer recharge rates. Other methods for estimating recharge are based on 

physical or chemical data on ground water, water in the unsaturated zone, or 

surface water (Scanlon and others, 2002). 1/1/04 4/1/04 7/1/04 10/1/04 1/1/05

1/1/04 4/1/04 7/1/04 10/1/04 1/1/05

DEPTH, IN CENTIMETERS DEPTH, IN CENTIMETERS

0

−20

20

40

60

80

100

120

140

160

Cumulative water input

Cumulative evapotranspiration

Cumulative recharge

Daily change in storage

EXPLANATION

0

20

−20

40

60

80

100

120

140

160

A. Indiana

B. California

Box F Ground-Water Recharge 33
 G Estimating Aquifer Hydraulic Conductivity

Estimation of hydraulic conductivity is an essential, yet problematic, activity in ground-water hydrology. Values of hydraulic 

conductivity vary over more than 10 orders of magnitude for common earth materials (fig. G–1). Very permeable material such as karst 

limestone, fractured basalts, and coarse gravel can have hydraulic conductivities as large as 1,000 meters per day. Shale, marine clay, 

and glacial tills, on the other hand, may have conductivities on the order of 10–6 meters per day. Difficulties in estimating hydraulic conductivity arise from the highly variable manner in which geologic material was formed and deposited, the limited accuracy with which 

this parameter can be measured, and the small spatial scale over which measurements are made. 

Figure G–1. Approximate ranges in 

hydraulic conductivity for selected earth 

materials. A total range of 13 orders of 

magnitude is shown, which is indicative 

of the range for more common earth 

materials. In general, the average hydraulic 

conductivity of earth material in the same 

hydrogeologic terrain can vary by orders of 

magnitude (after Heath, 1983).

Roadcuts and outcrops provide insight to the geologic complexities of 

the subsurface.

10-8 10-7 10-6 10-5 10-4 10-3

HYDRAULIC CONDUCTIVITY, IN METERS PER DAY

10-2 10-1 1 10 102 103 104

IGNEOUS AND METAMORPHIC ROCKS

BASALT

SANDSTONE

SHALE

CLAY

SILTY SAND

CLEAN SAND

GLACIAL TILL GRAVEL

SILT, LOESS

CARBONATE ROCKS

Unfractured Fractured

Unfractured Fractured Lava flow

Unfractured Fractured

Fractured Semiconsolidated

Fine Coarse

Fractured Cavernous

34 Water Budgets: Foundations for Effective Water-Resources and Environmental Management
Methods for estimating hydraulic conductivity fall into two categories: direct and indirect. Direct methods are based on hydraulic tests done in the laboratory or the field. Laboratory tests are conducted on sediment cores obtained during drilling of boreholes. 

The cores are generally several centimeters in diameter and may be a few centimeters to more than a meter in length. Exacting 

laboratory procedures can produce very accurate measurements of hydraulic conductivity. However, because of the small size of the 

sample, the representativeness of the measured values to the aquifer as a whole is largely unknown. Field methods, using single or 

multiple wells, provide estimates that are integrated over larger volumes than those of laboratory tests. The most common single well 

test is the slug test, whereby a known volume of water is instantaneously withdrawn from (or injected into) the well, and the resulting change in water level in the well is monitored over time. This 

test samples the aquifer material within perhaps a meter of the 

well screen. Multiple well tests, sometimes referred to as aquifer 

or pumping tests, are labor intensive and expensive. One well is 

pumped and water-level changes are monitored in observation 

wells at various distances from the pumped well. These tests can 

run for days or weeks and produce values of hydraulic conductivity 

integrated over the distances over which drawdown is measured, 

typically tens to hundreds of meters. Field data are processed by 

using analytical or numerical mathematical models, models that 

have inherent assumptions on aquifer boundaries and uniformity, 

to produce estimates of hydraulic conductivity.

Indirect methods for estimating hydraulic conductivity are 

generally less expensive than direct methods, although they 

may not provide the same level of accuracy. The simplest such 

approach is to consult the literature; many reports contain tables 

of hydraulic conductivity for various consolidated and unconsolidated earth materials, such as shown in figure G–1. Geophysical 

measurements, made within boreholes, on land surface, or from 

aircraft or satellites, provide information that can be used to 

infer values of hydraulic conductivity on the basis of correlations 

developed at specific sites. Inverse ground-water-flow modeling 

uses best-fit algorithms to determine parameter values (including 

hydraulic conductivity) that produce simulated results that most 

closely match measured water levels and fluxes.

All methods are tied to a distinct spatial scale (fig. G–2). 

Direct measurements, in particular, are made over a relatively 

small volume. If the aquifer is heterogeneous, many measurements 

may be required to adequately describe that variability. Indirect 

approaches may be applied over much larger spatial scales. 

10–5

10–7

10–5

10–3

10–1

10–9

10–11

10–13

10–3 10–1 10

VOLUME OF TESTED MATERIAL, IN CUBIC METERS

HYDRAULIC CONDUCTIVITY,

IN METERS PER SECOND

103 105 107 109

Permeameter tests

Piezometer tests

Pumping tests

High capacity pumping tests

Regional flow model

Figure G–2. Hydraulic conductivity as a function of sample size 

(after Schulze-Makuch and others, 1999).

Performing slug tests to determine aquifer hydraulic 

conductivity.

Box G Estimating Aquifer Hydraulic Conductivity 35
Exchange of Water Between 

Compartments of the Hydrologic Cycle 

Water moves from the atmosphere to the surface 

through the process of precipitation. Water in the subsurface 

is obtained from land surface either as infiltration of direct 

precipitation or as seepage from streams or other water bodies. 

Subsurface water discharges to the surface naturally at springs, 

streams, lakes, or wetlands and in response to human activities such as a pumping well. Subsurface water also discharges 

directly to the atmosphere by evapotranspiration. Surface 

water discharges to the oceans, infiltrates the subsurface, or 

returns to the atmosphere by evaporation. An understanding of 

these exchange processes is useful in discussions of water budgets, especially when considering how changes to one process 

may affect other exchange rates. Figure 13 shows annual rates 

for North America of water movement from the atmosphere 

to the land surface (precipitation), from the land surface to the 

subsurface (infiltration), from the subsurface back to the land 

surface (base flow), and from the subsurface and land surface 

back to the atmosphere (evapotranspiration). 

Precipitation

Precipitation, in the form of rain, snow, dew, or fog drip, 

is the ultimate source of all water on the Earth’s landmasses. 

Average annual precipitation rates vary across the world 

(table 3). Within the United States, annual rates exceed 10 m 

in parts of Hawaii and are as low as 50 mm in Death Valley. 

Figure 14 shows how annual precipitation rates are distributed 

across the conterminous United States. Precipitation patterns 

vary with location and season. Although some areas of the 

United States (such as coastal areas in the Northwest) experience steady, predictable precipitation patterns during some 

months, precipitation in most of the United States is episodic 

with no precipitation on most days and rainfall rates up to 

100 mm/hr for short periods on other days. Local, convectivecell type thunderstorms can produce several centimeters over 

an hour in one locale, while no rain may be falling a short distance away. Such variability complicates efforts to determine 

precipitation rates for any study area. 

Precipitation

Infiltration

Evapotranspiration

670

460

380

80 Base flow

Figure 13. Average annual water exchange rates for North 

America (from Lvovitch, 1973) in millimeters of depth per unit 

area of land surface. 

J. C. Koch

Rain storm.

Agricultural Research Service

Weighing-bucket precipitation gage.

36 Water Budgets: Foundations for Effective Water-Resources and Environmental Management
The largest source of precipitation data in the United 

States is the National Climatic Data Center (http://www.ncdc.

noaa.gov). It holds daily precipitation data for thousands of 

sites across the country. Precipitation at these sites is measured 

with a weighing-bucket gage, a cylindrical container with an 

opening at the top that is 20.32 cm in diameter. Accumulated 

water in the gage is measured either manually or with an 

automated sensor that monitors the weight of the container. A 

standard, manually read gage measures the total precipitation 

between readings. Another widely used gage is the tippingbucket gage. 

Snowfall is difficult to measure directly. Instead, snow 

accumulation, or snow depth, is measured. At fixed reporting 

stations, such as those operated by the National Weather Service or the Natural Resources Conservation Service (NRCS), 

snow depth is determined by manual observation or by sonic 

sensors. Some stations are equipped with snow pillows. These 

are electronic balances that determine the weight of accumulated snow; they can provide hourly information on equivalent 

water content of the snow. SNOTEL is a network of remote 

snowpack stations maintained by NRCS in the western United 

States (http://www.nrcs.usda.gov). Using satellite communications, near real-time data on snow accumulation is available 

for more than 600 sites. A limitation to measuring snow depths 

at a fixed location is that wind may move the snow around 

after it has fallen. To compensate for this, agencies such as 

NRCS make repeated manual measurements of snow depth at 

multiple points along set courses throughout the snow season. 

Uncertainty in precipitation estimates arises from inaccuracies in gage measurements and a limited number of gages. 

High winds or heavy rainfalls can lead to underestimation of 

rainfall rates. Gages can get clogged with debris or freeze over. 

Proper placement of precipitation gages is critical. Nearby 

objects, such as trees and buildings, may produce a shadow 

effect, essentially blocking rainfall from the gage.

Table 3. Average annual precipitation rates for various 

locations (BBC Weather, http://www.bbc.co.uk/

weather, accessed on February 12, 2007).

Precipitation,

in millimeters per year

Atlanta, Ga. 1,500

Nashville, Tenn. 1,220

Cleveland, Ohio 1,040

Denver, Colo. 400

Death Valley, Calif. 50

Hilo, Hawaii 3,200

Fairbanks, Alaska 250

Tokyo, Japan 1,570

Singapore, Singapore 2,400

London, UK 580

Cairo, Egypt 25

Barcelona, Spain 580

Sydney, Australia 1,200

Buenos Aires, Argentina 940

0–559

560–945

946–1,303

1,304–2,020

2,021–7,089

Precipitation, in millimeters

EXPLANATION

Figure 14. Average annual precipitation in the conterminous 

United States for the period 1980–1997 as determined by the 

DAYMET model (http://www.daymet.org). Snow gage in Idaho.

Agricultural Research Service

Exchange of Water Between Compartments of the Hydrologic Cycle 37
With a collection area of about 324 cm2

, a rain gage samples only a very small part of a landscape’s surface area. To 

generate an average depth of precipitation for a specific area, 

data from gages within that area can be combined using one of 

several methods. Results from using three methods (arithmetic 

mean, Thiessen method, and isohyetal method; Linsley and 

others, 1982) for a simple example are within 18 percent of 

each other (fig. 15). Geostatistical techniques, such as kriging, 

can also be used to integrate precipitation over an area (Seo, 

1998). Hevesi and others (1992) made use of the correlation 

between elevation and precipitation rates to improve estimates 

of rainfall rates for an area in southern Nevada.

The density of rain gages within a watershed affects the 

accuracy with which the average rainfall for the entire watershed can be estimated. Increasing the number of gages should 

increase the accuracy of that estimate, especially for short 

periods of time. At a site in Illinois, Huff and Schickedanz 

(1972) found that a gage density of 50 mi2/gage resulted in a 

17-percent sampling error for a 3-hour sampling period and 

a 6-percent sampling error for a monthly period. National 

Weather Service weather stations have an average density of 

250 mi2/gage. Studies in mountainous terrain in Idaho indicate 

that a gage density of about 2 mi2 per gage is needed to obtain 

reasonably accurate estimates of precipitation (Molnau and 

others, 1980).

0.65

1.92

1.46

1.54

1.75

Arithmetic mean: 3.09 inches Thiessen method: 2.84 inches Isohyetal method: 2.61 inches

1.95

2.98 5.0

2.69

2.82

4.50

1''

2''

3''

4''

5''

Figure 15. Estimating average precipitation for an area from precipitation gage records using three methods 

(after Linsley and others, 1982).

Fog drip collector in Hawaii. In some areas, fog drip is the

 primary form of precipitation.

38 Water Budgets: Foundations for Effective Water-Resources and Environmental Management
An alternative method for measuring precipitation, 

one whose usefulness has yet to be fully integrated into 

water-budget studies, is Doppler radar. A single radar installation can provide virtually instantaneous estimates of rainfall 

over areas of more than 70,000 km2. The National Weather 

Service has a radar network that permits estimation of rainfall 

on a 4-km grid over most areas of the United States, but some 

biases may impair estimates. A recently developed program 

called MPE uses a network of real-time standard rain gages 

and satellite imagery to remove these biases (Seo, 1998; Seo 

and others, 1999). The program produces hourly to daily total 

precipitation estimates on a 4-km grid across the conterminous 

United States (fig. 16). Used with various hydrologic models, 

these estimates have greatly improved our ability to predict 

runoff and streamflow, to provide warnings of severe weather 

and floods, and to manage reservoir systems. These estimates 

are particularly useful for large river basins and for areas that 

have few or no standard gages. However, the 4-km grid may 

not provide sufficient detail for studies conducted on areas of 

less than a few square kilometers.

Figure 16. Daily rainfall for August 30, 2005 on a 4-kilometer spatial grid as determined by the National Weather Service MPE program. 

(National Weather Service, http://www.srh.noaa.gov/rfcshare/precip_analysis_new.php accessed on Feb. 13, 2007) National Weather Service

Doppler radar station.

Exchange of Water Between Compartments of the Hydrologic Cycle 39
Infiltration and Runoff

Precipitation falling on land surface can evaporate, be 

stored on the surface, run off to another point on the surface, 

or infiltrate the subsurface. Surface storage is mainly in the 

form of snow. Precipitation falling directly on surface-water 

bodies or on small surface depressions and precipitation intercepted by vegetation also constitute surface storage. Surface 

storage is relatively short lived, with the exception of glaciers 

and ice fields. During periods of rainfall, rates of evaporation and storage change are usually much less than those of 

infiltration and runoff. If these two processes can be ignored, 

the sum of infiltration and surface runoff is equal to precipitation. A common practice in many hydrologic studies is to 

measure precipitation and either infiltration or surface runoff 

and to calculate the third value by difference. Factors such as 

soil properties, vegetation, land use, slope, climate (especially 

precipitation rate and temperature), and water-table depth can 

affect infiltration and runoff rates.

If the rate of precipitation on bare soil is less than the 

rate at which the soil can absorb water, then all precipitation 

will infiltrate. Runoff is initiated once the rate of precipitation exceeds the rate at which the soil can absorb water. The 

time at which this occurs, the time of ponding, is an important 

parameter in many hydrologic models. Although there are 

no theoretical means for determining this time beforehand, 

empirical equations have been developed for this purpose. 

Figure 17 shows hypothetical rates of infiltration as a function 

of precipitation rate and time for a homogeneous soil. When 

the precipitation rate is greater than the saturated hydraulic 

conductivity of the soil, the infiltration rate is steady until the 

time of ponding. It then decreases over time and approaches a 

magnitude equivalent to the hydraulic conductivity. 

Direct measurements of natural infiltration rates are not 

common but can be made with certain kinds of lysimeters. 

Infiltrometers are used to determine steady-state infiltration 

rates after the soil has been saturated (these rates should be 

similar to values for saturated hydraulic conductivity), but 

those rates may be substantially less than early time rates 

of infiltration into initially dry soils. Indirect methods for 

estimating infiltration may be based on measurement of 

pressure head and moisture content at different depths below 

land surface or on measurements of precipitation and runoff. 

