Agriculture and urban activities are major sources of phosphorus and nitrogen to aquatic ecosystems. Atmospheric deposition further contributes as a source of N. These nonpoint inputs of nutrients are difficult to measure and regulate because they derive from activities dispersed over wide areas of land and are variable in time due to effects of weather. In aquatic ecosystems, these nutrients cause diverse problems such as toxic algal blooms, loss of oxygen, fish kills, loss of biodiversity (including species important for commerce and recreation), loss of aquatic plant beds and coral reefs, and other problems. Nutrient enrichment seriously degrades aquatic ecosystems and impairs the use of water for drinking, industry, agriculture, recreation, and other purposes.

Based on our review of the scientific literature, we are certain that (1) eutrophication is a widespread problem in rivers, lakes, estuaries, and coastal oceans, caused by overenrichment with P and N; (2) nonpoint pollution, a major source of P and N to surface waters of the United States, results primarily from agriculture and urban activity, including industry; (3) inputs of P and N to agriculture in the form of fertilizers exceed outputs in produce in the United States and many other nations; (4) nutrient flows to aquatic ecosystems are directly related to animal stocking densities, and under high livestock densities, manure production exceeds the needs of crops to which the manure is applied; (5) excess fertilization and manure production cause a P surplus to accumulate in soil, some of which is transported to aquatic ecosystems; and (6) excess fertilization and manure production on agricultural lands create surplus N, which is mobile in many soils and often leaches to downstream aquatic ecosystems, and which can also volatilize to the atmosphere, redepositing elsewhere and eventually reaching aquatic ecosystems.

If current practices continue, nonpoint pollution of surface waters is virtually certain to increase in the future. Such an outcome is not inevitable, however, because a number of technologies, land use practices, and conservation measures are capable of decreasing the flow of nonpoint P and N into surface waters.

From our review of the available scientific information, we are confident that: (1) nonpoint pollution of surface waters with P and N could be reduced by reducing surplus nutrient flows in agricultural systems and processes, reducing agricultural and urban runoff by diverse methods, and reducing N emissions from fossil fuel burning; and (2) eutrophication can be reversed by decreasing input rates of P and N to aquatic ecosystems, but rates of recovery are highly variable among water bodies. Often, the eutrophic state is persistent, and recovery is slow.

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Nonpoint pollution of surface waters with phosphorus and nitrogen

Riparian zones possess an unusually diverse array of species and environmental processes. The ecological diversity is related to variable flood regimes, geographically unique channel processes, altitudinal climate shifts, and upland influences on the fluvial corridor. The resulting dynamic environment supports a variety of life-history strategies, biogeochemical cycles and rates, and organisms adapted to disturbance regimes over broad spatial and temporal scales. Innovations in riparian zone management have been effective in ameliorating many ecological issues related to land use and environmental quality. Riparian zones play essential roles in water and landscape planning, in restoration of aquatic systems, and in catalyzing institutional and societal cooperation for these efforts.

The Ecology of Interfaces: Riparian Zones

The Soil and Water Assessment Tool (SWAT) model is a continuation of nearly 30 years of modeling efforts conducted by the USDA Agricultural Research Service (ARS). SWAT has gained international acceptance as a robust interdisciplinary watershed modeling tool as evidenced by international SWAT conferences, hundreds of SWAT-related papers presented at numerous other scientific meetings, and dozens of articles published in peer-reviewed journals. The model has also been adopted as part of the U.S. Environmental Protection Agency (USEPA) Better Assessment Science Integrating Point and Nonpoint Sources (BASINS) software package and is being used by many U.S. federal and state agencies, including the USDA within the Conservation Effects Assessment Project (CEAP). At present, over 250 peer-reviewed published articles have been identified that report SWAT applications, reviews of SWAT components, or other research that includes SWAT. Many of these peer-reviewed articles are summarized here according to relevant application categories such as streamflow calibration and related hydrologic analyses, climate change impacts on hydrology, pollutant load assessments, comparisons with other models, and sensitivity analyses and calibration techniques. Strengths and weaknesses of the model are presented, and recommended research needs for SWAT are also provided.

