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

The Soil and Water Assessment Tool (SWAT) model has emerged as one of the most widely used water quality watershed- and river basin-scale models worldwide, applied extensively for a broad range of hydrologic and/or environmental problems. The international use of SWAT can be attributed to its flexibility in addressing water resource problems, extensive networking via dozens of training workshops and the several international conferences that have been held during the past decade, comprehensive online documentation and supporting software, and an open source code that can be adapted by model users for specific application needs. The catalyst for this special collection of papers was the 2011 International SWAT Conference & Workshops held in Toledo, Spain, which featured over 160 scientific presentations representing SWAT applications in 37 countries. This special collection presents 22 specific SWAT-related studies, most of which were presented at the 2011 SWAT Conference; it represents SWAT applications on five different continents, with the majority of studies being conducted in Europe and North America. The papers cover a variety of topics, including hydrologic testing at a wide range of watershed scales, transport of pollutants in northern European lowland watersheds, data input and routing method effects on sediment transport, development and testing of potential new model algorithms, and description and testing of supporting software. In this introduction to the special section, we provide a synthesis of these studies within four main categories: (i) hydrologic foundations, (ii) sediment transport and routing analyses, (iii) nutrient and pesticide transport, and (iv) scenario analyses. We conclude with a brief summary of key SWAT research and development needs.

Applications of the SWAT Model Special Section: Overview and Insights.

[1] Many processes lead to variability of catchment concentration-discharge relationships, but exports of geogenic (weathering derived) solutes and nutrients (nitrogen and phosphorus species) from agricultural basins display relatively constant concentrations despite large variations in streamflow. These “chemostatic” responses are hypothesized to arise when a large mass store, the parent material for geogenics or chemically recalcitrant legacies of fertilization in agricultural catchments, buffers concentration variability. This hypothesis implies that (1) chemostatic behavior should be a general response to elevated external inputs to a catchment and (2) chemostatic behavior should be predictable from theory. Data- and model-based analyses were used to explore these hypotheses. We evaluated concentration variability relative to discharge (expressed as the ratio of the coefficients of variation of concentration and flow, or CVC/CVQ) across a gradient of increasing exported load, as a proxy for an external impact gradient. The CVC/CVQ of multiple solutes declined with increasing exported load. Exceptions included the geogenic solutes, which showed chemostatic responses for all sites, phosphorus, and some nitrogen species. Nitrate showed a suggestive pattern in CVC/CVQ with export, but further data are needed to confirm its generality. A simple model of runoff generation and solute export suggested that the decline in CVC/CVQ arises if the internal mass store is distributed homogeneously in space and there is sufficient time for mass transfer to reach steady state between runoff events. Export from catchments may become more predictable in impacted watersheds, simplifying water quality prediction but inducing strong hysteresis in recovery and making restoration efforts challenging.

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Relative dominance of hydrologic versus biogeochemical factors on solute export across impact gradients

This study was conducted under the USDA-Conservation Effects Assessment Project (CEAP) in the Cheney Lake watershed in south-central Kansas. The Cheney Lake watershed has been identified as ‘impaired waters’ under Section 303(d) of the Federal Clean Water Act for sediments and total phosphorus. The USDA-CEAP seeks to quantify environmental benefits of conservation programmes on water quality by monitoring and modelling. Two of the most widely used USDA watershed-scale models are Annualized AGricultural Non-Point Source (AnnAGNPS) and Soil and Water Assessment Tool (SWAT). The objectives of this study were to compare hydrology, sediment, and total phosphorus simulation results from AnnAGNPS and SWAT in separate calibration and validation watersheds. Models were calibrated in Red Rock Creek watershed and validated in Goose Creek watershed, both sub-watersheds of the Cheney Lake watershed. Forty-five months (January 1997 to September 2000) of monthly measured flow and water quality data were used to evaluate the two models. Both models generally provided from fair to very good correlation and model efficiency for simulating surface runoff and sediment yield during calibration and validation (correlation coefficient; R2, from 0·50 to 0·89, Nash Sutcliffe efficiency index, E, from 0·47 to 0·73, root mean square error, RMSE, from 0·25 to 0·45 m3 s−1 for flow, from 158 to 312 Mg for sediment yield). Total phosphorus predictions from calibration and validation of SWAT indicated good correlation and model efficiency (R2 from 0·60 to 0·70, E from 0·63 to 0·68) while total phosphorus predictions from validation of AnnAGNPS were from unsatisfactory to very good (R2 from 0·60 to 0·77, E from − 2·38 to 0·32). The root mean square error–observations standard deviation ratio (RSR) was estimated as excellent (from 0·08 to 0·25) for the all model simulated parameters during the calibration and validation study. The percentage bias (PBIAS) of the model simulated parameters varied from unsatisfactory to excellent (from 128 to 3). This study determined SWAT to be the most appropriate model for this watershed based on calibration and validation results. Copyright © 2008 John Wiley & Sons, Ltd.