Empirical methods for estimating infiltration also exist (Chow 

and others, 1988).

Measurement of surface runoff, sometimes referred to as 

overland or Hortonian flow, is difficult but has been accomplished in some studies by building berms around a small 

area to funnel runoff to a central collector or measurement 

device. Surface runoff flows overland, eventually entering an established stream channel. As discussed previously 

in the “Streams” section, runoff is only one component of 

streamflow. It is sometimes possible to analyze a streamflow 

hydrograph to estimate rates of runoff and base flow (see, for 

example, Rutledge, 1998). Empirical equations for estimating 

runoff, most notably the NRCS Curve Number method, are 

also in widespread use (Chow and others, 1988). 

10 100 1,000 10,000 100,000

TIME SINCE START OF INFILTRATION, IN SECONDS

0

0.002

0.001

0.003

0.004

0.005

0.006

0.007

0.008

0.009

INFILTRATION RATE, IN CENTIMETERS PER SECOND

0.008

0.004

0.0008

0.001

Precipitation rate,

 in centimeters

 per second

Ponding

Ponding

Figure 17. Infiltration rates generated for a one-dimensional 

uniform soil column with a variably saturated flow model as 

a function of time for four precipitation rates. The saturated 

hydraulic conductivity of the soil is 0.001 centimeter per second.

Infiltrometer.

Surface runoff 

collection 

apparatus.

40 Water Budgets: Foundations for Effective Water-Resources and Environmental Management
Evapotranspiration

Evaporation is the conversion of liquid or solid water 

into a vapor. It is the process by which water is transferred 

from a surface-water body or land surface to the atmosphere. 

Evaporation that occurs through the stoma of plants is called 

transpiration. In a typical terrestrial setting, it is difficult 

to measure plant transpiration separately from evaporation 

from bare soil or water bodies. Therefore, it is common for 

these two processes to be lumped into a single term—evapotranspiration. 

Evapotranspiration, when averaged over one-year periods, is usually second in magnitude among water-budget components to precipitation, representing about 65 percent of precipitation that falls on global landmasses (about 540 mm/yr, 

table 2). Evapotranspiration may not vary spatially as much as 

precipitation, but accurate estimates of evapotranspiration are 

generally more difficult to obtain. There is no national network 

of evapotranspiration monitoring sites within the United States 

such as exist for precipitation and streamflow. However, at 

select National Weather Service sites, pan evaporation (evaporation from 1.2-m-diameter Class A pan) is measured daily. 

As the common link between the water and energy budgets, evapotranspiration is dependent upon the availability of 

both water and energy. In arid regions, water availability is the 

major limitation on evapotranspiration rates. In humid regions, 

there is generally an excess of water relative to available 

energy, so rates are energy limited. Evapotranspiration rates 

generally follow a trend similar to that of net radiation: highest 

in the summer and lowest in winter. The importance of energy 

on evapotranspiration rates is also apparent on a daily time 

scale. Rates are essentially 0 during night hours when no solar 

radiation is arriving at the site and highest during the daylight 

hours of peak net radiation. 

Potential evapotranspiration is the evapotranspiration that 

would occur if water were plentiful. Figure 18 shows estimates 

of annual potential evapotranspiration rates for the conterminous United States as calculated with the Hamon method using 

average temperatures from 1961 to 1990 generated by the 

PRISM model (Daly and others, 1994) . Estimates of potential 

evapotranspiration are used in the planning and management 

of irrigation systems. The difference between precipitation 

and potential evapotranspiration (fig. 19) provides an index of 

areas where evapotranspiration is water limited (differences 

less than 0) and energy limited (differences greater than 0).

Measurement of evapotranspiration rates at specific 

locations are made with lysimeters or micrometeorological 

techniques (Rosenberg and others, 1983). These latter techniques, which include eddy correlation, Bowen ratio/energy 

budget, and aerodynamic profile methods, measure or estimate the vertical flux of water vapor from land surface to the 

atmosphere. These methods may provide accurate estimates 

of evapotranspiration rates, but lysimeters and micrometeorological instrumentation are expensive and delicate and require 

frequent maintenance.

Climatological methods (Rosenberg and others, 1983) 

provide estimates of potential evapotranspiration. Although 

not as sophisticated as the micrometeorological techniques, 

they are much easier to apply, usually requiring only data 

that are available from National Weather Service stations 

(for example, daily temperature, relative humidity, or solar 

radiation). Included in this class are the Thornthwaite, 

0–564

565–709

710–877

878–1,074

1,075–1,662

Potential evapotranspiration (PET),

 in millimeters per year

EXPLANATION

–1,600–0

1–318

319–868

869–1,838

1,839–6,656

Precipitation-PET, in millimeters per year

EXPLANATION

Figure 18 . Annual potential evapotranspiration rates across 

the conterminous United States as calculated with the Hamon 

equation. 

Figure 19. Difference between annual precipitation and potential 

evapotranspiration rates across the conterminous United States. 

Exchange of Water Between Compartments of the Hydrologic Cycle 41
Jensen-Haise, Hamon, and Penman-Monteith methods. Actual 

evapotranspiration can be estimated from potential rates by 

application of a correction factor. In the agricultural literature, this factor is referred to as the crop coefficient. The crop 

coefficient is a related to crop type and maturity and climate; 

values for different crops and some native vegetation can be 

found in Jensen and others (1990). Obtaining estimates of 

actual evapotranspiration from pan evaporation rates requires 

application of a second correction factor called a pan coefficient (Doorenbos and Pruitt, 1975) as well as a crop coefficient.

Other techniques for estimating evapotranspiration 

deserve mention even though they are not yet as widely used. 

Light detection and ranging (LIDAR) is a ground-based 

system capable of making accurate measurements of sensible 

and latent heat fluxes over areas as large as several hectares. 

The expense of the LIDAR equipment limits its use to select 

research studies. Satellite and aerial remote sensing offers 

no direct method of measuring evapotranspiration. However, 

progress has been made in correlating point micrometeorological measurements with variables that can be mapped from 

space, such as vegetation type and cover (Liu and others, 

2003), soil moisture, and surface temperature (Quattrochi and 

Luvall, 1999). Models incorporating these variables can be 

used to generate regional evapotranspiration estimates from 

the point measurements. 

Tower to measure evapotranspiration in a forest.

Instruments used in eddy correlation measurements of 

evapotranspiration.

Instrumentation to measure evaporation from a lake.

42 Water Budgets: Foundations for Effective Water-Resources and Environmental Management
Exchange of Surface Water and Ground Water

Streams and ground-water bodies exchange water in all 

types of hydrologic settings. Streams gain water from inflow 

of ground water through the streambed and banks (gaining 

stream, fig. 20A); they lose water to ground water by outflow 

through streambed and banks (losing stream, fig. 21A). A 

stream can be gaining in some reaches and losing in other 

reaches. For ground water to discharge into a stream channel, the altitude of the water table in the vicinity of the stream 

must be higher than the altitude of the stream-water surface. 

Conversely, for surface water to seep to ground water, the 

altitude of the water table in the vicinity of the stream must be 

lower than the altitude of the stream-water surface. Contours 

of water-table elevation indicate gaining streams by pointing in an upstream direction (fig. 20B) and losing streams by 

pointing in a downstream direction (fig. 21B).

Losing streams can be connected to an aquifer by a 

continuous saturated zone (fig. 21A) or separated from it by 

an unsaturated zone (fig. 22). Ephemeral streams flow only 

in response to snowmelt and storms. Generally, ephemeral 

streams are not directly connected to an aquifer. However, 

GAINING STREAM

Flow direction

Unsaturated zone

Water table

Shallow aquifer

A

Stream

Ground-water flow line

B

70

50

50

40

40

30

30

20

20

60

60

Water-table contour

B

Stream

100

90

80

70

Ground-water flow line

Water-table contour

LOSING STREAM

Flow direction

Water table Unsaturated zone

A

Figure 20. Gaining streams receive water from the groundwater system (A). This can be determined from water-table 

contour maps because the contour lines point in the upstream 

direction where they cross the stream (B).

Figure 21. Losing streams lose water to the ground-water 

system (A). This can be determined from water-table contour 

maps because the contour lines point in the downstream 

direction where they cross the stream (B).

Big Spring, Missouri.

Exchange of Water Between Compartments of the Hydrologic Cycle 43
during periods of flow, these losing streams can be important 

sources of ground-water recharge.

In many stream settings, surface water flows through 

short segments of the streambed and banks and back into the 

stream. These segments, which may exist along the entire 

reach of a stream, constitute the hyporheic zone (figs. 23 and 

24). Ground water and surface water mix within the hyporheic 

zone, providing a unique environment for important biological 

and chemical reactions. The size and geometry of hyporheic 

zones surrounding streams vary in time and space. Streams 

Stream

Stream

Interface of local and regional

ground-water flow systems,

hyporheic zone, and stream

Direction of

ground-water

flow

Direction of

ground-water

flow

Water table 

H y p o r h e i c z o n e

A

B Meandering

stream

Pool and riffle

stream

Flow in

hyporheic

zone

Flow in

hyporheic

zone

Figure 23. Surface-water exchange with ground water in the hyporheic zone is associated with abrupt changes in streambed 

slope (A) and with stream meanders (B).

Figure 24. Streambeds and banks are unique environments because they are where ground water that drains much of the 

subsurface of landscapes interacts with surface water that drains much of the surface of landscapes.

flowing over sand and gravel may have hyporheic zones up to 

2 m thick. 

Bank storage, the temporary storage of stream water in 

the subsurface (fig. 25), occurs when stream stage rises as 

a result of precipitation, snowmelt, or release of water from 

a reservoir upstream. As long as the rise in stage does not 

overtop streambanks, most stored water returns to the stream 

a few days or weeks after the stage returns to normal level. 

Bank storage tends to reduce flood peaks and supplement 

streamflow when stage recedes. If the rise in stream stage is 

sufficient to overtop the banks and flood large areas of the land 

surface, widespread recharge to the water table can take place. 

In this case, the time it takes for the recharged floodwater 

to return to the stream by ground-water flow may be weeks, 

months, or even years. Many stream-aquifer systems are in 

continuous readjustment from exchanges of water related to 

bank storage and overbank flooding. 

Lakes and wetlands interact with ground water in a manner similar to that of streams. There are some differences, 

though. Evaporation is generally a larger component of the 

water budget for a lake or wetland than for a stream. Bank 

storage is usually of minor importance because water levels do 

not fluctuate as much as they do in streams. Important exceptions are surface-water reservoirs in arid and semiarid regions. 

DISCONNECTED STREAM

Flow direction

Water table

Unsaturated zone

Figure 22. Disconnected streams are separated from the 

water table by an unsaturated zone.

44 Water Budgets: Foundations for Effective Water-Resources and Environmental Management
Reservoir stage can change substantially over the course of 

a year, rising as spring rain or snowmelt fills the reservoir 

and falling through summer and fall as water is distributed to 

users. Wetlands may be present in many different parts of the 

landscape, whereas lakes and streams occupy local topographic low regions. 

Ground water contributes to many lakes, wetlands, and 

streams. Ground-water discharge can account for more than 

90 percent of total annual streamflow (table 4). These contributions can be critical to the maintenance of diverse ecosystems. Wetlands and riparian zones provide wildlife habitat, 

mitigate floods, and process nutrients and contaminants; the 

existence of these areas may depend on a steady discharge of 

ground water. Understanding the water budget for these areas 

can aid in assessing how changes to one water-budget component will affect other components. For example, an understanding of the water budget could help determine if diversion 

of ground water to a domestic water-supply well will reduce 

the rate of ground-water discharge to a wetland and, if so, 

what effect that would have on plants and wildlife. 

BANK STORAGE

Flow direction

Water table

during base flow

Bank storage

High stage

Water table at

high stage

Table 4. Base flow as a percentage of total streamflow 

for selected streams across the United States (Winter and 

others, 1998).

Stream State

Percentage of 

ground-water

contribution

Dismal River Nebraska 94

Forest River North Dakota 13

Sturgeon River Michigan 90

Ammonoosuc River New Hampshire 61

Brushy Creek Georgia 68

Homochitto River Mississippi 36

Dry Frio River Texas 58

Santa Cruz River Arizona 35

Orestimba Creek California 23

Duckabush River Washington 65

Figure 25 . If stream levels rise higher than adjacent groundwater levels, stream water moves into the streambanks as bank 

storage.

Techniques are available for estimating exchange rates 

between surface and ground waters over various space and 

time scales. Seepage meters provide point measurements over 

an area of about 1 m2 for periods of seconds to days. Discharge measurements can be made at different locations along 

a reach of stream; the difference in discharge between any two 

points will be equal to the net stream loss or gain along that 

reach. At the watershed scale, hydrograph separation methods 

and streamflow duration curves can be used to estimate base 

flow in gaining streams. Solute- and energy-budget approaches 

have been used over a variety of scales to estimate exchange 

rates of ground water with lakes and streams.

Stream disappearing into sinkhole in karst terrain in Texas.

Seepage meters 

are used to 

measure the 

exchange of water 

between surface 

water and the 

subsurface.

Exchange of Water Between Compartments of the Hydrologic Cycle 45
Water-Budget Studies

Space and time scales associated with water-budget 

studies largely determine the appropriateness of methods for 

estimating fluxes and changes in storage. Different methods 

are applicable over different scales. Some techniques provide 

estimates at a single point in space, such as the use of standard 

rain gages. Because rainfall rates vary with location, multiple 

gages may be needed to determine an average rate for a watershed. Other techniques provide estimates that are integrated 

over large areas (for example, a stream-discharge measurement provides an estimate of runoff for the entire area that 

drains to the measurement point). Time scales are also important. For estimating ground-water recharge, the water-table 

fluctuation method provides an estimate for each recharge 

event, of which there could be many during a year. Groundwater age-dating techniques, on the other hand, provide a 

single estimate of recharge that is averaged over several years 

or decades. Prudence dictates that the time and space scales of 

measurement and estimation methods match the needs of the 

water-budget study at hand.

Four intensive water-budget studies are presented in this 

section to illustrate the different approaches and exacting 

procedures that have been applied in studies of water budgets. 

Detailed studies such as these are not commonly undertaken, 

mostly because of monetary and time constraints. The examples convey the level of complexity inherent in conducting 

such studies and show that results from even the most detailed 

studies of water budgets in natural hydrologic systems contain 

some uncertainty. This uncertainty arises from the natural variability in hydrology, geology, climate, and land use and inaccuracies in the techniques used to collect and interpret data. 

Water Budget for a Small Watershed: 

Beaverdam Creek Basin, Maryland

The Beaverdam Creek basin is situated on the Atlantic 

Coastal Plain in the Delmarva Peninsula of Maryland (fig. 26). 

The water budget of the basin was studied for 2 years in the 

early 1950s (Rasmussen and Andreasen, 1959) to determine 

the apportionment of precipitation among ground-water 

recharge, subsurface runoff to ponds and streams, groundwater evapotranspiration, and ground-water storage. The

19.5-mi2 drainage basin ranges in elevation from 10 to 85 ft 

above sea level and receives on average 43 inches of precipitation annually. In the subsurface, Quaternary-age surficial sands 

and silts, as much as 70 ft thick, overlie aquifers of Tertiaryage sand. The water table is generally within 12 ft of land surface. The study area was in a natural setting, largely unaffected 

by human activity. Understanding such a natural hydrologic 

system is fundamental to evaluating human influences on this 

and other systems. 

