SWAT

SOFTWARE REVIEW by Paula Jo Lemonds and John E. McCray

Soil and Water Assessment Tool (SWAT) is a watershed-scale water quality model developed by the USDA Agricultural Research Service (ARS) to predict the impact of management practices on water, sediment, nutrient, and chemical yields in watersheds that have different soils, land uses, and management conditions over long durations. SWAT uses physically based data, that is, instead of using regression equations, it utilizes theory-based hydrologic and climate equations. Data from the watershed is used as input to these equations. Other important attributes of SWAT include its computational efficiency that allows for complex watersheds to be modeled in a straightforward manner, its incorporation of easily accessible data that is available for most watersheds from government agencies, and its ability to simulate long-term impacts of pollutant buildup and downstream impact. SWAT can simulate many different processes, including surface runoff, return flow, evapotranspiration, pond and reservoir storage, crop growth, reach routing, nutrient and pesticide loading from point and nonpoint sources, chemical transformations, inter-basin water transfers, irrigation, fertilization, and several types of tillage operations. Model development involves splitting a basin into sub-watersheds based on topography from a digital elevation model (DEM). Model output is easily compared to watershed data for calibration with the built-in SWAT calibration tool. A limitation of SWAT is that it does not rigorously simulate groundwater flow and transport. Model development may require significant time, depending on the user’s modeling background and knowledge of surface water and groundwater systems. SWAT is available free of charge. It may be downloaded from the SWAT web page or from the EPA’s BASINS web page. Both formats operate on a PC platform in an ArcView GIS environment, but other interfaces have been developed. Technical assistance is available in the form of beginner and intermediate training workshops and a web-based user’s forum (more information available from SWAT web page).

Last Update: November 2002