Transactions of the ASABE
Vol. 53(5): 1423-1431 2010 American Society of Agricultural and Biological Engineers ISSN 2151-0032 1423
S
OIL AND WATER ASSESSMENT TOOL
(SWAT) MODEL: CURRENT
DEVELOPMENTS AND APPLICATIONS
K. R. Douglas‐Mankin, R. Srinivasan, J. G. Arnold
ABSTRACT. This article introduces a special collection of 20 research articles that present current developments and
applications of the Soil and Water Assessment Tool (SWAT). The first objective is to review and introduce the research
addressed within this special collection. The second objective is to summarize and synthesize the model performance statistics
and parameters published in these articles to provide a succinct guide to complement a previous SWAT model summary.
Recent SWAT developments in landscape representation, stream routing, and soil P dynamics are presented in this collection.
Numerous critical applications of the SWAT model were conducted across a variety of landscape scales, climatic and
physiographic regions, and pollutant sources. In this article, model performance in terms of coefficient of determination,
Nash‐Sutcliffe efficiency, and percent bias across all the studies is summarized and found to be satisfactory or better in all
cases. These results are then compiled with a previous synthesis of results to generate a comprehensive assessment of SWAT.
Model parameters used to calibrate the model for streamflow, sediment, N, and P in numerous studies are also summarized.
This collection demonstrates that research in development and application of the SWAT model and associated tools continues
to grow internationally in a wide range of settings and applications.
Keywords. Hydrologic modeling, Hydrology, SWAT, Water quality, Watershed.
he Soil and Water Assessment Tool (SWAT) model
is a physically based, deterministic, continuous,
watershed‐scale simulation model developed by
the USDA Agricultural Research Service (Arnold
et al., 1998; Neitsch et al., 2004, 2005). SWAT has evolved
from numerous individual models over a 30‐year period and
has been tested for a wide range of regions, conditions, prac‐
tices, and time scales (Gassman et al., 2007). Gassman et al.
(2007) summarized more than 250 refereed journal articles
reporting research using SWAT around the world. Evaluation
of daily, monthly, and annual streamflow and pollutant out‐
puts indicate that SWAT functioned well in a wide range of
watersheds.
This article introduces an effort to present recent develop‐
ments of the SWAT model and current applications of the
model to address a range of issues. The ASABE SWAT 2010
Special Collection assembles 20 research articles, largely se‐
Submitted for review in August 2010 as manuscript number SW 8724;
approved for publication by the Soil & Water Division of ASABE in
September 2010.
Contribution No. 11‐044‐J from the Kansas Agricultural Experiment
Station, Manhattan, Kansas.
The authors are Kyle R. Douglas‐Mankin, ASABE Member
Engineer, Professor, Department of Biological and Agricultural
Engineering, Kansas State University, Manhattan, Kansas; Raghavan
Srinivasan, ASABE Member, Professor, Department of Biological and
Agricultural Engineering, and Director, Spatial Science Laboratory,
Department of Ecosystem Science and Management, Texas A&M
University, College Station, Texas; and Jeffrey G. Arnold, ASABE
Member Engineer, Agricultural Engineer, USDA‐ARS Grassland Soil and
Water Research Laboratory, Temple, Texas. Corresponding author: Kyle
R. Douglas‐Mankin, Department of Biological and Agricultural
Engineering, 129 Seaton Hall, Kansas State University, Manhattan, KS
66503; phone: 785‐532‐2911; fax: 785‐532‐5825; e‐mail: krdm@ksu. edu.
lected from 113 papers, presentations, and posters presented
at the 2009 International SWAT Conference (Twigg et al.,
2009). New research in this special collection summarizes re‐
sults of streamflow, sediment, nitrogen, phosphorus, and
bacteria simulation at watershed scales ranging from 0.004
to 491,665 km
2
.
The objectives of this article are to review and introduce
the research addressed by this SWAT special collection and
to summarize and synthesize the model performance statis‐
tics and parameters reported in these articles, and thus to pro‐
vide a succinct guide to complement the SWAT model
summary by Gassman et al. (2007).
SWAT MODEL
Five versions of the SWAT model are currently being dis‐
tributed: SWAT2009 (documentation currently is not avail‐
able), SWAT2005 (Neitsch et al., 2004, 2005), SWAT2000
(Di Luzio et al., 2002; Neitsch et al., 2002), SWAT99.2
(Neitsch et al., 1999b), and SWAT98.1 (Neitsch et al.,
1999a). SWAT uses spatially distributed data on topography,
soils, land cover, land management, and weather to predict
water, sediment, nutrient, pesticide, and fecal bacteria yields.
In the current versions, a modeled watershed is divided spa‐
tially into subwatersheds using digital elevation data accord‐
ing to the density specified by the user. Subwatersheds are
further subdivided into lumped, nonspatial hydrologic re‐
sponse units (HRUs) consisting of all areas within the subwa‐
tershed having similar landscape characteristics. Versions
2000 and earlier model subwatersheds as having uniform
slope and climatic conditions, and HRUs as having similar
soil, land use, and land management characteristics. Versions
2005 and 2009 allow slope to be included at the HRU level.
T