The amount and kinds of data collected are unusual for a 

small watershed. Ground-water levels were measured weekly 

in 25 observation wells. Stream discharge was monitored 

by means of a sharp-crested weir at the outlet of the basin. 

Changes in water storage in the two ponds within the basin 

were calculated from stage readings and a table (developed by 

bathymetric survey) that related pond volume to stage. Twelve 

precipitation gages were deployed across the basin and monitored on a weekly basis. A 4-ft Class A evaporation pan was 

part of a weather station that also included an anemometer, 

barometer, wet- and dry-bulb thermometers, and a thermistor for measuring soil temperature. Soil moisture content 

was determined weekly by electrical resistance (individually 

calibrated Bouyoucos blocks) at depths of 4, 12, and 39 inches 

at three locations within the basin.

Major components of the water budget are shown in 

figure 27. Precipitation for the 2-year period totaled 83 inches. 

Sixty percent, or 50 inches, was returned to the atmosphere 

through evaptranspiration; 31 inches (37 percent of precipitation) left the basin as streamflow. Two inches remained in the 

basin, augmenting surface and subsurface storage. The quasilinear nature of the precipitation curve in figure 27 indicates 

relatively uniform precipitation rates throughout the study 

Beaverdam

Creek,

Maryland

Figure 26. Location map for Beaverdam Creek basin. 

Typical stream in Delmarva Peninsula, Maryland.

46 Water Budgets: Foundations for Effective Water-Resources and Environmental Management
period. There is no indication of the seasonality in precipitation that is common in many regions. Evapotranspiration, on 

the other hand, shows a distinct seasonal trend, with highest 

values occurring in summer months and negligible rates for 

most winter months. Evapotranspiration was not measured 

directly. All other water-budget components were measured or 

estimated independently. Evapotranspiration was then determined to balance the water-budget equation. 

Ground-water recharge, the only source of inflow to the 

shallow ground-water system, was calculated by application 

of the water-table fluctuation method, using weekly average 

water levels from the 25 observation wells. Recharge was then 

partitioned into its different components:

R = ∆Sgw + Qbf + ETgw

where:

 R is recharge,

ΔSgw is change in ground-water storage,

Qbf is base flow,

and

ETgw is evapotranspiration from ground

 water. 

Change in ground-water storage was calculated from 

the difference in head between the end and the beginning of 

the week. Base flow was determined by a stream hydrograph 

separation method. Evapotranspiration from ground water 

was then calculated as the residual of the equation. Figure 28 

shows plots of calculated recharge components. For the 2-year 

period, ground-water recharge was 42.6 inches and was partitioned into 21.5 inches of base flow, 1.7 inches of increase in 

ground-water storage, and 19.5 inches of evapotranspiration 

of ground water. In summer months, evapotranspiration is the 

largest draw on ground water. For the rest of the year, base 

flow is the predominant mechanism of ground-water discharge.

Few studies before the Beaverdam Creek watershed 

study devoted as much effort to the comprehensive examination of the water budget of a small watershed. Measurements 

of water-budget components can be made more easily and 

more accurately with the improved instrumentation that has 

been developed over the decades since the original study was 

conducted. Even so, conducting a similar study today would 

require a substantial commitment of funding and manpower.

April

1950

September

1950

March

1951

September

1951

March

1952

0

20

40

60

80

WATER, IN INCHES

Precipitation

Evapotranspiration

Streamflow

Change in storage

Ground water

Surface water

–10

0

10

20

30

40

50

WATER, IN INCHES

Recharge

Base flow

Evapotranspiration from ground water

Storage change

April

1950

September

1950

March

1951

September

1951

March

1952

Figure 27. Water budget for Beaverdam Creek Basin, 

April 1950 to March 1952.

Figure 28. Ground-water budget for Beaverdam Creek 

Basin, April 1950 to March 1952. Agriculture is the main industry in the Delmarva Peninsula.

Farm pond, Delmarva Peninsula.

Exchange of Water Between Compartments of the Hydrologic Cycle 47
Soil-Water Budgets for Prairie and Farmed 

Systems in Wisconsin

Water budgets of soil zones are important for management of agricultural fields. They also are used to estimate rates 

of evapotranspiration and ground-water recharge. Agricultural 

practices can greatly affect soil-water budgets. Irrigation and 

cultivation techniques (conventional, minimum, or no tillage) 

can influence surface runoff, erosion, infiltration, and transport of applied agricultural chemicals. Brye and others (2000) 

determined water budgets for a 132-week period in 1995–98 

at three sites in Columbia County in southern Wisconsin: a 

restored natural prairie, maize under no-tillage, and maize with 

chisel-plow tillage (fig. 29). The objectives of the study were 

to evaluate the usefulness of newly designed instrumentation 

and to assess the effects of agricultural practices on drainage 

beneath the root zone. 

Precipitation was measured with rain and snow gages. 

Soil-moisture content profiles were measured weekly during the growing season and at 3-week intervals during winter 

with a neutron moisture meter. Readings were taken to a depth 

of 1.4 m in four access holes in each field. Water movement 

through the unsaturated zone is very difficult to measure 

directly. In this study, drainage beneath the root zone was 

measured with equilibrium tension lysimeters (ETLs) (Brye 

and others, 1999). These novel devices have a porous stainless steel surface that allows collection and measurement of 

drainage. Soil-matric potential sensors and a vacuum system 

allowed the tension within the ETL to be set slightly greater 

than that recorded in the bulk soil surrounding the lysimeter. 

Thus, flow into the ETLs should be similar to natural drainage rates. The ETLs were installed through a 2-m-deep trench; 

the 75-cm by 25-cm top surface was set at a depth of 1.4 m. 

Evapotranspiration (ET) was equal to the differences in inputs 

and outputs and storage changes:

ET = P – RO – D – ∆S

where:

 P is precipitation,

RO is surface runoff,

D is drainage below the root zone,

and

∆S is change in storage (soil and

 surface).

Runoff only occurred immediately after large precipitation events or snowmelt and was estimated by a procedure 

described by Brye and others (2000). Calculations were made 

on a weekly basis. 

Columbia County, Wisconsin

Figure 29. Location map of Columbia County, Wisconsin.

K.R. Brye

Agricultural site.

Prairie site.

K.R. Brye

48 Water Budgets: Foundations for Effective Water-Resources and Environmental Management
 Water budgets for the three sites are shown in figure 30. 

Precipitation rates were similar for the three sites. The prairie 

site had greater soil-moisture contents, more evapotranspiration, and less drainage compared to the maize fields at 

the other two sites. Evapotranspiration rates for the prairie 

site were slightly greater than those from the no-tillage site, 

which were slightly greater than those from the chisel-plow 

site. Drainage occurred from late January to mid-June at all 

sites. However, drainage totals for the 132-week period were 

substantially different: 199 mm for the prairie, 563 mm for 

no-tillage, and 793 for chisel-plow. It appears that infiltration 

rates increased with increasing disturbance of the land surface. 

Runoff, because of its episodic nature, is not included in figure 

30. For the study period, runoff totaled 197 mm at the Prairie 

site, 182 mm at the no-tillage site, and –5 mm (due to drifting 

snow) at the chisel-plow site.

Rates of infiltration, drainage, and evapotranspiration are 

important in terms of plant health and ground-water quantity 

and quality. It is difficult to accurately determine the effects of 

different agricultural practices on local water budgets, but new 

and improved instrumentation, such as that used in this study, 

can provide valuable insight into complex processes.

Figure 30. Water budget for (A) prairie site; (B) no-tillage maize site; and (C) chisel-plow maize site in central Wisconsin 

(Brye and others, 2000). Imbalance in the water budgets is attributed to runoff for the prairie and no-tillage site and to 

runon from melting snow at the chisel-plow site. 

K.R. Brye

K.R. Brye

Equilibrium tension lysimeter. The lysimeter requires careful installation.

Exchange of Water Between Compartments of the Hydrologic Cycle 49

Goose Pond Prairie

1,800

1,575

1,350

1,125

900

675

450

225

0

–225

26 35 45 3 13 23 33 43 1 11 21 31 41 49 1

WEEK

CUMULATIVE WATER-BUDGET COMPONENT 

IN MILLIMETERS

A B C

1995 1996 1997 1998

No-tillage maize

26 35 45 3 13 23 33 43 1 11 21 31 41 49 1

WEEK

1995 1996 1997 1998

Chisel-plow maize

26 35 45 3 13 23 33 43 1 11 21 31 41 49 1

WEEK

Precipitation

Winter surface-water storage

Soil-water storage changes

Drainage

Evapotranspiration

1995 1996 1997 1998
Water Budget of Mirror 

Lake, New Hampshire

The hydrology and chemistry of Mirror Lake (fig. 31) 

have been studied since the late 

1970s to develop an understanding of the hydrological processes 

associated with the lake and to 

examine uncertainties in estimates of water and chemical budgets for the lake (Winter, 1984). 

To accomplish this, all components of the water budget were 

determined independently, either 

by direct measurement or by 

calculation from measurements 

of environmental variables. 

Precipitation was measured at 

two gages within 400 m of the 

east and west shores of the lake. 

Evaporation was calculated by 

using an energy-budget method 

with data from meteorological instruments located on a raft 

in the lake and a land station near the lake. Stream inflow and 

outflow were measured by using Parshall flumes and weirs 

equipped with stage recorders. Ground-water inflow and 

outflow were calculated by using Darcy’s equation with water 

levels in the lake and in numerous wells near the lake together 

with measured hydraulic conductivity. Water storage in the 

lake was estimated from measured lake stage and a stage-volume relation.

Monthly and annual water and chemical budgets were 

determined for Mirror Lake for the 20-year period from 1981 

to 2000. Streams provided the largest inflow of water to the 

lake during this period; seepage to ground water was the largest loss of water from the lake (table 5). The largest uncertainty associated with the water budget is in the ground-water 

fluxes and is related to the complexity of the geologic deposits 

and the associated variability in hydraulic conductivity. 

For initial calculations of ground-water fluxes, hydraulic conductivity was determined by single-well aquifer tests, 

which test only small volumes of the aquifer in close proximity to the wells. The rate of ground-water inflow to the 

lake was estimated to be 47,000 m3/yr. A second estimate of 

ground-water inflow was generated by using data on oxygen 

isotopes (J.W. LaBaugh, U.S. Geological Survey, oral commun., 2006). The ratio of oxygen-18 to oxygen-16 in water 

can be used to estimate the rate of ground-water discharge to 

the lake. The isotopic ratio of lake water must be balanced by 

the ratios in incoming water and the change in the ratio that 

takes place as a result of evaporation. The rate of ground-water 

inflow to the lake can be computed by using measured isotopic 

Mirror Lake, New Hampshire

Table 5. Initial and final water budgets for Mirror Lake in 

New Hampshire. Values are in 1,000 cubic meters per year.

Initial Final

Inflows

Precipitation 182 182

Surface-water inflow 417 417

Ground-water inflow 47 113

Outflows

Evapotranspiration 77 77

Surface-water outflow 257 257

Ground-water outflow 281 347

Lake volume change 16 16

Imbalance 15 15

Figure 31. Location map of Mirror Lake, New Hampshire.

Mirror Lake, New Hampshire.

50 Water Budgets: Foundations for Effective Water-Resources and Environmental Management
ratios of ground water, precipitation, streamflow, and the lake 

itself, along with independently measured rates of precipitation, streamflow, and evaporation. This approach yielded an 

estimate of 95,000 m3/yr of ground-water inflow to Mirror 

Lake, about twice the value originally calculated.

A third estimate of ground-water inflow was generated 

from a separate study of the ground-water basin within which 

Mirror Lake lies (Tiedeman and others, 1997). As part of 

this study, a computer model of ground-water movement was 

constructed, producing a water budget for the ground-water 

basin (fig. 32); therefore, water budgets were available for two 

distinct accounting units. One accounting unit, Mirror Lake, 

is actually nested within another accounting unit, the groundwater basin in which the lake resides. A common component 

in the two water budgets is inflow to the lake. The mutual 

benefit of these concurrent studies was that data from one 

could be used to refine the other. For example, the groundwater-flow model was calibrated by using stream discharge 

and water levels in wells that were part of the lake study. 

Conversely, ground-water flow to and from the lake could 

be calculated from the ground-water-flow model. The model 

results indicated that 133,000 m3/yr were discharged to Mirror 

Lake, a value 2.8 times greater than the initial calculation.

The lack of agreement among the three approaches used 

to estimate ground-water inflow to the lake clearly illustrates 

some of the uncertainties in water-budget studies. Which estimate of ground-water inflow is correct? Researchers at the site 

ultimately settled on a rate of 113,000 m3/yr. To balance the 

final water budget for the lake (table 5), ground-water outflow 

from the lake was adjusted to compensate for the change in 

inflow; estimates of precipitation, stream inflow and outflow, 

and evaporation measurements were all thought to be acceptable. 

Results from the Mirror Lake study demonstrate the value 

of applying multiple approaches to quantify water-budget 

components. However, it is clear that all water budgets, 

including the most detailed budgets, contain some degree of 

uncertainty related to measurement inaccuracies and to our 

limited ability to make measurements in sufficient spatial and 

temporal detail. Methods used to calculate the water budget 

of Mirror Lake were state-of-the-art and the most accurate 

available. Estimated uncertainties are 5 to 10 percent for precipitation, 10 to 15 percent for evaporation, 5 to 10 percent for 

streamflow into and out of the lake, and 30 to 50 percent for 

ground-water inflow and outflow (T.C. Winter, U.S. Geological Survey, and G.E. Likens, Institute of Ecosystem Studies, 

oral commun., 2006). The overall uncertainty in the water 

budget is considered to be about 13 percent.

Figure 32. Model calculated water budget for the Mirror Lake 

ground-water basin from Tiedeman and others (1997). Error bars 

indicate approximate 95-percent confidence interval.

200

150

100

50

0

1,000 CUBIC METERS PER YEAR

Precipitation to bedrock

Precipitation to glacial deposits

Streams to glacial deposits

Glacial deposits to streams

Glacial deposits to Mirror Lake

Lake sediments to Mirror Lake

Bedrock to Mirror Lake

Inflows Outflows

Surface-water outflow from Mirror Lake was measured with a 

combined weir and flume.

Weather station.

Exchange of Water Between Compartments of the Hydrologic Cycle 51
Water Budget at a Waste Disposal Site in 

Illinois 

A water budget was developed for a radioactive-waste 

disposal site in Bureau County in northwestern Illinois 

(fig. 33) for the period of July 1982 through June 1984 (Healy 

and others, 1989b). The 8-ha site is situated in complexly 

layered glacial and eolian sediments that range from 10 to 

30 m in thickness and overlie a thick sequence (140 m) of lowpermeability shales. The disposal site was in operation from 

1967 to 1978. During that time, waste was placed in shallow 

trenches that were subsequently covered with compacted clay 

covers. The main objective of the study was to estimate rates 

of water percolation through the trench covers, an issue of 

concern for regulatory agencies seeking to minimize potential 

for ground-water contamination. A secondary objective was to 

assess the utility of water-budget methods for estimating those 

rates. 

Precipitation was measured at three locations (two tipping bucket and one weighing gage). Evapotranspiration rates 

were estimated on an hourly basis by using a combination 

Bowen-ratio/aerodynamic profile method. Data collected from 

the onsite weather station included net radiation, shortwave 

and longwave radiation, wet- and dry-bulb air temperatures 

at three heights, soil temperatures and heat flux, and windspeed and direction (Healy and others, 1989a). Flumes and 

weirs were used to measure runoff in the ephemeral streams 

that drained the site. Change in water storage in trench covers was measured with nuclear moisture probes at weekly 

intervals at three locations. At those same locations, percolation through trench covers (which was considered equivalent 

to recharge) was estimated with the Darcy method by using 

pressure heads measured with vertical clusters of tensiometers. 

