SWAT — The Complete Guide: A Free Watershed Model for Runoff, Erosion and Farm Pollutants
SWAT (Soil and Water Assessment Tool) is a watershed model developed at Texas A&M and USDA-ARS. It does not focus on a single storm but on decades: rainfall and evapotranspiration, surface runoff and baseflow, soil erosion and the transport of nutrients and pesticides from field to stream.
SWAT
Texas A&M AgriLife / USDA-ARS
A watershed model — runoff, erosion, nutrients and pesticides over decades.
Get it from the vendorWhat you get
- Hydrologic response units — the basin is divided into combinations of soil, slope and land use.
- Long-term water balance — at a daily time step.
- Sediment — soil loss from fields and transport in channels.
- Nutrients and pesticides — nitrogen and phosphorus by source.
- Scenarios — changes in land use, fertiliser or farm management.
What to know
The model is free with open source code, and you work with it through an interface in QGIS (QSWAT) or ArcGIS. Its default data and soil parameters are built for American conditions, so anywhere else you must supply local climate and soil data and calibrate against measured flows. A restructured version, SWAT+, is also available.
Related tools
For the hydrograph of a single storm — HEC-HMS. For urban drainage — SWMM. To prepare elevation models and catchments — WhiteboxTools.
HEC-HMS
US Army Corps of Engineers
Rainfall-runoff hydrological model — infiltration, unit hydrograph and routing. Pairs with HEC-RAS.
Get it from the vendor
SWMM
US Environmental Protection Agency
Urban drainage model — runoff, sewers, detention and infiltration, including runoff quality.
Get it from the vendor
QGIS
QGIS.org
Open-source GIS for spatial analysis, layer work and map production.
Get it from the vendorWhiteboxTools
Whitebox Geospatial
Hundreds of geospatial analysis tools, especially strong on terrain and LiDAR point clouds.
Get it from the vendorThe bottom line
SWAT is the leading free tool for long-term water quantity and quality questions at basin scale — provided you invest in local calibration.