These regional SPARROW modeling assessments

•	Provide the most comprehensive studies to date of the sources, 

transport, and fate of nutrients in the streams of six major 

regions of the conterminous United States (U.S.), based 

on national compilations of water-resource geospatial data 

from federal, state, and local water agencies. Advances in 

scientific understanding and modeling tools made from these

assessments can be used by decision-makers to evaluate the 

sources and environmental factors that control stream nutrients 

at scales ranging from small catchments and counties to large 

watersheds, states, and regions.

•	Confirm the importance of urban and agricultural sources as

major contributors of nutrients to U.S. streams but provide new 

information about local and regional differences in nutrient 

contributions from contrasting types of agricultural (farm 

fertilizers and animal manure) and urban (wastewater effluent

and diffuse runoff from developed land) sources. 

Regional Assessments of the Nation’s Water Quality—

Improved Understanding of Stream Nutrient Sources Through 

Enhanced Modeling Capabilities

The U.S. Geological Survey (USGS) recently completed assessments of stream nutrients in six major regions extending 

over much of the conterminous United States. SPARROW (SPAtially Referenced Regressions On Watershed attributes) models 

were developed for each region to explain spatial patterns in monitored stream nutrient loads in relation to human activities 

and natural resources and processes. The model information, reported by stream reach and catchment, provides contrasting 

views of the spatial patterns of nutrient source contributions, including those from urban (wastewater effluent and diffuse 

runoff from developed land), agricultural (farm fertilizers and animal manure), and specific background sources (atmospheric 

nitrogen deposition, soil phosphorus, forest nitrogen fixation, and channel erosion).

•	Indicate that highly diffuse background sources of nutrients 

that may be natural or anthropogenic in origin are the 

dominant sources of nutrients to many U.S. streams 

where other large sources are not present. This adds 

new information that has been difficult to quantify in

local water-quality assessments yet is needed to develop

comprehensive and realistic nutrient-management goals.

•	Describe the sources and quantities of nutrients that are

delivered to downstream sensitive water bodies, such as 

reservoirs and estuaries, where nutrient inputs can contribute 

to excessive algal growth and degraded water-quality

conditions. These results account for the effects of natural 

factors (such as climate, soils, and hydrology) and humanrelated factors (such as farm tile drainage) that influence the

attenuation and transport of nutrients at local scales in small 

drainages as well as at regional scales in large watersheds.

Forest

Atmospheric

Cropland

Manure

Point source

Developed land

Largest sources of 

 nitrogen to streams

Major river

 basin boundary

Largest sources of 

 phosphorus to streams

Background

Cropland

Manure

Point source

Developed land

Major river

 basin boundary

>200

<50

Total phosphorus yields—

In kilograms per square

 kilometers per year

50 to 100

100 to 150

150 to 200

EXPLANATION

Total nitrogen yields—

EXPLANATION

In kilograms per square

 kilometers per year

>2,000

<500

500 to 1,000

1,000 to 1,500

1,500 to 2,000

Figure 1. Estimates developed from separate regional models for six large regions of the conterminous United States illustrate the spatial distribution 

of total nitrogen and total phosphorus yield from individual stream-reach drainages and the largest contributing source to each stream reach drainage. 

Fact Sheet 2011–3114

October 2011

U.S. Department of the Interior

U.S. Geological Survey Printed on recycled paper
2

Urban and Agricultural Sources 

of Nutrients to Streams

On a regional basis, urban sources 

(wastewater effluent, diffuse runoff from

developed land) contribute from 12 to 

48 percent of the nitrogen to streams and 

from 19 to 63 percent of the phosphorus; 

the largest contributions occur in the 

Northeast and Mid-Atlantic regions. 

Wastewater effluent contributes the largest

share of nutrient mass to urban streams 

(more than 69 percent) and can compose 

a relatively large percentage of the total 

load to downstream receiving waters. In 

some areas, nutrients from wastewatereffluent discharges are transported over

hundreds of kilometers in streams and are 

the largest contributor among all sources 

to distant downstream water bodies. Urban 

diffuse runoff contributes less overall 

mass of nutrients, but given its highly 

diffuse nature, it affects a larger number 

of urban streams and is the larger local 

source to 43 to 98 percent of urban streams, 

depending on the region. 