Tensiometer readings were obtained electronically with pressure transducers and data recorders at intervals ranging from 

5 to 60 minutes.

Bureau County, Illinois

Figure 33. Location map of Bureau County, Illinois.

The waste disposal site near Sheffield, Illinois, was in operation from 1967 to 1978.

Data from onsite weather station were used to estimate 

evapotranspiration.

52 Water Budgets: Foundations for Effective Water-Resources and Environmental Management
Results of this study 

illustrate the day-to-day, 

season-to-season, and 

year-to-year variability in 

individual water-budget 

components. They also 

demonstrate the interdependence among all 

components. Similar seasonal trends are apparent 

between the 2 years, but 

there are some distinct 

differences (fig. 34). 

Precipitation totals for 

the 2 years were similar. 

The average of 948 mm 

is close to the long-term 

average of 890 mm; 

however, monthly values 

varied between years. July of the first year was quite wet, but 

July in the second year saw very little rain. May and June had 

little rain in the first year but were extremely wet the second 

year. This variability in precipitation affected other waterbudget components, especially runoff and recharge. Runoff 

was episodic, occurring only in response to large precipitation events, and was more likely to occur if the soil-moisture 

contents were high. About one-half of the average annual 

recharge of 208 mm occurred during the months of March and 

April. Recharge was 54 percent of precipitation for those 2 

months in the first year and 55 percent the second year. Other 

months did not display such consistency: July had recharge 

of 28 mm in the first year and 0 mm in the second, and May 

and June had 1 mm in the first year and 64 mm in the second. 

Evapotranspiration showed consistent seasonal patterns over 

both years, but daily values during peak summer months were 

influenced by the availability of soil moisture. 

The residual error in the water budget, defined as the difference between precipitation and all other components of the 

water balance, was –81 mm/yr on average (table 6). Although 

the magnitude of this number is small relative to precipitation 

and evapotranspiration, it is large relative to all other waterbudget components. Therefore water-budget methods may be 

suitable for estimating evapotranspiration at this site but problematic for estimating other components of the water-budget 

equation. Given the variability in weather patterns, the 2-year 

study period may have been too short to adequately determine 

a long-term average water budget.

Precipitation

Evapotranspiration

Runoff

Change in storage

Recharge

0

200

400

600

800

1,000 WATER, IN MILLIMETERS

0

200

400

600

800

1,000 WATER, IN MILLIMETERS

A

B

July Aug Sept Oct Nov Dec Jan Feb Mar Apr May June

Figure 34. Water-budget components for (A) July 1982 

through June 1983 and (B) July 1983 through June 1984 for 

a site in northwestern Illinois (Healy and others, 1989b). 

Table 6. Annual values of water-budget components in millimeters for a site in northwestern Illinois (Healy and others, 1989b).

Year Precipitation Evapotranspiration Runoff Storage 

change

Percolation 

into trench Residual

July 1982 – June 1983 927 606 206 –11 216 –90

July 1983 – June 1984 969 667 113 60 201 –72

2-Year average 948 637 160 24 208 –81

Flumes and weirs were used to 

measure runoff.

Soil moisture content and pressure head were measured.

Exchange of Water Between Compartments of the Hydrologic Cycle 53
 H Uncertainty in Water-Budget Calculations

All water-budget calculations contain 

some uncertainty. There are two general 

sources of this uncertainty: natural variability 

of the hydrologic cycle and errors associated 

with measurement techniques. Natural variability occurs in all aspects of the hydrologic cycle. 

Precipitation patterns are affected by altitude; 

evapotranspiration and runoff are affected by 

soil properties, vegetation type and density, 

surface slope and aspect, depth to ground 

water, and other factors. Temporal variability 

in storage and fluxes is largely tied to diurnal, 

seasonal, and long-term trends in weather. 

On a daily basis, the pattern of solar radiation 

generally limits evapotranspiration to daylight 

hours. Evapotranspiration also is affected by 

seasonal trends in solar radiation; rates are 

low during winter months when solar radiation is low, and rates are high during summer 

months. Seasonal patterns in precipitation exist 

in many regions. Perhaps the most extreme 

example is in South Asia. In Mumbai, India, 

storms during the monsoon season of June 

through September account for 94 percent of the average annual precipitation of 180 cm (BBC Weather, http://www.bbc.co.uk/weather,

accessed on February 12, 2007). Long-term climate change has a large effect on the hydrologic cycle. Ground water from the middle Rio 

Grande aquifer is as old as 30,000 years (Plummer and others, 2004), indicating that some recharge to the aquifer occurred when the 

Southwestern United States was experiencing a much wetter climate.

The water-budget equation commonly is used to estimate rates of evapotranspiration or ground-water recharge. A simple analysis 

of this approach illustrates the importance of considering measurement errors. In this approach, all but one of the water-budget components are measured or estimated independently. The remaining component is assumed equal to the residual of the equation. Consider, 

for example, an arid region with coarse-grained soils. Typically, there is no surface runoff; precipitation infiltrates the subsurface and 

is either removed by evapotranspiration or percolates through the unsaturated zone to recharge the underlying aquifer. An appropriate 

water-budget equation would be: 

 Precipitation = Evapotranspiration + Recharge (H1)

Suppose an estimate of evapotranspiration is needed. For 

one year, precipitation was measured at 25 cm and recharge was 

measured at 3 cm. An evapotranspiration estimate of 22 cm is 

then derived. If the recharge estimate were in error by 10 percent 

(recharge was actually 3.3 cm), the uncertainty in the evapotranspiration estimate would be small, less than 2 percent. Even if the recharge 

estimate were in error by 100 percent (recharge was actually 6 cm 

and evapotranspiration was 19 cm), the evapotranspiration uncertainty would be less than 15 percent. Now suppose, instead, that 

we are interested in estimating recharge as the difference between 

precipitation and evapotranspiration. If evapotranspiration were 

independently measured at 24 cm, a recharge estimate of 1 cm 

would be derived. If measurement uncertainty was 10 percent for 

evapotranspiration, recharge for that year may have been as high as 

3.4 cm, a 240-percent difference from the original estimate. As can be 

seen from this example, if the magnitude of the water-budget residual 

is small compared to those of the other components, then small 

uncertainties in other components can result in very large uncertainties in the residual.

Multiple gages reduce the inaccuracy of precipitation 

estimates.

An instrumented field in north-central Oklahoma.

U.S. Department of Energy Atmospheric Radiation Measurement Program

54 Water Budgets: Foundations for Effective Water-Resources and Environmental Management
Humans and the Hydrologic Cycle

By our very existence, humans, along with all other 

animals and plants, are a part of the Earth’s hydrologic cycle. 

Therefore, any human activity affects the natural hydrologic 

cycle. Drinking a glass of water, taking a bath, washing the 

car—these activities involve a small amount of water, yet 

they alter the course of that water within its cycle, though 

perhaps only in minor ways. The activities of humans that 

affect the hydrologic cycle can be grouped into three overlapping categories: construction of water storage and conveyance 

structures, land use, and extraction of ground water. These 

activities are reflected in the hydrologic cycle as a redistribution of water within the atmosphere, land surface, and subsurface, in changes in rates of water flow within and among these 

compartments, and in a relocation of points of water inflow 

to and outflow from them. Alterations to the hydrologic cycle 

may, in turn, lead to changes in natural environments, such as 

creation of new habitat for fish or loss of wetlands. The following three sections provide a brief overview of the effects 

humans can have on the hydrologic cycle. 

Water Storage and Conveyance Structures

Surface-water reservoirs serve many beneficial purposes, 

providing water for irrigation, domestic use, navigation, 

hydroelectric power, and recreation. Dams and the reservoirs 

they create alter the natural movement of water in streams, 

reducing the number of extreme events, such as floods, and 

possibly changing stream temperatures (Collier and others, 

1996). These alterations may affect downstream ecosystems, 

benefiting some species but stressing others. Reservoirs create 

whole new ecosystems, often providing valuable fish habitat. 

They generally lead to an increase in evapotranspiration and 

the flow of surface water to the subsurface and to a decrease in 

total streamflow relative to natural conditions.

Reservoirs, along with an infrastructure of pipelines, 

canals, and ditches, facilitate the transport of water between 

watersheds. For some basins, this artificial export or import 

may be the single largest component of its water budget. 

In Colorado approximately 475,000 acre-ft of water from 

the Colorado River basin is transferred eastward across the 

Continental Divide each year for agricultural and domestic 

use (http://www.water.denver.co.gov/). This water would have 

originally flowed to the Pacific Ocean; it is now diverted to 

the Gulf of Mexico. Canals and ditches and other conduits for 

transporting water provide the opportunity for exchange of 

water with the subsurface and the atmosphere.

Other conveyance structures are designed to remove 

rather than supply water. The Corn Belt of the United States, 

running through Indiana, Illinois, and Iowa, boasts some of 

the most productive agricultural land in the world. Yet less 

than 200 years ago, much of this fertile farmland was natural 

wetland. Since the early 1800s, farmers have installed tiles and 

dug ditches to facilitate drainage of the wetlands. Tile drains 

effectively lower the water table to about 1 meter below land 

surface, thus providing an adequate environment for crops 

to grow. 

Glen Canyon Dam and Lake Powell.

Bureau of Reclamation

Canal and irrigated fields.

Bureau of Reclamation

Tile drains are 

used to lower the 

water table in 

some agricultural 

fields.

Humans and the Hydrologic Cycle 55
Land Use

Land use is perhaps the most important phenomenon 

affecting water exchange between land surface and the 

atmosphere. Conversion of native forests, grasslands, and 

wetlands to agricultural uses constitutes the largest land-use 

change (in terms of area, at least) in the United States and 

most other countries. Replacement of native vegetation with 

agricultural crops leads to changes in patterns of infiltration, 

evapotranspiration, and ground-water recharge. Irrigation of 

crops in arid regions has produced an inflow of water to the 

atmosphere through evapotranspiration that was absent under 

natural conditions. Clearcutting of rain forests has reduced 

evapotranspiration rates in large areas of the Amazon River 

basin. Because of the importance of this region in global circulation patterns, potential climatic effects could extend beyond 

South America. 

Urbanization accounts for the second largest change 

in land use within the United States. Urban features such 

as buildings, roads, and parking lots are all impermeable. 

Thus, they tend to enhance surface runoff of precipitation and 

Urban development alters water’s course through the 

hydrologic cycle.

reduce infiltration. Runoff from these features may be channeled through storm sewers to streams, leading to increased 

streamflow and flooding in the worst situations. That runoff 

also could be funneled to an infiltration gallery leading to an 

increase in ground-water recharge. Water in urban areas is 

conveyed to and from users in networks of underground pipes. 

Invariably, there is some leakage from these pipes. That leakage can be a substantial source of ground-water recharge. 

Terra Photographics, Bruce F. Molnia

Satellite image of irrigated fields in western Kansas.

National Aeronautics and Space Administration

56 Water Budgets: Foundations for Effective Water-Resources and Environmental Management

Trickle irrigation conserves water.

Ground water being pumped for irrigation of rice field.

Image provided courtesy Rain Bird Corp.

Natural Resources Conservation Service
Development of a new irrigation well in west-centrl Florida 

triggered hundreds of sinkholes over a 20-acre area. 

(See person in center for scale.)

Natural Resources Conservation Service

A 1942 photograph of a reach of the Santa Cruz River south of Tucson, Arizona, shows stands of mesquite and cottonwood 

trees along the river (left photograph, Arizona Game and Fish Department). A replicate photograph of the same site in 1989 

shows that riparian vegetation has largely disappeared (right photograph, Robert H. Webb). Data from two nearby wells 

indicate that the water table has declined more than 30 meters due to pumping; this pumping appears to be the principal 

reason for the loss of vegetation.

Ground-Water Extraction

Throughout history, humans relied primarily upon 

surface water to satisfy their needs for water. Storage reservoirs were constructed, streams were diverted, and canals 

were built to convey the water to the areas of need, usually 

agricultural fields or urban areas. Over the past 200 years, 

humans have become more reliant on ground water to supply 

their needs. Extraction of ground water, whether for domestic, 

agricultural, or industrial uses, is balanced by a reduction in 

ground-water storage, a reduction in natural discharge, or an 

increase in recharge. For any particular aquifer, all of these 

phenomena can occur simultaneously, but change in storage 

(indicated by changing ground-water levels) is usually more 

easily determined than changes in discharge or recharge. 

Many aquifers within the United States have experienced 

widespread declines in ground-water levels over the last 

several decades. Declining water levels indicate a reduction 

in subsurface water storage, and they may result in reduced 

ground-water flow to wetlands and streams. Streams that normally gain water from the subsurface could be transformed 

into losing streams. Effects such as these can sometimes be 

seen instantaneously—for example, a stream drying up when 

a well pump is turned on. More commonly, the effects are 

prolonged in time and difficult to quantify. Similarly, the 

effects of reduced ground-water discharge on stream and wetland ecosystems may become apparent only over extended 

periods of time. 

Humans and the Hydrologic Cycle 57

Most ground water that is extracted for irrigation is 

evapotranspired back to the atmosphere shortly after it is 

applied to the land surface. However, a percentage of irrigation water, called irrigation excess or return flow, may run off 

to a stream or may infiltrate, percolate through the unsaturated zone, and eventually become ground water again. Most 

ground water removed from the subsurface for domestic use 

eventually ends up returning to the saturated zone through 

septic leach fields or is discharged to surface-water bodies 

from wastewater-treatment plants. 
I Water Use and Availability

Humans need water. But just how much water do we need? Every day in the United States 345 billion gallons on average is withdrawn from ground- and surface-water sources for human use (Hutson and others, 2004). This is equivalent to more than 1,000 gal/d for 

every person in the country—about 40 bathtubs full. We do not usually take that many baths, so how is this water used? The largest use 

(48 percent) is by thermoelectric power plants, for cooling and steam generation. Other uses, as shown in figure I–1, are for irrigation of 

agricultural lands, domestic needs, industry, mining, aquaculture, and livestock. Water satisfies a myriad of thirsts; a daily bath is but a 

few drops in the water-use bucket.

Figure I-1. Percent of total water withdrawals for major categories within the United States (from Hutson 

and others, 2004).

Public Supply, 11 percent

Irrigation, 34 percent

Aquaculture, less than 1 percent

Mining, less than 1 percent

Livestock watering, Rio Arriba County, New Mexico Gated-pipe flood irrigation, Fremont County, Wyoming

World’s largest trout farm, Buhl, Idaho

Livestock, less than 1 percent

Domestic, less than 1 percent

Industrial, 5 percent

Thermoelectric Power, 48 percent

Paper mill, Savannah, Georgia

Richard L. Marella, USGS

Alan M. Cussler, USGS Alan M. Cussler, USGS Alan M. Cussler, USGS 

Nancy L. Barber, USGS Courtesy of Clear Springs Foods, Inc. Courtesy of Jeff Vanuga, USDA NRCS

Courtesy of Jeff Vanuga, USDA NRCS

Public supply water intake, Bay County, Florida

Spodumene pegmatite mine, Kings Mountain, North Carolina

Domestic well, Early County, Georgia

Cooling towers, Burke County, Georgia

58 Water Budgets: Foundations for Effective Water-Resources and Environmental Management
Ground water

Surface water

Total

Population

 POPULATION, IN MILLIONS

0

50

100

150

200

250

300

350

400

1950 1955 1960 1965 1970 1975 1980 1985 1990 1995 2000

0

50

100

150

200

250

300

 WITHDRAWALS, IN BILLION GALLONS PER DAY

Figure I-2. Freshwater withdrawal and population in the United States, 1950–2000 (from Hutson and others, 2004).

