1. General Model Information

Name: Soil and Water Assessment Tool

Acronym: SWAT

Contact:

Author(s):

Abstract:

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.

Processes: 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.

Adaption: 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.

• 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: http://www.brc.tamus.edu/swat/swatdoc.html

II. Technical Information

II.1 Executables:

• SWAT Software: http://www.brc.tamus.edu/swat/swatmod.html
• SWAT-GRASS
• SWAT Online - New WWW-Interface to SWAT (New Link Sep 97)

II.2 Source-code:

II.3 Manuals: