Watershed models are powerful tools for simulating the effect of watershed processes and management on soil and water resources. However, no comprehensive guidance is available to facilitate model evaluation in terms of the accuracy of simulated data compared to measured flow and constituent values. Thus, the objectives of this research were to: (1) determine recommended model evaluation techniques (statistical and graphical), (2) review reported ranges of values and corresponding performance ratings for the recommended statistics, and (3) establish guidelines for model evaluation based on the review results and project-specific considerations; all of these objectives focus on simulation of streamflow and transport of sediment and nutrients. These objectives were achieved with a thorough review of relevant literature on model application and recommended model evaluation methods. Based on this analysis, we recommend that three quantitative statistics, Nash-Sutcliffe efficiency (NSE), percent bias (PBIAS), and ratio of the root mean square error to the standard deviation of measured data (RSR), in addition to the graphical techniques, be used in model evaluation. The following model evaluation performance ratings were established for each recommended statistic. In general, model simulation can be judged as satisfactory if NSE > 0.50 and RSR < 0.70, and if PBIAS + 25% for streamflow, PBIAS + 55% for sediment, and PBIAS + 70% for N and P. For PBIAS, constituent-specific performance ratings were determined based on uncertainty of measured data. Additional considerations related to model evaluation guidelines are also discussed. These considerations include: single-event simulation, quality and quantity of measured data, model calibration procedure, evaluation time step, and project scope and magnitude. A case study illustrating the application of the model evaluation guidelines is also provided.

Model Evaluation Guidelines for Systematic Quantification of Accuracy in Watershed Simulations

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 State of Texas has initiated the development of a Total Maximum Daily Load program in the Bosque River Watershed, where point and nonpoint sources of pollution are a concern. Soil Water Assessment Tool (SWAT) was validated for flow, sediment, and nutrients in the watershed to evaluate alternative management scenarios and estimate their effects in controlling pollution. This paper discusses the calibration and validation at two locations, Hico and Valley Mills, along the North Bosque River. Calibration for flow was performed from 1960 through 1998. Sediment and nutrient calibration was done from 1993 through 1997 at Hico and from 1996 through 1997 at Valley Mills. Model validation was performed for 1998. Time series plots and statistical measures were used to verify model predictions. Predicted values generally matched well with the observed values during calibration and validation (R2≥ 0.6 and Nash-Suttcliffe Efficiency ≥ 0.5, in most instances) except for some underprediction of nitrogen during calibration at both locations and sediment and organic nutrients during validation at Valley Mills. This study showed that SWAT was able to predict flow, sediment, and nutrients successfully and can be used to study the effects of alternative management scenarios.

Validation of the SWAT Model on a Large River Basin With Point and Nonpoint Sources

Three watershed-scale hydrologic and nonpoint-source pollution models, all having the three major components
(hydrology, sediment, and chemical), were selected based on a review of eleven models (AGNPS, AnnAGNPS, ANSWERS,
ANSWERS-Continuous, CASC2D, DWSM, HSPF, KINEROS, MIKE SHE, PRMS, and SWAT) presented in a companion article.
Those selected were SWAT, a promising model for long-term continuous simulations in predominantly agricultural watersheds;
HSPF, a promising model for long-term continuous simulations in mixed agricultural and urban watersheds; and
DWSM, a promising storm event (rainfall) simulation model for agricultural and suburban watersheds. In this article, applications
of these three models, as reported and found in the literature, are reviewed and discussed. Seventeen SWAT, twelve
HSPF, and eighteen DWSM applications are compiled. SWAT and HSPF require a significant amount of data and empirical
parameters for development and calibration. DWSM has efficient physically (process) based simulation routines and therefore
has a small number of calibration parameters. SWAT and HSPF were found suitable for predicting yearly flow volumes,
sediment, and nutrient loads. Monthly predictions were generally good, except for months having extreme storm events and
hydrologic conditions. Daily simulations of extreme flow events were poor. DWSM reasonably predicted distributed flow hydrographs,
and concentration or discharge graphs of sediment, nutrient, and pesticides at small time intervals resulting from
rainfall events. Combined use of these complementary models and perhaps other models having different strengths is warranted
to adequately address water quantity and quality problems and their solutions.

Watershed-scale hydrologic and nonpoint-source pollution models: review of applications

Agriculture, including livestock production, has been implicated as a major source of pollution to streams and
lakes. This study was conducted to assess the effect of dairy production on water quality within Upper North Bosque
River Watershed (UNBRW) of north central Texas. The UNBRW encompasses an area of approximately 93 250 ha and
included 94 dairies at the time of the study. A river basin model (Soil and Water Assessment Tool, SWAT) was applied in
two phases. During the first phase, SWAT was validated for the baseline condition within UNBRW. The baseline condition
within UNBRW was simulated from 1988 through 1996; model output was compared to flow, sediment, and nutrient
measurements for 11 stream sites within the watershed for the period of October 1993 to July 1995 for SWAT model
validation. The Nash-Sutcliffe coefficient evaluating model efficiency of SWAT for predicting average monthly flow,
sediment, and nutrient loading (organic-N, NO3-N, organic-P, and PO4-P) at 11 stream sites over the validation period
ranged from 0.65 to 0.99, indicating reasonable predicted values. SWAT also adequately predicted monthly trends in
average daily flow, sediment, and nutrient loading over the validation period with Nash-Sutcliffe coefficients ranging
from 0.54 to 0.94 except for NO3-N which had a value of 0.27. Nutrient loadings were consistently highest in the subwatersheds
with the most dairies operations. In phase two, to evaluate the effect of dairies, SWAT was executed by
replacing the manure waste application fields with grassland. The results from this phase of study indicate that loading
from the watershed could be reduced about 33% for total-N (organic-N plus NO3-N) and 79% for total-P (organic-P plus
PO4-P) in the UNBRW if dairy waste application fields were replaced by grassland. Empirical equations based on the
output with and without dairy loading were developed to illustrate the impact of cow density and percent area covered by
dairy waste application fields on sediment and nutrient loading.

Application of swat for the upper north bosque river watershed

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

Nutrient losses from agricultural nonpoint sources are a key component of surface water impairment across the United States. Nitrogen is clearly the primary pollutant problem in many agricultural areas. However, development of management practices that reduce phosphorus loadings is becoming more important in many watersheds because phosphorus is often the limiting nutrient for fresh water eutrophication. This study presents the results of computer simulations performed to assess the impacts of various management practices on phosphorus losses from dairy farms in a watershed in north central Texas. The results show that moving from nitrogen to phosphorus-based waste application rates could significantly reduce phosphorus losses at moderate cost to producers. Composting solid manure for end uses outside the impacted watersheds provides even greater phosphorus load reductions and requires less land, but results in significantly higher cost to producers. The choice for each watershed depends on such key factors as available land area and the load reduction sought.

Economic and Environmental Impacts of Alternative Practices on Dairy Farms in an Agricultural Watershed

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.

W. D. Rosenthal

Papers

Application of swat for the upper north bosque river watershed

Sediment and nutrient modeling for tmdl development and implementation

Economic and Environmental Impacts of Alternative Practices on Dairy Farms in an Agricultural Watershed

Sediment and Nutrient Modeling for TMDL Development and Implementation