Total water withdrawals in the United States have been stable since the mid-1980s. However, on a per capita basis, total withdrawals have decreased over the same period (fig. I–2). This is likely the result of improved techniques that require less water for power 

generation and advances in irrigation efficiency. Technologies such as low-flow bathroom fixtures and water-saving appliances in 

homes aid in conserving local water supplies. In New York, for example, water use has declined from about 200 gal/d per person in 1990 

to less than 140 gal/d in 2003 (City of New York, Department of Environmental Protection, http://www.nyc.gov/html/dep/html/droughthist.

html accessed on December 18, 2006). On the national scale, however, the savings realized in the home are minimal when compared to 

thermoelectric and agricultural use. Conservation and improved efficiency of water use may be driven by economic and water-quality 

issues as well as by water supply. Energy costs for pumping water continue to rise, and stricter water-quality standards for water discharges have been put in place at the Federal and State levels. Periods of drought may prompt water managers to impose limits on water 

use. Farmers, ranchers, manufacturers, energy providers, and individual consumers adapt their water use to technological advances, 

changing regulations, and market forces. 

“Compilation of water-use information on a regular basis provides information on the amount of water used by humans, yet there 

has been no corresponding assessment of water availability within the United States. In addition, the water needs of biota that inhabit 

waterways, wetlands, flood plains, and other environments are largely unknown” (National Science and Technology Council, 2004). 

Water budgets of watersheds, aquifers, and surface-water bodies are essential tools for assessing the availability of water for both 

human and environmental needs. It is useful, therefore, to look at the way water budgets are determined, the uncertainty inherent in 

those budgets, and the effect of human activity on water budgets. Episodes of drought and floods are reminders that water availability 

can change substantially over time. It is therefore important to consider the temporal variability in the movement and storage of water in 

the hydrologic cycle and the relation of that variability to water use.

Box I Water Use and Availability 59
J Water Budgets of Political Units

Water in most areas is managed by governmental units, 

be it countries, States, counties, or water districts. Concerns of 

these entities include how much water they have, how much 

water they use, and how, when, and at what rate water supplies 

are replenished. Throughout this report, the uncertainty inherent 

in water-budget calculations is demonstrated. That uncertainty 

is compounded when boundaries of an accounting unit are not 

aligned with hydrologic boundaries. Such is often the case with 

governmental units where humans have defined political boundaries that may crisscross natural watershed boundaries and partition aquifers. These boundaries may also follow rivers, resulting 

in the rivers being shared by competing entities. Measurement 

of surface- and ground-water flow across political boundaries 

presents unique and sometimes contentious challenges.

The average annual water budget for the State of Kansas is 

depicted in figure J–1 for a period from the 1970s into the 1980s. 

Average annual precipitation within the State is about 27 inches. 

Equating inflow and outflow, with the assumption that annual 

change in storage is relatively small, the water budget can be 

expressed as:

 Precipitation + Surface-Water Inflow =

 Evapotranspiration + Surface-Water Outflow (J1)

State boundaries cross tens, if not hundreds, of streams that 

flow from Colorado and Nebraska into Kansas and a similar number of streams that flow out of Kansas to Oklahoma and Missouri 

(fig. J–2). Only a small percentage of those streams have stream 

gages to monitor flow, and few of those gages are located at State 

lines. Thus, uncertainty in estimating total surface inflows and 

outflows for the State of Kansas may be quite high. 

Conspicuous by their absence in the above equation and in 

figure J–1 are ground-water inflow and outflow. These processes 

do occur. At the State level, those rates were deemed insignificant relative to other terms. At a local scale, however, groundwater flow into Kansas from Colorado in the High Plains aquifer 

(McGuire and others, 2003), for example, may be an important 

component in the water budget of some western Kansas counties. 

BRF 28.7

SWI 1.45

P 106

SWO 11.6

ET 91

CU 4.7

K A N S A S

Beaver Cr

Saline River

River

River

River

Walnut R

River

River

Pawnee

Rattlesnake Cr

Smoky

Crooked

Arkansas

Arkansas

Little Arkansas

Solomom River

River

River

Creek

Big Kansas

Wakarusa

des

Republican

River

River

River R

Cimarron R

Cr

Hill Blue

Marais

Verdigris

Missouri

Cygnes

Stranger

Figure J–1. Average-annual water budget for 

Kansas, in million gallons per day, 1970s and early 

1980s. Abbreviations: BRF, boundary-river flow; CU, 

consumptive use (evapotranspiration related to human 

activities, mostly irrigation); ET, evapotranspiration from 

native plants and nonirrigated agricultural fields; P, 

precipitation; SWI, surface-water inflow; SWO, surfacewater outflow (Carr and others, 1990).

Figure J–2. Principal rivers, Kansas (Paulson and others, 

1991).

The Missouri River flows in or along seven States. The High Plains aquifer lies in parts of eight States. U.S. Army Corps of Engineers

Natural Resources Conservation Service

60 Water Budgets: Foundations for Effective Water-Resources and Environmental Management
Water Budgets and Management of 

Hydrologic Systems 

The global hydrologic cycle is a continuum; the atmosphere, land surface, and subsurface are intrinsically linked by 

the water cycling through them. Each of these compartments 

is influenced in one way or another by human activities, either 

directly or as a complementary reaction to changes in another 

compartment. The following examples are presented to show 

why water budgets are important and how they can be used in 

management of water resources.

Large River System: Colorado River Basin

The Colorado River basin covers 637,000 km2 in the 

Southwestern United States (fig. 35). Although its headwaters 

are in the Rocky Mountains, where snowmelt and rainfall 

provide substantial quantities of water, the river traverses 

largely semiarid and arid regions, where relatively small 

quantities of water are added to the river. Irrigated agriculture 

and major metropolitan areas in southern California, southern 

Nevada, southern Arizona, and the Front Range in Colorado 

make major demands on Colorado River water. In California 

and Colorado, water from the Colorado River is exported from 

Dillon Reservoir, Colorado.

Water Budgets and Management of Hydrologic Systems 61
WYOMING

UTAH

NEVADA

ARIZONA

NEW MEXICO

COLORADO

CA

CONTINENTAL DIVIDE

CONTINENTAL DIVIDE

Green R

Green R

Big 

Sandy 

Res.

Meeks

Cabin

Res.

Yampa R

Little Snake River

Fontenelle

Reservoir

Viva

Naughton

Reservoir

River

Gila

San Pedro River

Gila River

Verde River

Agua Fria River

Bill Williams River

Reudi

Res.

Stateline

Reservoir

Fremont

Lake

Sandy

San Francisco River

Flaming

Gorge Dam

Hoover Dam

Davis Dam

Glen 

Canyon

Dam

Alamo 

Dam Parker

Dam

Colorado

River

Aqueduct

Area of map

in figure 36.

Palo Verde

Diversion

Dam

Senator Wash Dam

Imperial Dam

Laguna Dam

All American Canal Painted Rock Dam

New 

Waddell 

Dam

Horseshoe Dam

Bartlett DamRoosevelt Dam

Horse Mesa Dam

Mormon Flat Dam

Stewart Mountain Dam

Coolidge 

Dam

Navajo Dam

Azotea 

Tunnel

Vallecito

Dam

Lemon

Dam McPhee

Dam

Silver Jack

Dam

Blue Mesa 

Dam

Crystal

Dam

Dallas

Creek

Dam

Taylor

Park

Dam

Moon

Lake

Dam

Upper 

Stillwater

Dam

Currant 

Creek 

Dam

Soldier

Creek Dam

(Strawberry Res.)

Scofield

Dam

Diamond

Fork

System

Duchesne

Tunnel

Joe’s

Valley

Dam

Red

Fleet

Dam

Steinaker

Dam

Roberts 

Tunnel

Big Sandy River

Big

R

HAMS FK

Green River

White River

Price 

San 

Muddy Cr

Fremont R

Escalante R

Dirty Devil

Colorado River

Colorado River

Dolores R

San Miguel R

Gunnison R, N.F.

Colorado River

River

River

Rafael

Muddy Cr

Green Mountain

Reservoir

Grand Lake

Virgin River

Paria River Little Colorado River

Headgate

Rock Dam

0 50 100 150 200 MILES

0 50 100 150 200 250 KILOMETERS

u Pinedale

u

u

u

u

u

u

u

Aspen

Lee’s Ferry

Flagstaff

Phoenix

Tucson

Yuma

Las Vegas

FryingpanArkansas 

Moffat

Tunnel

Alva B. Adams

Tunnel

Rivers

Dams

Federal tunnels

Diversion out of

 Colorado River

 basin

EXPLANATION

Colorado

River basin

N

Figure 35. Map of the Colorado River basin showing locations of major surface-water reservoirs and diversions.

62 Water Budgets: Foundations for Effective Water-Resources and Environmental Management
the basin. Because water demands can exceed what the river 

supplies, technically complex and politically contentious water 

compacts have evolved over many decades.

Water budgets have been an essential tool for apportioning the Colorado River water among various claimants. 

Budgets are needed for various types of accounting units that 

are nested within the basin, including headwater watersheds, 

agricultural fields, reservoirs, and aquifers. The water budgets 

for all of these nested units ultimately interact to balance the 

overall water budget of the basin. The challenge is to manage 

water resources while explicitly accounting for the inherent 

uncertainties in water-budget estimates. Two accounting units 

of the Colorado River basin are briefly discussed in the following sections.

Watersheds and Reservoir Management

 Reservoirs are key components in managing the water 

resources of the Colorado River basin. Reservoirs are used to 

store water when and where it is abundant so the water can be 

released to downstream areas or exported out of the basin as 

needed. Reservoirs serve this purpose in many different environments but are perhaps most prominent in semiarid and arid 

regions. Over the course of each year, operators of reservoirs 

in the Colorado River basin must make decisions on release 

rates from reservoirs. Determining how much water to release 

involves some risk—release too much water and there may not 

be enough later in the year to satisfy thirsty customers; release 

too little water and large spring runoff from snowmelt could 

cause severe flooding.

Selection of optimum water-release rates from a reservoir requires an understanding of the reservoir’s water budget. 

Rates and timing of releases depend on the amount of water 

currently stored in the reservoir, the anticipated rate and timing 

of inflow, needs for power generation or flood control, irrigation demands, and other legal and environmental requirements. 

Watershed models provide a convenient means for predicting 

rates and timing of inflow; thus, they have become important 

decision-support tools for many reservoir operators. Watershed 

models basically are water budgets of watersheds; they calculate water input, storage, and losses of water from a watershed. 

But like all water budgets, there are uncertainties in watershed 

model results.

The Bureau of Reclamation (BOR) and the U.S. Geological Survey (USGS) have developed a decision-support 

system to allow BOR to manage some reservoirs in the Upper 

Colorado River basin in Colorado (Frevert and others, 2006). 

The Bureau of Reclamation runs watershed models, developed 

by the USGS, to predict discharge from the major streams 

that flow into these reservoirs. A novel approach allows 

uncertainty in predicted reservoir inflows to be factored into 

reservoir operations. The watershed model uses current data 

on snowpack, soil moisture, stream discharge, and historical 

climate data, such as temperature and precipitation to generate synthetic discharge hydrographs, one for each historical 

year of data. The idea behind the approach is to estimate what 

discharge rates are likely to occur in the future under climate 

conditions similar to those of the past. Results from the 

watershed model are used to generate probabilities associated 

with different discharges (for example, 50 percent of the time 

the volume of discharge would exceed 500,000 m3; 10 percent 

of the time peak discharge would exceed 1,000,000 m3). The 

reservoir operator factors these discharges and probabilities, 

along with estimates and uncertainties of other components 

of the reservoir’s water budget, into decisions on reservoir 

releases. 

Streamflow in the Colorado River is derived primarily from 

snowfall in the mountains within its drainage.

Central Arizona Project aqueduct.

Lake Mead, Arizona and Nevada.

D.H. Campbell

Bureau of Reclamation

Bureau of Reclamation

Water Budgets and Management of Hydrologic Systems 63
Aquifers in Arizona

 The limited amount of surface water in Arizona has 

led to substantial use of ground water, especially for agriculture. With an arid to semiarid climate and very low rates 

of recharge, ground-water withdrawals caused water levels 

in Arizona to decline as early as the 1920s. Declines were 

more rapid after the 1940s because of the increased availability in rural areas of electricity to power deep-well turbine 

pumps. Ground-water withdrawals have resulted in reduced 

discharge to streams and wetlands (Webb and others, 2007) 

and water-level declines that have caused land subsidence 

in some areas (Galloway and others, 1999). To help manage ground-water resources, ground-water flow models of 

aquifers were constructed for many parts of the State. These 

models, which in effect are water-budget models, were used to 

predict how ground-water levels would be affected by future 

aquifer-management practices. 

To assess the predictive capabilities of a ground-water 

flow model of an aquifer in central Arizona (fig. 35), Konikow (1986) compared water levels in 77 wells measured in 

1974 with water levels predicted for that year with a model of 

the Salt River and lower Santa Cruz River basins (Anderson, 

1968). The original model was calibrated by using water-level 

and pumping data collected from 1923 to 1964. Between 1923 

and 1964, average ground-water levels declined by an average 

of about 120 ft. Water levels measured in 1974, 10 years after 

the ground-water model was completed, differed from those 

predicted by 50 to 200 ft in large parts of the area (fig. 36). 

N

> 0

Error, in feet

– 50 to 0

– 100 to – 50 

– 200 to – 100 

< – 200 

0 20 MILES

Salt River 

Gila River Santa Cruz R

Figure 36. Map of central Arizona showing error in water levels 

predicted by a ground-water flow model (after Konikow, 1986). 

Errors are determined as predicted minus measured water levels. 

Location shown in figure 35.

Agricultural crops require irrigation in Arizona.

Natural Resources Conservation Service

64 Water Budgets: Foundations for Effective Water-Resources and Environmental Management
The discrepancies were attributed to three main factors: 

(1) differences between where ground-water development was 

predicted to occur and where it actually occurred, (2) differences in the amount of water predicted to be withdrawn 

and the amount actually withdrawn, and (3) changes in the 

hydraulic properties of the aquifer and confining beds caused 

by dewatering, including land subsidence in a few areas. 

The Arizona experience illustrates the value of continually 

updating ground-water-budget models because of the inherent 

uncertainties in aquifer characterization and in future patterns 

of ground-water development. 

The State of Arizona enacted the Arizona Groundwater 

Management Code (http://www.azwater.gov/dwr/Content/

Publications/files/gwmgtovw.pdf, accessed on March 13, 2007) 

in 1980 to address the sustainability of its limited groundwater resources. The code established a program of 

ground-water rights and permits with some very demanding 

provisions. A series of water-management plans created comprehensive conservation targets for areas of severe groundwater depletion. Developers were required to obtain a 100-yr 

water supply for any new growth. A program for reporting 

ground-water withdrawal and use was established. The restrictive nature of these provisions reflects serious concern for the 

availability of water within Arizona. Effective implementation 

of the code requires development and continual revision of 

water budgets of the State’s aquifers.