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The Soil and Water Assessment Tool: Historical Development, Applications, and Future Research Directions

The Soil and Water Assessment Tool (SWAT) model is a continuation of nearly 30 years of modeling efforts conducted by the U.S. Department of Agriculture (USDA), Agricultural Research Service. SWAT has gained international acceptance as a robust interdisciplinary watershed modeling tool, as evidenced by international SWAT conferences, hundreds of SWAT-related papers presented at numerous scientific meetings, and dozens of articles published in peer-reviewed journals. The model has also been adopted as part of the U.S. Environmental Protection Agency's BASINS (Better Assessment Science Integrating Point & Nonpoint Sources) software package and is being used by many U.S. federal and state agencies, including the USDA within the Conservation Effects Assessment Project. At present, over 250 peer-reviewed, published articles have been identified that report SWAT applications, reviews of SWAT components, or other research that includes SWAT. Many of these peer-reviewed articles are summarized here according to relevant application categories such as streamflow calibration and related hydrologic analyses, climate change impacts on hydrology, pollutant load assessments, comparisons with other models, and sensitivity analyses and calibration techniques. Strengths and weaknesses of the model are presented, and recommended research needs for SWAT are provided.

Soil and Water Assessment Tool: Historical Development, Applications, and Future Research Directions, The

SWAT (Soil and Water Assessment Tool) is a comprehensive, semi-distributed river basin model that requires a large number of input parameters, which complicates model parameterization and calibration. Several calibration techniques have been developed for SWAT, including manual calibration procedures and automated procedures using the shuffled complex evolution method and other common methods. In addition, SWAT-CUP was recently developed and provides a decision-making framework that incorporates a semi-automated approach (SUFI2) using both manual and automated calibration and incorporating sensitivity and uncertainty analysis. In SWAT-CUP, users can manually adjust parameters and ranges iteratively between autocalibration runs. Parameter sensitivity analysis helps focus the calibration and uncertainty analysis and is used to provide statistics for goodness-of-fit. The user interaction or manual component of the SWAT-CUP calibration forces the user to obtain a better understanding of the overall hydrologic processes (e.g., baseflow ratios, ET, sediment sources and sinks, crop yields, and nutrient balances) and of parameter sensitivity. It is important for future calibration developments to spatially account for hydrologic processes; improve model run time efficiency; include the impact of uncertainty in the conceptual model, model parameters, and measured variables used in calibration; and assist users in checking for model errors. When calibrating a physically based model like SWAT, it is important to remember that all model input parameters must be kept within a realistic uncertainty range and that no automatic procedure can substitute for actual physical knowledge of the watershed.

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SWAT: Model Use, Calibration, and Validation

RIPARIAN ecosystem—big words and scientific jargon for the land farmers usually call bottomland or “that wet area down by the creek.” In the past decade or two, scientists have become interested in these wet areas. There have been many studies to see how streamside areas work—what animals and plants live there, how the areas affect streamflow, how they capture and retain nutrients and agricultural chemicals, and how they fit into a well-managed agricultural land-scape.

Some definitions

“Riparian” comes from the Latin rip meaning bank (of a stream). The first scientific use of the word may have been in 1758 when Linnaeus named the bank swallow Riparia riparia because it nests in stream banks ( 11 ). In the eastern United States (east of the 100th meridian) state surface water law generally is based on riparian rights. Under this legal system owners of land adjacent to water courses have the right to reasonable use of the flow of water, undiminished in quantity or quality.

“Ecology” is the study of interactions among organisms and their environment. The “ecosystem” is the basic functional unit in the science of ecology; it includes both the …

Managing riparian ecosystems to control nonpoint pollution

Sediment deposition from 1880 to 1979 was estimated for the riparian zone of a coastal plain agricultural watershed. Two approaches were used: (1) deposition estimates based on changes in depth to the argillic horizon along transects from fields to streams and (2) calculations of mass of deposition derived from estimated 100-year upland erosion based on the universal soil loss equation and a sediment delivery ratio. Estimates of changes in depth to the argillic horizon along nine transects yielded a mean of 52 Mg·ha·−1yr−1, with a range 7.6 to 92 Mg·ha·−1yr−1. The estimated average annual rate of gross erosion minus sediment transport from the watershed was 35 Mg·ha·−1yr−1. Thus, the average annual rate of sediment deposition on this watershed during the 1880-1979 period was 35 to 52 Mg·ha·−1yr−1. These data suggest that riparian ecosystems arc important sinks for sediments.

