Soil and Water Assessment Tool (SWAT)
The Soil & Water Assessment Tool (SWAT) is a river basin scale model developed to quantify the impact of land management practices in large, complex watersheds. SWAT is a public domain hydrology model with the following components: weather, surface runoff, return flow, percolation, evapotranspiration, transmission losses, pond and reservoir storage, crop growth and irrigation, groundwater flow, reach routing, nutrient and pesticide loading, and water transfer.
SWAT is a continuous time model that operates on a daily time step at basin scale. Its objective is to predict the long-term impacts of management and of the timing of agricultural practices within a year (i.e., crop rotations, planting and harvest dates, irrigation, fertilizer, and pesticide application rates and timing). It can be used to simulate at the basin scale water and nutrients cycle in landscapes whose dominant land use is agriculture. It can also help in assessing the environmental efficiency of best management practices and alternative management policies.
SWAT uses a two-level dissagregation scheme; a preliminary subbasin identification is carried out based on topographic criteria, followed by further discretization using land use and soil type considerations. Areas with the same topographic characteristics, soil type, land use and management form a Hydrologic Response Unit (HRU), a basic computational unit assumed to be homogeneous in hydrologic response to land cover change.
SWAT is perhaps the most comprehensive environmental modeling software available. All equations used are described in the very comprehensive Theoretical Documentation Manual.
Important SWAT modules and resources are:
ArcSWAT: ArcGIS-ArcView extension and graphical user input interface for SWAT.
Global Weather Data for SWAT: allows to download daily Climate Forecast System Reanalysis (CFSR) data (precipitation, wind, relative humidity, and solar) in SWAT file format for a given location and time period.
QSWAT: a GIS interface linking SWAT to the public domain GIS software QGIS.
SLEEP: a tool to help SWAT users generate a soil database at sub-catchment level from point field observations, or legacy soil maps.
SWAT2012: The SWAT Input/Output File Documentation reviews all processes simulated with the model. and provides definitions for all input variables. It is absolutely essential for beginning to work with SWAT.
SWAT-MODFLOW: an integrated hydrological model that couples SWAT land surface processes with spatially-explicit groundwater flow processes.
SWAT Output Viewer: a tool to quickly view and analyze the outputs of a SWAT model on-the-fly.