Land surface subsidence in Eloy area, central Arizona.

Water well.

Water Budgets and Management of Hydrologic Systems 65
0 50 100 MILES

0 50 100 KILOMETERS

Base from U.S. Geological Survey digital data, 1:2,000,000

Albers Equal-Area projection

Standard parallels 29 ̊ 30' and 45 ̊ 30', central meridian –101 ̊

NEBRASKA

KANSAS

OKLAHOMA

TEXAS

NEW

MEXICO

COLORADO

WYOMING SOUTH DAKOTA

North Platte River

South Platte River

Platte

River

Republican River

Arkansas River

Canadian River

Large Aquifer System: High Plains Aquifer

The High Plains aquifer stretches over an area of about 

443,000 km2 in the central plains of the United States (fig. 37). 

It consists of unconsolidated and consolidated materials, mixtures of sand, silt, and clay, that overlie a sedimentary bedrock 

surface. The large areal extent and thickness of the aquifer 

provide storage for huge volumes of water; estimates are as 

high as 3 billion acre-ft (McGuire and others, 2003). Since the 

1940s, large volumes of ground water have been withdrawn to 

irrigate crops such as corn, soybeans, and cotton. These withdrawals have resulted in substantial declines in ground-water 

levels beneath large areas, which have in turn led to concerns 

about ground-water depletion and the future of the aquifer.

The climate in the High Plains varies from arid to semiarid to subhumid. Average annual precipitation rates range 

from less than 14 inches in the southwest to about 32 inches in 

the northeast. There are few perennial streams in the southern 

and central parts of the High Plains aquifer. Perennial streams, 

however, are common across the northern part. 

Figure 37. Location map of High Plains aquifer.

Water in the High Plains aquifer has accumulated over 

thousands of years. Before human development, inflow to 

the aquifer was some small portion of precipitation that 

infiltrated, traversed the unsaturated zone, and recharged the 

aquifer. Estimated recharge rates at that time ranged from 

0.10 to 0.50 inch/yr for the southern and central parts and 

1 to 2 inches/yr for the northern part of the aquifer. Outflow 

prior to development was in the form of discharge to streams 

(or to plants in riparian zones) and hillslope seepage along 

lateral and eastern aquifer boundaries where the earth materials of the High Plains aquifer form an easily discernible 

topographic escarpment. While hillslope seepage is a common 

hydrologic phenomenon, the large spatial scale of this occurrence in the High Plains is noteworthy.

Human development of the High Plains aquifer resulted 

in large withdrawals of ground water from wells. Total withdrawals and the rate of decline of water levels increased into 

the mid-1970s (figs. 38, 39) at which time the limitations of 

this resource became apparent. Since then, improved irrigation efficiency (furrow irrigation has largely been replaced by 

sprinkler systems) has slowed the rate of decline of groundwater levels. A water budget for a part of the High Plains 

aquifer was constructed for a time prior to development and 

another for 1997 (fig. 40; Luckey and Becker, 1999). Over 

that time, there was an 18-percent reduction in water storage caused primarily by extraction of ground water. There 

was also a decrease in natural discharge and an increase in 

recharge from irrigation return flow. 

1949 1954 1959 1964 1969 1974 1980 1985 1990 1995

0

6

12

18

24

GROUND-WATER PUMPAGE FOR 

IRRIGATION, IN MILLION ACRE-FEET

Colorado

Kansas

Nebraska Texas

New Mexico, Oklahoma,

 South Dakota, and Wyoming

EXPLANATION

Figure 38. Ground-water pumpage from the High Plains 

aquifer for irrigation by State for selected years, 1949 to 1995 

(McGuire and others, 2003).

66 Water Budgets: Foundations for Effective Water-Resources and Environmental Management
0 50 100 MILES

0 50 100 KILOMETERS

Base from U.S. Geological Survey digital data, 1:2,000,000

Albers Equal-Area projection

Standard parallels 29o 30' and 45o 30', central meridian –101o

U

U

U

U U U

U

U

NEW MEXICO

COLORADO

TEXAS

KANSAS

NEBRASKA

WYOMING SOUTH DAKOTA

OKLAHOMA

High

Plains

Aquifer

Boundary

Declines

 More than 150

 100 to 150

 50 to 100

 25 to 50

 10 to 25

Rises

 10 to 25

 25 to 50

 More than 50

 

No substantial change

 –10 to 10

 

Water-level change, in feet

EXPLANATION

Area of little or no saturated

 thickness

Fault—U, upthrown side

Land surface

Water table

Base of aquifer

19501960

1970

198019902000

3,300

2,700

2,800

2,900

3,000

3,100

3,200

ALTITUDE, IN FEET

ABOVE NGVD 29

Hansford County, Texas 1950 1960 1970 1980

1990

2000

Land surface

Water table

Base of aquifer

2,700

2,800

2,900

3,000

3,100

ALTITUDE, IN FEET

ABOVE NGVD 29

Grant County, Kansas

2000

Land surface

Water table

Base of aquifer

Scott County, Kansas

ALTITUDE, IN FEET

ABOVE NGVD 29

2,800

2,900

3,000

1930

194019501960

1970

19801990

Land surface

Water table

Base of aquifer

ALTITUDE, IN FEET

 ABOVE NGVD 29

2,000

2,200

2,400

193019401950

19601970

198019902000

2,500

2,300

2,100

Gosper County, Nebraska

Figure 39. Water levels over time (1950 – 2005) in observation wells in the High Plains and map showing water-level changes in the 

High Plains aquifer, predevelopment to 2000 (McGuire and others, 2003).

Water Budgets and Management of Hydrologic Systems 67
Recharge 

from 

precipitation

and irrigation

return flow

0.60

Decrease in water 

in storage, 

predevelopment to 1997

87 million acre-feet

Recharge 

from 

precipitation

0.22

Discharge

to streams

and springs

0.22

Water in storage,

predevelopment

480 million acre-feet

Water in storage,

1997

393 million acre-feet

Predevelopment Conditions

Discharge 

to streams

and springs

0.15

Discharge

to wells

2.67

Development Conditions in 1997

Boundary of 

High Plains

aquifer

TEXAS

KANSAS

WYOMING

NEW MEXICO

COLORADO

NEBRASKA

OKLAHOMA

SOUTH DAKOTA

South-central 

High Plains

aquifer study

area

100 200 MILES

0 200 KILOMETERS

0 

100 

110°

104°

38°

44°

32°

26°

98°

Figure 40. Ground-water budget in the south-central 

High Plains aquifer study area during predevelopment 

and in 1997. Values without units are in million acrefeet per year (Luckey and Becker, 1999).

“Accurate estimates of ground-water withdrawals are difficult to obtain. Farmers are not always required to report 

ground-water withdrawals from their irrigation wells to 

regulatory agencies, and it is not practical to place flow 

meters on every well. Indirect methods are often used to estimate withdrawal rates. One such method is based on typical 

water use for specific crops; another method relates power 

consumption to pumping rates.”

(McGuire and others, 2003)

The loss of water from the entire High Plains aquifer 

from predevelopment to the year 2000 is estimated to be about 

200 million acre-ft, or about 6 percent of its original volume 

(table 7). Most of this loss occurred in Texas. Ground-water 

levels have declined over most of the High Plains, but some 

northern areas have actually experienced a rise in water levels 

in recent years (fig. 39). These rises are attributed to the use 

of surface-water supplies for irrigation; excess irrigation water 

percolates through the unsaturated zone, enhancing recharge 

rates and augmenting aquifer storage.

Table 7. Change in water in storage in the High Plains 

aquifer, predevelopment to 2000 (McGuire and others, 2003).

State Change in water in storage,

in million acre-feet

Colorado –11

Kansas –47

Nebraska 4

New Mexico –8

Oklahoma –11

South Dakota 0

Texas –124

Wyoming 0

Eight States –197

68 Water Budgets: Foundations for Effective Water-Resources and Environmental Management
Aerial view of center pivot irrigation systems in the Texas Panhandle.

High-capacity irrigation well.

1961

1994

Figure 41. Major perennial streams in Kansas, 1961 and 

1994. Parts of the High Plains aquifer occur in western 

Kansas where stream channels exhibiting perennial flow 

have been reduced in length (after Sophocleous, 2000).

Decades of large withdrawals of ground water have 

altered historical ground-water discharge patterns. Groundwater discharge to streams has decreased in some areas 

(fig. 41; Sophocleous, 2000), and hillslope seepage along 

lateral and eastern aquifer boundaries has been reduced. 

Effectively, the volume of pumped water, which would have 

eventually discharged to streams and hillslopes, has been intercepted, and a large percentage of the withdrawn water is lost 

to evapotranspiration by crops. 

Determining the changes in the volume of water stored 

in an aquifer system such as the High Plains is a difficult task 

because of its large size, complex geometry, and variable 

storage properties. Water-level changes shown in figure 39 

are based on water-level measurements made in more than 

8,000 wells—a large number of wells in an absolute sense, 

but not so large when the immense expanse of the aquifer 

is considered. Because pumping for irrigation is seasonal 

(during the growing season of March through September), 

these wells were measured in winter or early spring and thus 

represent maximum water levels for the year by allowing 

time for water levels to stabilize from the previous irrigation 

period. Regular synoptic surveys of ground-water levels provide the most useful data with which the health of the aquifer 

can be assessed. These measurements document changes 

in storage and provide essential information for calibrating 

ground-water-flow models.

Water Budgets and Management of Hydrologic Systems 69
Minimizing reduction in ground-water storage requires 

a balance between aquifer recharge and discharge. Water 

budgets are useful tools for assessing this balance. Thus, 

improved water-budget information can facilitate decisions on 

water-allocation issues. The accuracy of water budgets can be 

improved with monitoring of ground-water levels, measurement of streamflow, reliable information on pumping and irrigation rates, and better estimates of rates of natural recharge 

and of irrigation return flow. 

Unfortunately, water-resources management is more 

complicated than simply balancing the water budget (as if 

that balancing act were a simple task!). Political and economic issues also are important. The High Plains aquifer 

system lies within the borders of eight States. Although the 

States have a common goal of managing aquifer development 

to prevent depletion and ensure water availability in future 

years, each State has its own approach for attaining that goal. 

Irrigation is the economic lifeline of farmers within the High 

Plains, supporting a vibrant economy since the middle of the 

20th century. Cities and towns have grown in response to this 

economy. Decisions on water management have direct effects 

A corn field of one acre can give off 4,000 gallons of water per day in evaporation.

One inch of rain or irrigation on that field produces more than 27,000 gallons of water.

Installing an observation well.

on the lives and livelihoods of the people who reside in this 

region. Proper management of water resources at any level 

depends not only on sound science but also on cooperation 

among political entities, long-term water-management plans, 

and public involvement and education (Sophocleous, 2000). 

70 Water Budgets: Foundations for Effective Water-Resources and Environmental Management
Water Budgets and Governmental Units: Lake 

Seminole

Dams are constructed on rivers to meet a variety of needs. 

The reservoirs that result from impoundment of flowing waters 

provide water supply, recreational opportunities, flood control, 

and hydroelectric power. They also serve as a source of water 

to maintain navigation levels in waterways. Dams alter the 

natural flow of water and sediment in a river, thus affecting 

habitat for biota that live in and along river corridors. Balancing the water needs of humans with those of natural biological 

resources is an emerging area of concern of reservoir operation.

The Jim Woodruff Lock and Dam on the Apalachicola 

River in the southeastern United States, built in the late 1940s 

and early 1950s, impounds Lake Seminole (fig. 42). The lake 

was intended to aid river navigation, produce hydroelectric 

power, and provide recreational opportunities. It is fed primarily by flow from the Chattahoochee and Flint Rivers; the 

contributing watersheds include areas in Alabama, Florida, 

and Georgia. Outflow from the lake is the major source of 

freshwater, nutrients, and detritus to the lower Apalachicola 

River, its estuary, and Apalachicola Bay, an important shellfish 

fishery.

Water-resource managers in these three States must deal 

with competing demands for Lake Seminole water. Metropolitan Atlanta draws most of its water from the ApalachicolaChattahoochee-Flint basin. The rivers in the basin, as well 

as Lake Seminole itself, are hydraulically connected to the 

Upper Floridan aquifer system. Irrigation for agriculture in 

southwestern Georgia is largely derived from ground-water 

withdrawals. These withdrawals may affect streams flowing 

into Florida. Alteration of streamflow into Apalachicola Bay is 

a concern for the fishery in the bay. Allocation of basin water 

among the three States was of such concern that a compact 

regarding the water resources of the basin between the U.S. 

Congress and the States of Alabama, Florida, and Georgia 

was in effect from 1997 to 2003. The compact has expired and 

water allocation within the basin may ultimately be decided by 

the U.S. Supreme Court.

In an effort to understand the effects of the lake on the 

lower Apalachicola River basin, a detailed water budget of 

the lake was developed for the period April 2000 to September 2001 (Dalton and others, 2004). The conceptual water 

budget is depicted in figure 43. Precipitation was monitored at 

two locations on the lake. Surface-water inflows and outflows 

were measured at USGS stream-gaging stations. Several small, 

ungaged tributaries along arms of the reservoir contributed 

a minor amount of streamflow. Ground-water inflows and 

outflows were determined from a detailed ground-water-flow 

model for the area. Evaporation was estimated using an 

energy-balance method, and lake storage was determined on 

the basis of daily readings of stage. Jim Woodruff Lock and Dam and Lake Seminole.

30n

31n

32n

85n 84n

Map

area

0

0

10

10

20 MILES

20 KILOMETERS

SEMINOLE

MILLER

COLQUITT

WORTH

TURNER

CRISP

DOUGHERTY CALHOUN

RANDOLPH

STEWART

CLAY

TERRELL

LEE

MITCHELL

BAKER

DOOLY

SUMTER WEBSTER

GADSDEN

GULF

JACKSON

CALHOUN

HOUSTON

HENRY

BARBOUR

DECATUR

LIBERTY

FRANKLIN

GRADY

EARLY

Blountstown

Dothan

Chattahoochee

Bristol

Woods

Orange

Bainbridge

Cordele

Albany

Apalachicola

Eufaula

UPDIP

LIMIT

OF UPPER

FLORIDAN

AQUIFER

SOLUTION

ESCARPMENT

AL

FL

FL

GA

FALL

LINE

HILLS

GULF COASTAL LOWLANDS

DOUGHERTY

PLAIN

TIFTON

UPLAND

Base modified from U.S. Geological Survey

1:100,000-scale digital data

AL GA

Gulf of Mexico

A palachicola Bay

Lak e Seminole

ALABAMA

Apalachicola–

Chattahoochee–

Flint River Basin

GEORGIA

FLORIDA

EXPLANATION

Lower Apalachicola–

 Chattahoochee–

 Flint River Basin

Physiographic division 

 boundary

dut S y area

Walter F.

George

Lake

Chattahoochee R Apalachicola R

Flint River

Figure 42. Location of Lake Seminole, boundaries of the lower 

Apalachicola-Chattahoochee-Flint River Basin, and physiographic 

divisions of the Coastal Plain Province in southeastern Alabama, 

northwestern Florida, and southwestern Georgia (Dalton and 

others, 2004).