Agricultural sources (farm fertilizers, 

animal manure) contribute from 23 to 

70 percent of the nitrogen and 13 to 

59 percent of the phosphorus to streams; 

the largest contributions occur in the 

Midwest and central regions of the Nation. 

Over broad areas of these regions and in 

selected areas of the East, animal manure 

contributes larger quantities of nitrogen

and phosphorus compared to farm fertilizer 

(fig. 1). In most other areas, farm fertilizers

(and the associated runoff from cultivated 

lands) generally contribute more nitrogen 

to streams and nearly equal quantities of

phosphorus compared to animal manure. 

Background Sources of 

Nutrients to Streams

In all drainages, some of the nutrient 

mass transported to streams is due to highly 

diffuse or even ubiquitous sources that

could be natural or anthropogenic in 

origin. For example, nitrogen fixation or

plant degradation are naturally occurring 

processes that provide some nitrogen to 

streams broadly across the landscape. Soil 

erosion occurring across an entire region 

can add phosphorus to streams that could 

be naturally occurring or that could have 

originated from human activities. Lastly 

atmospheric deposition is a widespread 

source of nitrogen that is highly diffuse 

in nature but is entirely anthropogenic in 

origin. In general, we refer to these highly 

diffuse sources as “background” simply 

because they tend to occur ubiquitously

throughout the landscape and are difficult to

quantify without a model like SPARROW.

Further, these diffuse sources are important 

because they can be the largest sources in 

some drainages, and accounting for them 

is critical for establishing water-quality

management goals that are both appropriate 

and feasible. The SPARROW model 

results described herein provide unique

information for identifying and quantifying

background sources for such water-quality

management goals.

Natural background sources are the 

largest contributors of nutrients, among 

all sources, in many streams in the Pacific

Northwest, the Northeast and mid-Atlantic, 

and the Southeast where other major 

nutrient sources do not occur. In the Pacific

Northwest, forests contribute 51 percent 

of the nitrogen to streams and represent 

the largest source to 70 percent of the 

stream miles. Forests (especially red 

alder stands that foster nitrogen fixation)

contribute as much as 97 percent of the 

nitrogen to streams of the western Cascade 

Mountains. Forest land also was identified

as a source of phosphorus in a number of 

the regional models. However, forest land 

generally serves as a surrogate indicator 

of sources that are assumed to be from the 

erosion of soil or geologic material, or from 

plant degradation that may be associated 

with forests.

Background sources of phosphorus 

identified by the models include streamchannel erosion, geologic properties (soils), 

and forested lands. Stream-channel erosion 

is an especially important phosphorus 

source in the Missouri River basin, where it 

contributed 23 percent to stream reaches on 

average and as much as 97 percent to some 

streams. Phosphorus in eroded sediments 

may originate from both natural sources 

(such as soils) and human sources (such as 

wastewater-treatment effluent). Soils are a

major natural phosphorus source to streams 

of the Southeast, contributing 41 percent on 

average and more than 73 percent to some 

streams. Forests are important natural 

background sources in three regions 

including the Northeast and upper and 

lower Midwest.

Atmospheric deposition from local 

and distant anthropogenic sources is the 

largest contributor of nitrogen to many 

streams in the south-central, mid-Atlantic, 

and northeastern U.S. Across the six regions 

atmospheric deposition contributes from 5 to 

46 percent of the total mass of nitrogen to 

streams. The largest contributions occur to 

streams of the Southeast, where atmospheric 

deposition contributes up to 46 percent of 

the nitrogen and is the largest source to 

78 percent of the stream miles. Although 

atmospheric deposition contributes less than 

20 percent of the nitrogen to the streams 

of the Northeast, it is the largest source to 

nearly 50 percent of the stream miles in this 

region. Atmospheric sources in the models 

primarily reflect nitrogen deposition from

distant stationary emission sources, such 

as power plants, but also include minor 

contributions from local ammonia emissions 

associated with farm fertilizers and 

livestock and nitrogen from urban sources 

associated with automotive exhaust.

