1. General Model Information
Name: Soil and Water Assessment Tool
Main medium: terrestrial
Main subject: hydrology, biogeochemistry
Organization level: landscape, ecosystem
Type of model: not specified (3D)
Main application: decision support/expert system, research
Keywords: watershed, management, basin scale, spatially distributed, runoff, water quality, pollutant transport, climate change, vegetative changes, resevoir management, groundwater withdrawals, water transfer, nutrient cycling, erosion, sediment transport, continuous-time, multiple subbasins, capacity cascade soil water model , Priestley-Taylor evapotranspiration, Curve-Number-runoff, GIS-interface, soil database
The SWAT Team
Grassland, Soil & Water Research Laboratory,
808 East Blackland Road
Temple, Texas 76502 USA
Phone: (254) 770-6500
Fax: (254) 770-6561
Jeff Arnold: Hydraulic Engineer, Model Development
Nancy Sammons: Computer Assistant, PC version of SWAT, Windows Interface for SWAT, User Assistance
Susan Neitsch: Biological Technician (Soils), Model Documentation, User Assistance
Raghavan Srinivasan: Assistant Professor, GIS, modeling, application and interfaces
Mauro DiLuzio: Post-doctoral Research Associate, GIS, modeling, and application, SWAT/ArcView interface
Arnold, J.G., P.M. Allen, and G.T. Bernhardt (Arnold, Allen, Bernhardt, Srinivasan, Muttiah, Walker,Dyke, 1993, USDA & Texas A&M University),
Model Objectives: To predict effects of management (Climate and vegetative
changes, resevoir managemant, groundwater withdrawals, water transfer) on
water sediment and chemical yields on large river basins. SWAT can analyse
watersheds and river basins of 100 square miles by subdividing the area into
homogenous parts. Uses daily time step, continous for 1-100 years.
Approach: This model subdivides large river basins into
homogenous parts, then analyzes each part and its interaction with the whole.
SWAT is spatially distributed, so that these parts can interact. The model simulates hydrology, pesticide and
nutrient cycling, erosion and sediment transport . Input consists of files, information from databases and information from a GIS interface . More specific information can be entered singly, for each area or
for the watershed as a whole.
Backgrond: The model was developed by modifying the SWRRB , (Arnold
et al, 1990) and ROTO (Arnold, 1990) models for application to
large, complex rural basins. SWRRB is a distributed version of CREAMS ,which can be applied to a basin with a maximum of 10 subbasins, and SWAT is an extended and improved version of SWRRB, running
simultaneously in several hundred subbasins.
The SWAT hydrology model is based on the water balance equation. A distributed SCS curve number is generated for the computation of overland flow runoff volume, given by the standard SCS runoff equation (USDA, 1986). A soil database is used to obtain information on soil type, texture, depth, and hydrologic classification. In SWAT, soil profiles can
be divided into ten layers. Infiltration is defined in SWAT as precipitation minus runoff.
Infiltration moves into the soil profile where it is routed through
the soil layers. A storage routing flow coefficient is used to
predict flow through each soil layer, with flow occurring when
a layer exceeds field capacity. When water percolates past the bottom layer, it enters the shallow
aquifer zone (Arnold and others, 1993). Channel transmission loss and pond/reservoir seepage
replenishes the shallow aquifer while the shallow aquifer interacts
directly with the stream. Flow to the deep aquifer system is effectively lost and cannot return to the stream
(Arnold and others, 1993). The irrigation algorithm developed for SWAT allows
irrigation water to be transferred from any reach or reservoir
to any other in the watershed. Based on surface runoff calculated using the SCS runoff equation,
excess surface runoff not lost to other functions makes its way
to the channels where it is routed downstream. Sediment yield used for instream transport is determined from
the Modified Universal Soil Loss Equation (MUSLE) (Arnold, 1992). For sediment routing in SWAT, deposition calculation is based on fall velocities of various sediment sizes. Rates of channel
degradation are determined from Bagnold's (1977) stream power
equation. Sediment size is estimated from the primary particle
size distribution (Foster and others, 1980) for soils the SWAT
model obtains from the STATSGO (USDA 1992) database. Stream power also is
accounted for in the sediment routing routine, and is used for
calculation of re-entrainment of loose and deposited material
in the system until all of the material has been removed.
SWAT is currently adapted only for US watersheds
(using the specific data sets, particularly soil and weather data bases). The SWAT represents a component of the HUMUS project, where it is applied for 350 6-digit hydrologic unit areas
in the 18 major river basins in the U.S. (Srinivasan et al., 1993b).
Krysanova et. al (1996) adopted large parts of SWAT for their model SWIM which they designed for the Elbe river basin in Northern Germany.
Pre- and Postprocessing:
The SWAT/GRASS interface (Srinivasan, Arnold, 1993, Srinivasan et al., 1993a)extracts spatially distributed parameters of elevation, land use, soil types, and groundwater table. The interface creates a number of
input files for the basin and subbasins, including the subbasin routing structure file.
The SWAT-GIS linkage incorporates advanced visualization tools capable of statistical
analysis of output data.