Water Budgets and Management of Hydrologic Systems 71

U.S. Army Corps of Engineers
As shown in figure 44, surface-water flow dominates 

the Lake Seminole water budget. This is not surprising; the 

impoundment is operated for short-duration flow augmentation of the Apalachicola River for navigational purposes. The 

hydraulic connection between the reservoir and the underlying aquifer is reflected in the large flow of ground water 

into Lake Seminole and the leakage of water from the reservoir to ground water in the proximity of the dam. The direct 

contribution of rainfall to the reservoir is only 1 percent of the 

SWin

SWin

SWin

SWin

E

P

GWin

GWout

SWout

GWout

Solution feature

in Upper Floridan

aquifer

Lake Seminole

NOT TO SCALE

P + SWin – SWout + GWin – GWout – E = $S

P

SWin

SWout

GWin

GWout

E

$S

= precipitation

= surface-water inflows

= surface-water outflows

= ground-water inflows

= ground-water outflows

= evaporation

= change in lake storage

Figure 43. The water budget for Lake Seminole (Dalton 

and others, 2004).

Measuring streamflow.

12,000

14,000

10,000

8,000

6,000

4,000

2,000

0

CUBIC FEET PER SECOND

Surface-water inflow

Ground-water inflow

Precipitation

Surface-water outflow

Lake leakage (Ground-water outflow)

Evaporation

Change in lake storage

Budget difference

Inflows Outflows

Figure 44. Average monthly water budget for Lake Seminole for 

the period April 2000 through September 2001 (Dalton and others, 

2004).

Weather station in Lake Seminole.

total inflow to the lake. Loss of water from the reservoir to the 

atmosphere by evaporation is only 2 percent of the total loss of 

water. Because of uncertainties inherent in each method used 

to estimate water-budget components, the difference between 

water gain and loss does not exactly match the independently 

determined change in lake storage; the discrepancy is about 

4 percent of total inflow.

It is physically impossible to account for every drop of 

water entering and leaving a reservoir, a drainage basin, an 

aquifer, or a State. Thus, water managers are faced with making decisions about water in the context of some uncertainty. 

In the case of Lake Seminole, customary and state-of-the-art 

72 Water Budgets: Foundations for Effective Water-Resources and Environmental Management
techniques were used to quantify all water-budget components 

for the study period. Even so, each technique has some degree 

of uncertainty associated with it (Dalton and others, 2004). 

The 4-percent discrepancy in the water budget deserves some 

attention. In light of the uncertainty in each measurement technique, hydrologists would generally consider this to represent 

good agreement among the different measurements. However, 

for water managers, this 4 percent (about 560 ft3/s) is a substantial amount of water to be unaccounted for. Because the 

study of water budgets in hydrologic systems is not an exact 

science, sound management decisions based on good science 

also account for uncertainties inherent in that science. 

The study of Lake Seminole included an analysis of the 

relative importance of individual water-budget components 

to the overall accuracy of the water budget. This analysis, 

also known as a sensitivity analysis, informs water scientists 

as well as water managers whether the closure of the water 

budget could be improved by applying more accurate measurement techniques. For example, if evapotranspiration had 

been the dominant component of the water budget of Lake 

Seminole, then future measurements of evapotranspiration 

with detailed micrometeorological methods would be desirable. Because evapotranspiration was a small part of the water 

budget, it can be measured with a simpler, less accurate technique requiring fewer resources to implement. Surface-water 

flow dominates the water budget, so the accuracy of the water 

budget is closely tied to the accuracy of streamflow measurements. Independent measurements or estimates of groundwater flows also may lead to an improved water budget. 

Water-allocation issues among the three States may not 

be resolved for years to come. On a shorter time scale, water 

budgets are an integral part of day-to-day dam operations. The 

Woodruff Dam is the most downstream of a series of six dams 

in the Apalachicola-Chattahoochee-Flint basin. Releases of 

water through dams in this system are coordinated to satisfy, 

as best possible, a multitude of needs—hydroelectric power, 

navigation, water quality and quantity, flood control, environment, and recreation. River stage forecasts are provided by 

the National Weather Service which uses complex simulation models in conjunction with stream and precipitation data 

obtained in real time through satellite transmission and with 

predicted precipitation rates. Managers prioritize current water 

needs in light of these forecasts to set appropriate release rates 

(http://water.sam.usace.army.mil/narrativ.htm, accessed on 

March 5, 2007).

Twilight fishing.

U.S. Army Corps of Engineers

U.S. Army Corps of Engineers

Maintaining navigable waterways is one of the issues 

addressed by dam operators. Wildlife.

Water Budgets and Management of Hydrologic Systems 73
Agriculture and Habitat: Upper Klamath Lake

Upper Klamath Lake, in south-central Oregon (fig. 45), 

is the main source of water for the Bureau of Reclamation’s 

Klamath Project, a project designed to provide water for 

irrigation of about 180,000 acres of cropland and for two 

National Wildlife Refuges. The Williamson and Wood Rivers, 

two streams with a large amount of ground-water discharge, 

account for more than 50 percent of the water inflow to the 

lake. Small streams, ground-water seepage, and direct precipitation are secondary sources of inflow. The lake drains to 

the Link River, the natural outlet for the lake, and eventually 

contributes to the Klamath River. Water is also diverted from 

Upper Klamath Lake through the A Canal to provide irrigation water during the growing season. Evapotranspiration 

losses from the lake are also important. Flow to the Link River 

is controlled by the Link River Dam, which was completed 

in 1921. In addition to providing water for irrigation, wildlife refuges, and recreation, water in the lake and associated 

rivers provides aquatic habitat for a number of species, some 

that are endangered or threatened, such as the Lost River 

sucker (Deltistes luxatus), shortnose sucker (Chasmistes 

brevirostris), and coho salmon (Oncorhynchus kisutch), and 

some, such as chinook salmon (Oncorhynchus tshawytscha), 

that are of substantial commercial, cultural, and recreational 

value (Service, 2003).

Is there enough water to go around? This is an important 

question that water managers address in attempting to balance 

the needs of all parties in the Klamath basin where agriculture 

and fisheries are important parts of the economy. Farmers need 

water to grow crops. Native salmon and trout need adequate 

flows of cool water for reproduction during critical life stages. 

Endangered suckers need river riparian areas and shoreline 

habitat for spawning and rearing. National Wildlife Refuges in 

the basin need water to provide habitat for water fowl migrating along the Pacific Flyway. 

In the early 2000s, developing drought conditions in the 

Klamath basin created water shortages that affected several 

Native American tribes, the farming community, fishermen, 

and several aquatic species. In 2001, irrigation withdrawals 

from the lake were curtailed for hundreds of farmers in order 

to maintain lake levels to protect endangered sucker habitat 

and provide instream flows for coho salmon. In late September 2002, tens of thousands of adult prespawned salmon and 

other fish died in reaches of the Klamath River 200 miles 

downstream from Upper Klamath Lake even though the BOR 

released the amount of flow called for in a “dry” hydrologic 

period in the National Oceanic and Atmospheric Administration’s (NOAA) Fisheries’ Biological Opinion. The California 

Department of Fish and Game (2003) attributed the deaths 

to low streamflow, high water temperatures, a large salmon 

Upper Klamath Lake, Oregon

Figure 45. Map showing location of Upper Klamath Lake. 

Upper Klamath National Wildlife Refuge, Oregon.

U.S. Fish and Wildlife Service

Klamath River.

 U.S. Fish and Wildlife Service

74 Water Budgets: Foundations for Effective Water-Resources and Environmental Management
return, and a proliferation of two naturally occurring pathogens. Lynch and Risley (2003) quantified the September 2002 

streamflow and water temperature conditions in the main-stem 

Klamath River leading up to this fish die-off. A persistent 

drought in the region and warmer than average weather conditions resulted in below-average streamflows in September 

2002 (with a 10-year recurrence interval) and warmer-thanaverage water temperatures (with a 5-year recurrence interval). 

An important step in determining if there is enough water 

to go around is to develop an understanding of the water budget of Upper Klamath Lake. The most complete water budget 

presently available for Upper Klamath Lake is that of Hubbard 

(1970). Hubbard measured or estimated all major lake inflows 

and outflows and changes in lake storage during a 3-year 

period (fig. 46). Hubbard’s study has certain limitations: it 

spanned only 3 years, and the quantitative understanding of the 

relation between stage and volume of the lake has since been 

improved. However, because many components of the lake 

water budget that Hubbard measured have not been measured 

subsequently on a routine basis, his water balance for Upper 

Klamath Lake remains the most comprehensive analysis of all 

the inflows to and outflows from the lake and provides a good 

example of the value of this type of study.

Measured surface-water inflow and surface-water outflow 

represented the largest gains and losses of water for the lake. 

Precipitation falling on the lake accounted for just 7.4 percent of total water inputs. Evapotranspiration, determined 

from pan-evaporation measurements for the lake and by the 

Blaney-Criddle method for marshes, represented 15.7 percent of total water loss from the lake. That evapotranspiration 

losses exceeded gains from precipitation is a reflection of the 

semiarid climate of the region. The volume of water stored 

in the lake was determined by lake-stage measurements and 

a pre-established stage-volume table (subsequently revised). 

Lake storage is largely controlled by dam operations and irrigation withdrawals by BOR and private canals. When viewed 

on a monthly basis, storage decreased during the growing 

season (because of releases for downstream flow requirements, irrigation, and wildlife refuges) and increased from late 

fall to early spring. On an annual basis, there was little change 

September

August

April

May

June

July

March

January

November

December

October

February

September

August

May

June

July

March

April

January

November

December

October

February

September

August

April

May

June

July

March

January

November

December

October

February

0

50,000

100,000

150,000

200,000

250,000

AVERAGE MONTHLY INFLOWS, 

IN ACRE-FEET

AVERAGE MONTHLY OUTFLOWS, 

IN ACRE-FEET

AVERAGE MONTHLY CHANGE IN STORAGE, 

IN ACRE-FEET

Average surface-water 

 inflow

Average springs and 

 seeps

Average precipitation

Average surface-water outflow

Average evapotranspiration

A

B

C

250,000

200,000

150,000

100,000

50,000

150,000

100,000

50,000

0

–50,000

–100,000

0

Figure 46. Average monthly water budget for Upper Klamath 

Lake for October 1964 through September 1967 (Hubbard, 1970): 

(A) Average monthly inflows, (B) average monthly outflows, and 

(C) average monthly change in storage. 

Irrigated fields in the Klamath River Basin.

Bureau of Reclamation

Lost River sucker.

Chinook salmon.

U.S. Fish and Wildlife Service

Water Budgets and Management of Hydrologic Systems 75
in storage from year to year, largely because the lake refills in 

most years. The lake storage and evapotranspiration estimates 

of Hubbard (1970) have been revised in recent years, but these 

revisions have not substantially altered the basic understanding 

of the relative proportions of various inflows to and outflows 

from Upper Klamath Lake put forth in that report, and it continues to serve as a useful resource for hydrologists working in 

the basin. 

The present knowledge of ground-water inflow to Upper 

Klamath Lake is largely based on Hubbard’s water budget. 

Ground-water inflow from springs and seeps could not be 

measured directly; it was estimated as the residual of the water 

budget equation. Because of this, estimates of ground-water 

inflow have the largest uncertainties of any of the water-budget components; they reflect the uncertainties in all the other 

estimates. Month-to-month variability in surface-water flows, 

precipitation, and evapotranspiration are expected because of 

seasonal weather patterns. Ground-water inflow, on the other 

hand, has relatively little seasonal variability but does vary in 

response to interannual climate cycles. Measurements show 

that the discharge of large spring complexes near Upper Klamath Lake vary by a factor of 1.5 to 2 in response to decadal 

climate cycles (M.W. Gannett, U.S. Geological Survey, written 

commun., 2007). Ground-water discharge directly to the lake 

likely varies in a similar manner. The average monthly groundwater inflow to the lake for the 36-month study period of Hubbard (1970) was about 21,000 acre-ft. 

The Bureau of Reclamation’s Annual Operations Plan 

for Upper Klamath Lake (http://www.usbr.gov/mp/kbao/

news/2006_Klamath_Project_Operations_Plan.pdf) is developed in accordance with the U.S. Fish and Wildlife Service 

and NOAA Fisheries Biological Opinions and a U.S. District 

Court ruling. The complex plan relies on predictions of waterbudget components, especially inflow to the lake for April 

through September. Forecasts of inflow are provided on a 

monthly basis by the Natural Resources Conservation Service, 

which uses current data on snowpack, precipitation, and other 

meteorological parameters across the drainage basin, streamflow data, and predictions of weather patterns. These inflow 

forecasts have large uncertainties early in the calendar year 

when Annual Operation Plans are being developed because 

of the difficulty in forecasting weather patterns accurately 

many months in advance. Constraints in managing the system 

include maintaining an adequate stage in the lake, ensuring 

a minimum flow in the Klamath River, delivering water to 

National Wildlife Refuges at historical rates, and providing 

farmers with sufficient quantities of irrigation water. Satisfying all these needs is a difficult proposition, especially during 

extended periods of below-normal precipitation because Upper 

Klamath Lake does not have multiyear carry-over storage. 

Accurate and up-to-date assessments of the water budget 

of the lake would provide valuable support for important 

water-management decisions on BOR’s Klamath Project, 

particularly if lake-level and downstream-flow requirements 

are to be better coordinated with current hydrologic conditions 

in the basin. 

U.S. Fish and Wildlife Service

Lower Klamath National Wildlife Refuge.

76 Water Budgets: Foundations for Effective Water-Resources and Environmental Management
Water for Humans and Ecosystems: San Pedro 

River Ecosystem

The San Pedro River is one of the last free-flowing rivers 

between Mexico and the United States. Flowing northward 

from the Mexican State of Sonora through southeastern Arizona to the Gila River (fig. 47), the river maintains one of the 

most ecologically diverse desert riparian ecosystems on Earth 

(The Nature Conservancy Web site: http://www.nature.org). 

The ecosystem is home to more than 100 species of mammals 

(20 species of bats), 40 species of reptiles and amphibians, and 

100 species of butterflies. More than 300 species of birds live 

in or migrate through the river corridor. The U.S. Congress 

recognized the importance of this ecosystem when it created 

the San Pedro Riparian National Conservation Area in 1988 

(fig. 47) to protect the area. 

Protecting the ecosystem means protecting surface water 

in the river and ground water in the riparian zone. The ecosystem will continue to thrive only through the maintenance of its 

water resources. Human demands for water within the basin 

are growing as population increases, posing a classic challenge: Can a limited water resource be properly managed to 

satisfy the needs of humans and the needs of the environment 

they treasure? To address this question, a detailed analysis of 

the ground-water budget for the basin was undertaken by the 

U.S. Department of the Interior. 

The San Pedro River is fed primarily by ground water. 

Late summer monsoon storms may produce short-term runoff, 

but the perennial flow in many stream reaches is supported 

by slow, long-term ground-water discharge. Recharge to the 

ground-water system derives principally from precipitation 

falling in the mountains that bound the basin on three sides; 

this is typical of watersheds in arid and semiarid regions of 

the United States. Average annual precipitation in the basin is 

about 16 inches (Pool and Coes, 1999) with rates higher in the 

mountains than on the basin floor. Recharge water takes years 

to travel from mountain fronts through the subsurface to the 

riparian zone that borders the river. Ground water in the riparian zone discharges to the river or is taken up by phreatophytic vegetation.

The Sierra Vista Subwatershed occupies the upper part 

of the U.S. portion of the river basin. Population in this area 

was about 68,000 in 2002 and is expected to grow to more 

than 83,000 by 2011 (U.S. Department of the Interior, 2005). 