Environmental Processes Affecting 

Nutrient Delivery to Streams and 

Downstream Waters

Natural environmental processes and 

some human activities on the landscape 

can affect the delivery rates of nutrients 

to streams either by enhancing delivery 

or by reducing it through attenuation 

processes. Knowledge of the effects of 

these environmental factors can be used to 

identify high-priority watersheds with low 

levels of natural attenuation where nutrientsource reductions generally would be 

0 to 25

25 to 50

50 to 75

75 to 100

Incremental yield—

In percentage due to 

geologic material

Percentage of Phosphorus Yield Due to 

Naturally Occurring Geologic Material

Figure 2. A substantial 

amount of the phosphorus 

transported to streams 

in some areas of the 

Southeast is derived from naturally 

occurring geologic sources. Understanding 

the distribution of natural and human 

sources of phosphorus transported to 

streams is critical in developing realistic 

nutrient-reduction strategies.

ATLANTIC OCEAN

GULF OF MEXICO
3

remove nutrients before they can reach 

downstream waters.

Large differences in nutrient yields 

from west to east across the U.S. (fig. 1) are

explained in part by variations in climatic 

factors (precipitation and temperature). 

These are fundamental measures of water 

and energy availability that control the rates 

of nutrient movement and loss on the land 

and in streams and reservoirs. For example, 

nutrient yields are among the lowest 

throughout much of the West because of 

the prevailing arid conditions that limit 

nutrient runoff and delivery to streams 

and downstream waters. This occurs 

despite a diversity of important sources, 

expected to have greater downstream 

benefits—that is, the most efficient

response per unit of upstream reduction. 

Conversely, watersheds with high natural 

attenuation of nutrients could be given 

lower priority if the goal was to improve 

downstream water quality.

In most areas of the country, only 

a small fraction of the nutrient mass 

that is input to watersheds on an annual 

basis is actually delivered to streams 

and downstream water bodies. This 

is because nitrogen and phosphorus 

commonly are attenuated during transport 

as a result of the effects of many physical 

and biochemical processes. These may 

include nutrient uptake by plants, adsorption to soil particles, storage in the 

subsurface from leaching to groundwater, 

and denitrification (the conversion of

reactive nitrogen to an inert gaseous 

form that is released to the atmosphere). 

Human activities also can affect the rates 

of nutrient attenuation. For example, 

large quantities of nutrients on cultivated

lands (from farm fertilizers or natural 

nitrogen fixation) are assimilated in

crops and exported from watersheds in 

harvested grains and plant residues. In 

addition, natural processes in streams 

and reservoirs (such as assimilation, 

settling and storage of and denitrification)

In kilograms per square kilometers per year

Incremental yield—

<240

240 to 320

320 to 400

400 to 580

>580

0 to 30

30 to 60

60 to 90

>120

90 to 120

In kilograms per square kilometers per year

Incremental yield—

Water-resource managers and policy makers have relied on hydrologic and 

chemical models to estimate current water-quality conditions and predict how

conditions might change in response to a selected policy decision. It is often 

difficult, however, for decision makers to readily access model information and

use models directly to evaluate a range of alternative scenarios. The SPARROW 

Decision Support System provides easy access to SPARROW model results. 

By making this capability available over the Internet in a user interface with 

familiar controls, modelers and water-resource managers alike can experiment 

with hypothetical scenarios and develop science-based estimates regarding 

the effects that specific contaminant sources or changes may have on water

quality. These estimates can then be easily communicated to stakeholders

and the general public through the same website. Equally important, the

Decision Support System provides estimates of model uncertainty to inform 

managers about the range of variability in model predictions of stream loads 

that can be attributed to uncertainties about how well the models describe 

actual water-quality conditions and the factors that influence these conditions.

Comprehensive information on how the SPARROW models were 

developed, data sets used to calibrate the models, and instructional videos are 

available on the Decision Support System. Users also can save user-defined

management scenarios and share the session with colleagues. Options also are 

available to export model output 

data from selected areas for use in 

other applications.

The Decision Support System 

can produce a variety of maps 

to show the spatial variability in 

nutrient yields and the sources that 

contribute the largest amounts of 

nutrients throughout a large region, 

such as the southeastern U.S.

SPARROW Decision Support System—Informing Nutrient Management Decisions

Fertilizer

Point sources

Total yield

The SPARROW Decision 

Support System can be 

accessed at http://water.usgs.

gov/nawqa/sparrow/dss.