- Subbasin attributes: Using a given subbasin map, the program calculates area, resolution,
and coordinate boundaries for the basin and each subbasin. The
fraction of each subbasin area to the basin area is calculated.
- Topographic attributes: The program estimates the stream
length, stream slope and geometrical dimensions, accumulation
area, and aspect. The weighted average method is used to estimate
the overland slope and slope length. Finally, the channel
factors K and C of the Universal Soil Loss Equation (USLE)
are estimated using a standard table.
- Ground water attributes: The ground water parameters
are estimated for each subbasin using the alpha layer, which
defines the time lag needed to the groundwater flow as it leaves the
shallow aquifer to return to the stream (Arnold et al., 1993).
- Routing structure: This very important step in the SWAT/GRASS
interface creates the routing structure for subbasins, based on
the elevation map. Also, it defines the channel width and depth
using a neural network that is embedded in the interface, based
on the drainage area and average elevation of a subbasin.
Please check for the revised documentation of the current 99.2 version:
II. Technical Information
Operating System(s): UNIX (Solaris), PC (DOS, Windows)
Programming Language(s): Fortran to obtain the SWAT-codes please contact the authors
Raghavan Srinivasan email@example.com
see section II.2
III. Mathematical Information
Santhi, C., Arnold, J.G., Williams, J.R., Dugas, W.A., and Hauck, L.Validation of the SWAT model on a large river basin with point and nonpoint sources. J. of American Water Resources Association (in review).
Cruickshank, T.S., Fontaine, T.A., Arnold, J.G., and Hotchkiss, R.H. 2000Large scale hydrologic modeling of the snowfall-snowmelt system in mountainous terrain. J. of Hydrology (in review).
Fritch, T. G., McKnight, C. L., Yelderman, J. C. Jr., Dworkin, S. I., Arnold, J. G. 2000.A predictive modeling approach to assessing the groundwater pollution susceptibility of the Paluxy Aquifer, Central Texas, using a geographic information system. Environmental Geology (In Press).
Saleh, A., Arnold, J.G., Gassman, P.W., Hauck, L.W., Rosenthal, W.D., Williams, J.R., and McFarland, A.M.S. 2000.Application of SWAT for the upper north Bosque watershed. Transactions of the ASAE (In press)
Arnold, J.G., Srinivasan, R., Muttiah, R.S., Allen, P.M., and Walker, C. 1999. Continental scale simulation of the hydrologic balance. J. American Water Resources Association 35(5):1037-1052.
Srinivasan, R.S., Arnold, J.G., and Jones, C.A. 1998.Hydrologic modeling of the United States with the soil and water assessment tool. Water Resources Development 14(3):315-325.
Srinivasan, R., Ramanarayanan, T.S., Arnold, J.G., and Bednarz, S.T. 1998.Large area hydrologic modeling and assessment part II: model application. J. American Water Resources Association 34(1):91-101.
Arnold, J.G., Srinivasan, R., Muttiah, R.S., and Williams, J.R. 1998. Large area hydrologic modeling and assessment part I: model development. J. American Water Resources Association 34(1):73-89.
Arnold, J.G., 1992: Spatial Scale Variability in Model Developmentand Parameterization: Ph.D. Dissertation, Purdue University, WestLafayette, IN, 183 p.
Arnold, J.G., P.M. Allen, and G.T. Bernhardt, 1993: A ComprehensiveSurface-Groundwater Flow Model: Journal of Hydrology, v. 142,p. 47-69.
Arnold, J.G., B.A. Engel, and R. Srinivasan, 1993: A ContinuousTime, Grid Cell Watershed Model: in Proceedings of Applicationof Advanced Technology for the Management of Natural Resources,Sponsored by American Society of Agricultural Engineers, June17-19, 1993, Spokane, WA.
Arnold, J.G., J.R. Williams, R. Srinivasan, K.W. King, and R.H.Griggs, 1995: SWAT - Soil and Water Assessment Tool: Draft UsersManual, USDA-ARS, Temple, TX.
Srinivasan, R., and Arnold, J.G., 1993. Basin scale water qualitymodelling using GIS. Proceedings, Applications of AdvancedInform. Technologies for Manag. of Nat. Res. June 17-19, Spokane,WA, USA.
Srinivasan, R., Arnold, J.G., Muttiah, R.S., Walker C., Dyke P.T.,1993. Hydrologic Unit Model for the United States (HUMUS). In:Sam S.Y.Wang (ed.) Advances in Hydro-Science and -Engineering,Vol. I.
USDA, 1992. STATSGO - State soils geographic data base.Soil Conservation Service, Publ. Number 1492, Washington D.C.
V. Further information in the World-Wide-Web
other examples for GIS-based watershed modeling are described under
VI. Additional remarks
Information and remarks about the application of this model
Last review of this document by: Hailing Wang and T. Gabele: 25. 9. 1997
J. Bierwirth: 19.9.2000
Status of the document:
last modified by
Tobias Gabele Wed Aug 21 21:44:50 CEST 2002