SWAT stands for Soil and Water Assessment Tool. It was developed by United States Department of Agriculture. Notable fact is that SWAT is a public domain software.
SWAT is available as a GUI extension for ArcGIS in the form of ArcSWAT. ArcGIS is a leading GIS software developed by ESRI®. ArcSWAT can be used for watershed simulation and modelling. SWAT is accepted worldwide in a big way. This is evident from the large number of scientific papers published and international conferences held every year. For a successful simulation the designation of physical parameters like land use, soil, weather, water use management, and chemical parameters like water quality data along with the simulation period becomes necessary. Watersheds are divided into subbasins first and then into hydrological response units or HRUs.
The software is available for download from the SWAT user website. The latest version available is ArcSWAT 2009.93.3 beta released on January 27, 2010. Once the software is installed example datasets, inputs, weather folder, template for output databases, soils, text database files, layer files for the software are available as a part of the installation.
The spatial datasets required for the model are: Digital Elevation Model (DEM), landuse/landcover and soil in the ESRI grid/shapefile/feature class format. Those familiar with GIS will be aware of necessity of projecting the data. Projection is used to convert data in Geographic Coordinate System to Projected Coordinate System. In ArcSWAT any projection may be used but based on the precondition that same projection is used for all maps of the particular data. In addition to this DEM mask, streams, user-defined watersheds or streams are optional ArcSWAT datasets. These data again may be in the form of ESRI Grid, shapefile or feature class format.
Different tables like subbasin location table, watershed inlet location table, land use lookup table, soil look up table, solar radiation data table, point discharge data table, reservoir outflow data table, potential evapotranspiration table etc. can be used in the form of dBase or ASCII files for providing input data.
Excellent user support is available in the form of SWAT user website and groups plus discussion forums.
Official SWAT website: http://swatmodel.tamu.edu/
SWAT forums and user groups: http://swatmodel.tamu.edu/user-groups
ArcSWAT - Google groups: http://groups.google.com/group/ArcSWAT