Urban areas
4

–25 to –20

–20 to –15

–15 to –10

–10 to –5

–5 to 0

Change in total 

delivered load 

of nitrogen,

in percentage,

to Apalachicola Bay

> 420

0 to 75

75 to 150

150 to 300

300 to 420

In kilograms per square 

kilometers per year

Incremental

delivered yield—

> 140

0 to 25

25 to 50

50 to 100

100 to 140

In kilograms per square kilometers per year

Incremental delivered yield—

ATLANTIC

OCEAN

NC

FL

GA

AL

FL

GA

AL

TN

SC

GULF OF MEXICO

Atlanta

Apalachicola

River basin

Over past decades, human activities have accelerated nutrient inputs to U.S. estuaries, thus 

contributing to excessive algal growth and degraded conditions in the estuaries. About two-thirds of 

the 138 U.S. estuaries have moderate to high levels of eutrophication, with about one-half identified

as having impaired human or ecological uses that are associated with eutrophic conditions (Bricker 

and others, 1999). Managers can use the Decision Support System to identify the watersheds and 

sources that contribute the largest amounts of nutrients that are delivered to nutrient-sensitive estuaries. 

These capabilities are illustrated for the Apalachicola Bay where agriculture (fertilizer applications to 

cultivated lands) and atmospheric deposition account for about two-thirds of the total nitrogen load 

entering the bay, with wastewater effluent contributing nearly 20 percent of the total load. Additional

displays from the system can be used to identify inland watersheds that deliver the highest amounts 

of nitrogen to the bay and may, therefore, be useful for ranking management actions. Watersheds 

contributing the highest amounts 

to the bay are near Atlanta (from 

point sources) and in the southern 

parts of Georgia and Alabama (from 

the atmosphere and fertilizer). 

Reductions in the stream-nutrient 

loads leaving the watersheds with 

the highest contributions generally 

would be expected to lead to a 

more efficient reduction in loads

entering the bay—that is the

reduction in load to the bay per unit 

of load removed upstream would 

be expected to be larger for these 

watersheds compared to the reduced 

loads in watersheds with relatively 

low delivered yields. A notable 

management challenge faced in 

the Apalachicola Bay, as in other 

southeastern coastal waters where 

atmospheric deposition is a large 

source, is that atmospheric nitrogen 

frequently originates from distant

sources (such as power plants) 

located outside of the watershed.

The Decision Support System 

can be used to evaluate nutrientreduction scenarios. The model 

estimates that a 25-percent reduction 

in point, atmospheric, or fertilizer 

sources individually results in a 

less than 10-percent reduction 

in the nitrogen load delivered to 

Apalachicola Bay. Reducing all 

three sources by 25 percent would 

reduce the nitrogen load delivered 

to the bay by 21 percent.

Understanding the sources 

and which watersheds contribute 

the largest amounts of nutrients to 

nutrient-sensitive waters can assist 

managers in the development of 

nutrient-management strategies.

Nitrogen Delivery to Estuaries 

Nitrogen Yields Delivered from Upland Watersheds to Apalachicola Bay

Three Nitrogen Reduction Scenarios 

25-Percent Reduction in Nitrogen Inputs

Fertilizer

Fertilizer

Atmosphere

Atmosphere

5 percent 9 percent 7 percent

Point sources

Total yield Point sources

Reduction in nitrogen load delivered to Apalachicola Bay

Apalachicola

Bay

Apalachicola

Bay
5

ranging from agriculture and atmospheric 

deposition in drier areas of the Midwest to 

forest and other background sources in the 

Pacific Northwest. By contrast, nutrient

yields are generally higher in the more 

humid watersheds of the Midwest and East. 

Figure 3 illustrates precipitation patterns 

and phosphorus yields in the Missouri River 

basin where the highest delivered yields to 

the mouth of the river occur in the lower 

end where precipitation rates are higher. 

Nutrients generated in the upper portions 

of the drainage tend to have low delivery 

rates due in part to lower precipitation and 

other environmental characteristics that 

may affect delivery to streams or limit the 

amount transported downstream.