Long-term sediment deposition in the riparian zone of a coastal plain watershed

Riparian buffer zones are effective in mitigating nonpoint source pollution and have been recommended as a best management practice (BMP). The Riparian Ecosystem Management Model (REMM) has been developed for researchers and natural resource agencies as a modeling tool that can help quantify the water quality benefits of riparian buffers under varying site conditions. Processes simulated in REMM include surface and subsurface hydrology; sediment transport and deposition; carbon, nitrogen, and phosphorus transport, removal, and cycling; and vegetation growth. Management options, such as vegetation type, size of the buffer zone, and biomass harvesting also can be simulated. REMM can be used in conjunction with upland models, empirical data, or estimated loadings to examine scenarios of buffer zone design for a hillslope. Evaluation of REMM simulations with field observations shows generally good agreement between simulated and observed data for groundwater nitrate concentrations and water table depths in a mature riparian forest buffer. Sensitivity analysis showed that changes that influenced the water balance or soil moisture storage affected the streamflow output. Parameter changes that influence either hydrology or rates of nutrient cycling affected total N transport and plant N uptake.

REMM: The Riparian Ecosystem Management Model

The Better Assessment Science Integrating point and Nonpoint Sources (BASINS) system was developed by the
U.S. Environmental Protection Agency to facilitate developing total maximum daily loads (TMDLs). The Soil Water Assessment
Tool (SWAT) is one of the watershed-scale simulation models within BASINS. Because of the critical nature of the TMDL
process, it is imperative that BASINS and SWAT be adequately validated for regions on which they are being applied. BASINS
and SWAT were tested using six years of hydrologic data from a 22 km2 subwatershed of the Little River in Georgia.
Comparisons were made between water balance results obtained using high and low spatial resolution data as well as those
obtained using default initial parameters versus those modified for existing groundwater conditions. In general, all scenarios
simulated general trends in the observed flow data. However, for the years with lower precipitation, the total water yields
simulated with the low spatial resolution data and the default initial conditions were overpredicted by up to 27% of the annual
precipitation input. Total water yields simulated using the high spatial resolution input data were within 20% of the observed
yields for each year of the assessment. Nash-Sutcliffe model efficiencies (E) for monthly total water yields were 0.80 using
the high spatial resolution data with the modified initial conditions and 0.64 using the low spatial resolution data with the
default initial conditions. While the model simulated general streamflow trends, discrepancies were observed between
observed and simulated hydrograph peaks, time to peak, and hydrograph durations. A one-day time lag between the simulated
and observed time to peak was the primary cause of large errors in daily flow simulations. Model modification and more
extensive calibration may be necessary to increase the accuracy of the daily flow estimates for TMDL development.

Evaluation of the swat model on a coastal plain agricultural watershed

The effectiveness of mature riparian forests in reducing the impact of agriculture on the quality of the nation’s
water resources has been documented, but the impact of forest management practices implemented within riparian forest
buffers on their water quality function has not been evaluated. This article examines the effect of forest management
within a Coastal Plain riparian forest buffer system (RFBS) on runoff and sediment transport over a four year period. The
RFBS, which conformed to USDA-FS and USDA-NRCS best management recommendations, included a narrow strip of
undisturbed forest located adjacent to the stream drainage system (Zone 1), a wide managed pine forest downslope from
the grass filter (Zone 2), and a narrow grass filter strip immediately downslope from an agricultural field (Zone 3). Forest
management treatments evaluated within Zone 2 were mature forest, clear-cut, and selectively-thinned. Significant
reductions in runoff and sediment transport were measured under all three forest management treatments. The primary
zone of runoff and sediment reduction was within the grass filter portion of the RFBS. These results indicate that riparian
forests within a RFBS may be managed for economic return to the land owner without adversely affecting the runoff and
sediment reduction function performed by these buffer systems.

Joseph M. Sheridan

Papers

Managing riparian ecosystems to control nonpoint pollution

Long-term sediment deposition in the riparian zone of a coastal plain watershed

REMM: The Riparian Ecosystem Management Model

Evaluation of the swat model on a coastal plain agricultural watershed

Management effects on runoff and sediment transport in riparian forest buffers