Introduction
The SWAT (Soil Water Assessment Tool) model, first released in the 1990’s, is a small watershed to river
basin-scale model that simulates the quality and quantity of surface and groundwater and predicts the
environmental impacts of land use, land management practices, and climate change. The water balance, or the flow
of water in and out of a hydrologic system, informs all processes in the SWAT model because of its impact on
plant growth, sediment, nutrients, pesticides and pathogens. In order to model hydrologic processes, the SWAT
model first divides basin of interest into sub-basins, and then further into hydrologic response units (HRUs)
based on land use, management and soils. SWAT estimates runoff for each HRU separately, and then the total
runoff for the entire basin. The SWAT model is widely used for hydrologic studies, climate change studies, and
water quality studies including nutrient loading, total daily maximum loads, pesticides and bacteria. Key
features of the SWAT model include its ability to perform on multiple GIS platforms, including ArcGIS,
Mapwindows (MWSWAT), or QGIS (QSWAT). SWAT can also be integrated with the groundwater modeling software
MODFLOW.