Soil and topographic properties 

(soil permeability and depth, erodibility, 

acidity, organic matter, and land slope) 

were found to be important factors 

affecting nutrient delivery to streams 

in many of the regional models. Soil 

permeability was important in three of the 

six regions and was inversely related to 

nutrient loads in streams; the observance 

of lower nutrient loads in streams of 

watersheds with higher soil permeability may reflect the effects of greater

infiltration of water and nutrients in these

watersheds with subsequent storage and

(or) denitrification-related losses in soils

and groundwater. Farm-management 

activities also can affect soil drainage and 

nutrient delivery to streams. For example, 

the nitrogen transport to streams in the 

Midwest and the Missouri River basin 

was appreciably affected by the use of 

irrigation and tile drains. Stream nitrogen 

loads entering the Mississippi River from 

the Missouri River basin were estimated 

to be reduced by as much as 17 percent 

as a result of increased anoxia and 

denitrification on irrigated farm lands.

By contrast, nitrogen loads in streams of 

the upper Midwest region were found to 

increase with the percentage of land area 

drained by tile-drainage systems which are 

extensively used in the Midwest to support 

corn and soybean cultivation. 

Nutrient attenuation processes 

in streams and reservoirs, caused by 

denitrification and the nutrient storage

in particulates, reduce the quantities of

nutrients delivered to local and distant 

downstream waters in many of the 

regions. Nutrient removal in reservoirs 

was identified as important in five of the

six regions. Approximately 16 percent of 

the nitrogen load and 33 percent of the 

phosphorus load in the Missouri River 

were removed with nearly half of the total 

removal occurring in the eight largest 

reservoirs. The rates of nitrogen removal 

in streams (expressed as a fraction of the 

nutrient mass removed per unit of water 

travel time) declined with increases in 

stream depth (stream size) in many of 

the regions, thereby indicating that the 

largest rates of in-stream nutrient removal 

generally occur in the smallest streams. 

This relation reflects the reduced capability

of streams to remove nitrogen as water 

volumes increase with stream size, leading 

to less particulate settling (nitrogen and 

phosphorus storage) and less water and 

nitrogen contact with benthic sediments 

(where denitrification occurs).

Development of Regional 

SPARROW Models for the 

Conterminous United States

The USGS developed the spatial 

water-quality model SPARROW to assist

with the interpretation of available waterresource data and provide predictions of 

water quality in unmonitored streams.

The modeling framework was developed 

originally for and applied at the national 

scale, but models have since been 

developed for specific regions of the

U.S. and outside of the U.S. for specific

countries. SPARROW is a hybrid empirical 

and process-based mass-balance model 

that has been used previously in major U.S. 

river basins to estimate the primary sources 

and environmental factors that affect the 

supply, transport, and fate of contaminants 

in streams for mean annual conditions 

(see http://water.usgs.gov/nawqa/sparrow/ 

for a more detailed description).

Recently, the USGS developed new 

SPARROW models to assess nutrient 

conditions in streams of six large regions 

of the conterminous U.S. (Major River 

Basins; fig. 4). The documentation for

these models and the results and example 

nutrient-management applications were 

published as part of a featured collection 

in the Journal of the American Water 

Resources Association (Preston and others, 

2011). These regional models provide an 

updated assessment of nutrients in most 

streams of the conterminous U.S. based on 

a more current and refined set of national

geospatial data. Major advances were 

made in expanding the number of sites 

Precipitation—In millimeters

High: 2,875

Low: 144

Yield—In kilograms per square 

kilometers per year

<5

5 to 20

>20 to 50

>50 to 200

>200 to 1,000

>1,000 to 2,000

>2,000

Total phosphorus yields

from watersheds Precipitation

Missouri 

River

basin

Mississippi R

EXPLANATION

EXPLANATION

Minnesota

Iowa

South Dakota

North Dakota

Wyoming

Montana

Nebraska

Kansas Missouri

Colorado

Missouri R

Figure 3. The amount of phosphorus transported to streams in the Missouri River basin is related to climatic conditions. The highest 

phosphorus yields occur in the lower part of the basin in part because greater rainfall allows more agriculture and in part because 

greater rainfall increases the transport of phosphorus to streams. 
6

with stream loads derived from monitoring 

information, developing a data base 

for describing effluent discharges from

industrial and municipal wastewatertreatment facilities, and developing data 

sets for describing background nutrient 

sources. All of these data were used with 

other national geospatial data (for example 

farm fertilizer, animal manure, land-use, 

climate, soils, topography, stream-network 

properties) to calibrate the SPARROW 

models to provide accurate accounting 

of nutrient sources and environmental 

processes for reliable predictions of nutrient 

loads in unmonitored streams.