The population is split between dispersed rural residences 

and urban centers such as Sierra Vista, Bisbee, and Tombstone. Ground water is the only source of water for domestic, 

industrial, and agricultural use. Development of ground water 

began in earnest sometime after 1940; by 2002, pumpage was 

estimated at about 16,500 acre-ft/yr. The ground-water budget 

for the subwatershed (U.S. Department of the Interior, 2005) 

is illustrated schematically in figure 48 and can be expressed 

as: 

Natural Recharge + Additional Recharge + 

Ground-Water Inflow 

=

Ground-Water Outflow + San Pedro River Base Flow + 

Evapotranspiration + Pumping + 

Change in Ground-Water Storage.

San Pedro River.

The San Pedro River flows northward from the Mexican 

State of Sonora into Arizona.

Water Budgets and Management of Hydrologic Systems 77
The Narrows

Benson

St. David

Tombstone

Sierra Vista

Nicksville

Charleston

SPRNCA

Huachuca City

Fairbank

Fort Huachuca

Bisbee

Hereford

Palominas

Coronado

National

Forest

Coronado

National

Forest

Coronado

National

Forest

Coronado

National

Forest

UNITED STATES

MEXICO

Base from U.S. Geological Survey

digital data, 1:100,000, 1982

Universal Transverse Mercator

projection, Zone 12

? COCHISE COUNTY

Elgin

PIMA COUNTY

SANTA CRUZ

COUNTY

110°45'

32°15'

32°00'

31°45'

31°30'

31°15'

31°00'

110°30' 110°15' 109°45'

EXPLANATION

U.S. Geological Survey

 streamflow-gaging station

Cananea

mine

Cananea 0 10 MILES

0 10 KILOMETERS

92

90

80

90

82

10

ARIZONA

SONORA CORONADO

NATIONAL

MONUMENT 

Tombstone

Hills

Mountains

Huachuca

Mule Mountains

Whetstone Mounta ni s

BENSON

SUBWATERSHED

SIERRA VISTA

SUBWATERSHED

Babocomari River San Pedro River

San Pedro River 

basin, Arizona

Figure 47. Location of the upper San Pedro River basin showing the San Pedro Riparian National Conservation Area (SPRNCA).

78 Water Budgets: Foundations for Effective Water-Resources and Environmental Management

Warbler.

White-breasted Nuthatch.
Additional recharge refers to the return of pumped water to 

the aquifer through drainage of irrigation water, septic tanks, 

and enhanced recharge from routing of runoff from impervious areas. That routing could be unintentional, as a result 

of increased impervious areas such as roads, buildings, and 

sidewalks. It could also be the result of planned urban infiltration galleries that funnel runoff directly to the ground-water 

system, thus bypassing the soil zone and avoiding uptake by 

vegetation. 

Table 8 shows values for components of the groundwater budget for a time before ground-water development 

(1940) and after a period of more than 60 years of development (2002). For the water budget to balance, the increase 

in pumping between 1940 and 2002 must be offset by one or 

more other water-budget components. Results of computer 

Bedrock Bedrock

Mountain-front

recharge

Evapotranspiration

Agricultural

use Ground-water

withdrawals

East West

Ground water

in from Mexico

Artificial recharge detention pond

Fort Huachuca

Streamflow-gaging Sierra Vista

station at Charleston

Mule Mountains

Mountains Huachuca

Ground-water underflow

out of Sierra Vista

subwatershed

Streamflow-gaging station near Tombstone

Base flow out

of Sierra Vista

subwatershed

Figure 48. Simulated annual ground-water budget for the upper San Pedro River basin (U.S. Department of the Interior, 2005).

Water Budgets and Management of Hydrologic Systems 79

simulations of ground-water flow indicated that by 2002 there 

was a 65 percent decrease in annual ground-water discharge 

(base flow) to the river, 8,400 acre-ft of water was removed 

from ground-water storage each year, and there was a slight 

reduction in evapotranspiration rates. Interestingly, recent 

estimates of evapotranspiration based on field measurements 

indicate that current rates (about 10,800 acre-ft/yr) are greater 

than those estimated for the past (Scott and others, 2006). It 

is not clear if past estimates, which were not based on field 

estimates, are in error or if, indeed, riparian evapotranspiration 

rates have increased. In either regard, the ground-water flow 

simulations indicated that continued pumping at current rates 

with no additional recharge will eventually dry up the river 

(U.S. Department of the Interior, 2005). Such a result would 

have severe implications for the ecosystem.

Table 8. Annual ground-water budget (in acre-feet) for Sierra Vista subwatershed [predevelopment conditions (1940) from Corell 

and others (1996) and in 2002 (U.S. Department of the Interior, 2005)]. Net pumping is actual pumping minus that amount of pumped 

water that returned to the aquifer.

Year Natural 

recharge

Ground-water 

inflow

Ground-water 

outflow Evapotranspiration San Pedro River base flow Net pumping Storage change

1940 16,000 3,000 440 8,020 9,540 1,000 0

2002 15,000 3,000 440 7,700 3,250 15,000 –8,400
U.S. Geological Survey stream gage.

Riparian vegetation along the river stands out in the desert environment.

The possibility of such an occurrence led to the formation of the Upper San Pedro Partnership (USPP), a group of 

governmental and private agencies charged with achieving 

sustainable yield within the basin (U.S. Department of the 

Interior, 2005). Sustainable yield is defined as “development 

and use of ground water in a manner that can be maintained 

for an indefinite time without causing unacceptable environmental, economic, or social consequences” (Alley and Leake, 

2004, p. 12). Of course, determining what is or is not acceptable is a subjective matter that may lead to contentious debate. 

Regardless, the water budget in table 8 provides a starting 

point for determining sustainable yield. To predict consequences in time and space of future development, results from 

a ground-water model will be interpreted in the context of 

various completed and ongoing studies of basin hydrogeology 

and riparian water needs. In order to ensure continued flow 

in the San Pedro River and health of the ecosystem, managers are implementing measures designed to conserve water, 

thereby reducing the population’s ground-water demand. At 

the same time, the USPP seeks to enhance additional recharge 

by encouraging large-scale artificial recharge. The success of 

these efforts depends largely on the accuracy of the groundwater budget. Continual refinement of the ground-water 

budget, as new data become available, is an important aspect 

of the management plan. 

80 Water Budgets: Foundations for Effective Water-Resources and Environmental Management
Urban Water Supply: Chicago

A prolific confined aquifer system underlies a large portion of northern Illinois and southern Wisconsin (fig. 49). The 

system, composed of sandstones and dolomites of CambrianOrdovician age, lies at depths of several hundred feet below 

the urban areas of Chicago and Milwaukee and has long 

provided drinking water for those areas. Under predevelopment conditions, recharge to the system occurred in areas to 

the north and west of the cities where aquifer rocks are close 

to land surface; ground water flowed to the east and southeast, slowly discharging to Lake Michigan and to land surface 

southeast of Chicago. 

The response of this aquifer system to development 

provides an illustration of the dynamic nature of water budgets 

and the hydrologic cycle. The first deep well in the Chicago 

area was drilled in 1864 to a depth of 711 ft; it flowed at land 

surface at the rate of about 150 gal/minute (Visocky, 1997). 

In the following decades, ground-water withdrawals from the 

aquifer system increased sharply, coincident with rising population (fig. 50). The increased pumping resulted in substantial 

declines in ground-water levels in the aquifer (fig. 51). Around 

1980, fears of depleting the system led public water suppliers in the area to shift their source of water to Lake Michigan. 

Ground-water withdrawals decreased in much of the area, and 

Cambrian-Ordovician

aquifer system

Figure 49. Approximate location map of CambrianOrdovician-age aquifer system in northern Illinois and 

southern Wisconsin. 

Water Budgets and Management of Hydrologic Systems 81

ground-water levels showed some recovery through the early 

1990s (fig. 52). Burch (1991) predicts that by 2010, water levels will have rebounded substantially in the Chicago area, with 

rises of more than 600 ft in areas near former pumping centers. 

How did changing development practices affect the 

natural water budget of the aquifer system? The large total 

withdrawals had multiple effects. Obviously, there was a great J. Crocker
reduction in water stored in the system. The total change in 

storage could be roughly estimated by using the declines 

shown in figure 51 if a representative value of storage coefficient were available. Development has reduced the natural 

rates of outflow to Lake Michigan and other discharge points. 

In some cases, falling water levels resulted in reversals in 

hydraulic gradients so that some natural outflow boundaries, such as some lakes near Madison, Wis., actually became 

inflow boundaries (Burch, 1991). In terms of inflow to the 

system, predevelopment rates of recharge were likely limited 

by the ability of the aquifer to accept more water; that is, the 

system was essentially full and could not accept available 

recharge. Falling water levels brought on by development have 

created additional storage capacity, so it is likely that current 

rates of recharge exceed those of the past. (Of course, as previously discussed, many factors, in particular land-use changes, 

influence the recharge process.) 

1900 1910 1920 1930 1940 1950 1960 1970 1980 1990 2000

YEAR

50

100

150

MILLIONS OF GALLONS PER DAY

Public

Industrial

Total (industrial + 

public water supply)

0

200

Figure 50. Ground-water withdrawals from the CambrianOrdovician aquifer system in the eight-county Chicago area, 

1900–94 (Visocky, 1997).

82 Water Budgets: Foundations for Effective Water-Resources and Environmental Management

Recharge to deep aquifers that served the Chicago and Milwaukee areas occurs to the north and west of these areas 

where aquifer rocks are close to land surface.
Milwaukee

OZAUKEE

FOND DU LAC SHEBOYGAN GREEN

LAKE

MARQUETTE

COLUMBIA WASHINGTON

MILWAUKEE

DODGE

KENOSHA

RACINE

WAUKESHA

WALWORTH

DANE

ROCK

BOONE

WINNEBAGO

OGLE

LEE

BUREAU

McHENRY

KANE

DE KALB

DU PAGE

LAKE

COOK

LAKE

PORTER

JASPER

KANKAKEE

WILL

KENDALL

PUTNAM

LA SALLE GRUNDY

IROQUOIS

NEWTON

WISCONSIN

ILLINOIS

INDIANA

ILLINOIS

GREEN STEPHENSON

JEFFERSON

Chicago

Rock

River

Des

Fox

River

River

River

Plaines Kankakee

0

Base from U.S. Geological Survey 1:2,000,000 Digital Data

Albers Equal-area Conic projection

Standard parallels 33° and 45°, central meridian –89°

25 50 KILOMETERS

25

41

42°

43°

89° 88° 87°

0 50 MILES

EXPLANATION

Line of equal water-level decline, 1864–1980—Dashed

 where approximate. Interval, in feet, is variable

Major ground-water divide

LAKE MICHIGAN

700

800

600

700

800

500

300

100

200

200

400

350

375

300

100

50

50

50

100

500

600

300

400

500

200

50

100

100

100

100

Figure 51. Ground-water level declines from 1864 to 1980 

in the Cambrian-Ordovician aquifer system, Chicago and 

Milwaukee areas (Alley and others, 1999).

Pump house in northern Illinois.

1940

–50

0

50

100

150

200

250

1950 1960 1970

YEAR

WATER LEVEL, IN FEET ABOVE SEA LEVEL

1980 1990 2000

Figure 52. Trends in ground-water levels in the deep 

Cambrian-Ordivician aquifer system. Data are from a well in 

Cook County near Chicago (after Visocky, 1997).

Outcrops of the St. Peter Sandstone near Monticello, 

Wisconsin, July 26, 1907.

Water Budgets and Management of Hydrologic Systems 83
Concluding Remarks

A water budget states that the rate of change in water 

stored in an accounting unit, such as a watershed, is balanced 

by the rate at which water flows into that unit minus the rate at 

which water flows out of it. Universally applicable, water budgets can be constructed at any spatial scale—an agricultural 

field, a wetland, an aquifer, a lake, a watershed, and even the 

Earth itself and at any temporal scale, from seconds to years 

to millennia. While theoretically simple, water budgets, in 

practice, are often difficult to determine. Inherent uncertainties 

pervade all techniques used to measure water storage and flux. 

In addition, the dynamic nature of the hydrologic cycle implies 

that storage and flux terms change over time. 

As the human population on Earth continues to grow, 

so will its demands for water. Balancing the water needs of 

humans with those of the many ecosystems on Earth will 

continue to be a challenge. Water budgets provide a means for 

evaluating the availability and sustainability of a water supply. 

The link among all components of a water budget serves as a 

basis for predicting how a natural or human-induced change 

to one component, such as ground-water extraction, may be 

reflected in other components, such as streamflow or evapotranspiration. When viewed with an understanding of the 

underlying hydrologic processes and the uncertainties associated with quantifying those processes, water budgets form a 

foundation for evaluating water-resources and environmental 

planning and management options. 

Science and technology can assist water-resources and 

environmental management by addressing important questions 

related to the hydrologic cycle, water use, water needs, and 

water availability and sustainability. These questions include:

How much water do humans use?

How much water do ecosystems need to flourish?

How much water is available for humans and ecosystems? Where is this water?

How does the hydrologic cycle naturally change over 

time?

In what ways do human activities affect the hydrologic 

cycle?

How will changes in the hydrologic cycle affect water 

availability and use?

What effects do uncertainties in estimates of water storage and movement have on our understanding of water 

budgets in general and of the availability and sustainability of water resources in particular?

Humans are part of the hydrologic cycle, ...

...as are all animals and plants.

U.S. Fish and Wildlife Service

Chemical and energy budgets of lakes, rivers, and 

aquifers are directly linked to water budgets.

U.S. Army Corps of Engineers

84 Water Budgets: Foundations for Effective Water-Resources and Environmental Management
Steps to developing answers to these questions include 

monitoring domestic, agricultural, and industrial water use; 

conducting inventories of ecosystems and their water needs; 

and undertaking surveys and assessments of water resources. 

Insight into the variability, both natural and that induced by 

human activity, of hydrologic cycles can be obtained with 

improved methods for studying the hydrologic cycle—more 

accurate instruments, new designs for field studies, alternative 

methods for interpreting remotely sensed data, new simulation 

models of water movement through various parts of the hydrologic cycle, and improved methods for predicting water use by 

humans, plants, and animals. Science and technology can also 

assist in the development and improvement of decision support 

systems that allow managers to evaluate various operational 

options. 

Accessible freshwater is a limited resource that humans 

must share among themselves and with the environment. 

Supplies of freshwater are not available everywhere. Hence, 

throughout history humans have constructed systems for conveying and storing water. The resource gained when humans 

import water to one location is accompanied by a loss of water 

at another location. In terms of water amounts, every gain is 

balanced by an equal loss. In terms of economic, cultural, and 

ecological values, the question of whether gains balance losses 

must be evaluated by society as a whole. Perhaps the greatest 

challenge to maintaining a sustainable future for our water 

resources is the development of policies and laws that balance 

the many water needs of humans with the water needs of their 

environment. Water budgets form a foundation upon which 

those policies and laws can be developed.

The hydrologic cycle varies naturally over time.

Holgate Glacier 1909 Holgate Glacier 2004 

Human activity affects the hydrologic cycle.

Natural Resources Conservation Service 

Even the most accurate measurements of water fluxes 

contain some uncertainty.

Concluding Remarks 85
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“It is this backward motion toward the source,

Against the stream, that most we see ourselves in,

The tribute of the current to the source.

It is from this in nature we are from.

It is most of us.”

Robert Frost, from West-Running Brook (1928)

90 Water Budgets: Foundations for Effective Water-Resources and Environmental Management

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