Comparison of AnnAGNPS and SWAT model simulation results in USDA‐CEAP agricultural watersheds in south‐central Kansas

At present, there are over 34,000 impaired waters and over 58,000 associated impairments officially listed in the
U.S. Nutrients and sediment are two of the most common pollutants included in the list. States are required to identify and
list those waters within their boundaries that are not meeting standards, to prioritize them, and to develop Total Maximum
Daily Loads (TMDLs) for the pollutants of concern. Models are used to support development of TMDLs, typically to estimate
source loading inputs, evaluate receiving water quality, and determine source load allocations so that receiving water quality
standards are met. Numerous models are available today, and selection of the most suitable model for a specific TMDL project
can be daunting. This article presents a critical review of models simulating sediment and nutrients in watersheds and
receiving waters that have potential for use with TMDL development and implementation. The water quality models
discussed, especially those with sediment and/or nutrient components, include loading models (GWLF and PLOAD),
receiving water models (AQUATOX, BATHTUB, CE-QUAL-W2, QUAL2E, and QUAL2K), and watershed models having
both loading and receiving components (AGNPS, AnnAGNPS, CASC2D/GSSHA, DWSM, HSPF, KINEROS2, LSPC, MIKE
SHE, and SWAT). Additional models mentioned include another receiving water quality model (WASP), watershed models
(ANSWERS storm event, ANSWERS continuous, PRMS storm event, SWMM, and WEPP), and BMP models (APEX, REMM,
and VFSMOD). Model sources, structures, and procedures for simulating hydrology, sediment, and nutrients are briefly
described for the reviewed models along with an assessment of their strengths, limitations, robustness, and potentials for using
in sediment and/or nutrient TMDLs. Applications of AGNPS, APEX, BATHTUB, CE-QUAL-W2, GWLF, and SWAT in TMDL
developments are presented. Applications of some of the other models (DWSM, GSSHA, and KINEROS2) relevant to TMDL
studies are also presented. The models proved to be useful; however, they require a learning process. Simple models are easy
to use but have limitations; comprehensive models are labor and data intensive but offer extensive analysis tools. Finally,
recommendations are offered for advancing the sediment and nutrient modeling technologies as applied to TMDL
development and implementation. Advances could be made towards: making the best use of existing models, enhancing the
existing models, combining strengths of existing models, developing new models or supplemental components with physically
based robust routines, numerous field applications, sensitivity analyses, full documentation, and rigorous education and training.

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Sediment and nutrient modeling for tmdl development and implementation

This paper presents a critical review of models simulating sediment and nutrients in
watersheds and receiving waters that have potential for use with TMDL development and
implementation. The water quality models discussed, especially those with sediment and/or nutrient
components include two loading models, five receiving water models, and nine watershed models
having both loading and receiving simulation capabilities. Other potential TMDL models are also
mentioned. Applications of some of the models, including in TMDL studies, are presented. The
models proved to be useful, however, still a learning process. Recommendations are offered for
advancing the sediment and nutrient, as well as their basic hydrologic, modeling technologies as
applied to TMDL development and implementation. An extended abstract of the paper is presented
here while the full-length paper has been accepted for publication in the Transactions of the ASABE.

G. Yagow

Papers

Sediment and nutrient modeling for tmdl development and implementation

Sediment and Nutrient Modeling for TMDL Development and Implementation