Stream water-quality monitoring

data were obtained from approximately 

125,000 sites operated by 186 governmental 

agencies across the country through 

retrievals from public data bases and 

contacts with agency personnel. The data 

were screened to ensure that the number 

and length of the water-quality and

streamflow records were sufficient for

estimating long-term loads. This resulted in 

the selection of approximately 2,700 sites 

(from 73 monitoring agencies) with 

sufficient data to calibrate the models. Note

that one important reason for the sizeable 

loss of water-quality data from the screening

process reflects the common practice of

collecting water-quality samples from

stream locations without a nearby water 

flow gage; measures of streamflow are

necessary to calculate a reliable measure 

of the nutrient mass (load) as required for

source-transport models such as SPARROW. 

The estimates of long-term nutrient 

loads at monitoring sites used to calibrate 

the regional model reflect water-quality

conditions for 2002, and also reflect longterm mean streamflows over the 1975 to

2005 period. The use of the long-term mean 

streamflows to estimate the monitoring

site loads, rather than the flows during

only 2002, ensures that the SPARROW 

regional model estimates of stream nutrient 

load, source contribution to streams, and 

environmental processes that govern 

the mean rates of nutrient removal and 

transport in watersheds are representative 

of long-term hydrologic variability.

Estimates of the nutrient loads in 

effluent discharges from 23,481 industrial

and municipal wastewater-treatment 

facilities were based on data retrievals 

from the U.S. Environmental Protection 

Agency (USEPA) Permit Compliance 

System. The loads reflect the application

of systematic procedures by USGS to 

evaluate and enhance the quality of the

data for estimating loads. The procedures 

included the validation of facility locations, 

correction of erroneous data, and the use 

of “typical” effluent concentration values

By Stephen D. Preston, Richard B. Alexander, and Michael D. Woodside

References

Bricker, S.B., Clement, C.G., Pirhalla, D.E, 

Orlando, S.P., and Farrow, D.R.G., 

1999, National estuarine eutrophication 

assessment: Effects of nutrient 

enrichment in the Nation’s estuaries: 

Silver Springs, MD, National Oceanic 

and Atmospheric Administration, 

National Ocean Service, Special Projects 

Office and National Centers for Coastal

Ocean Science, 71 p.

to represent missing or unreported data. 

The resulting data base provides the 

discharge estimates and point locations 

for nearly all of the major facilities 

that contribute the large majority of 

the point-source nutrient loads to streams.

Preston, S.D., Alexander, R.B., and 

Wolock, D.M., 2011, SPARROW 

modeling to understand water-quality

conditions in major regions of the United 

States: A featured collection introduction: Journal of the American Water 

Resources Association (JAWRA), 4 p., 

available online at http://onlinelibrary.

wiley.com/doi/10.1111/j.1752-1688.2011.

00585.x/pdf.

Major river 

 basin boundary

Monitoring site

EXPLANATION

New England

and 

Mid-Atlantic

South Atlantic–Gulf 

 and Tennessee

Great Lakes, Ohio, Upper Mississippi, 

and Souris–Red–Rainy Missouri

Lower Mississippi,

Arkansas–White–Red,

and Texas–Gulf

Rio Grand, Colorado,

 and Great Basin

Pacific Northwest

California

Figure 4. Stream water-quality data obtained from 73 local, state, and federal agencies 

at 2,700 monitoring sites were used to calibrate the six regional nutrient SPARROW 

models. A model of dissolved solids was developed for the Rio Grande, Colorado, and 

Great Basin region, where stream salinity is a major water-quality concern, and nutrient 

models are currently under development for the California region.

For more information contact:

Stephen D. Preston, spreston@usgs.gov. The SPARROW model Decision Support 

System and articles describing the new regional SPARROW models and input data 

sets can be accessed at http://water.usgs.gov/nawqa/sparrow/mrb.

Major River Basins