About SWAT

Soil and Water Assessment Tool


SWAT (Soil and Water Assessment Tool) is continuous, semi-distributed model for simulating complex hydrologic processes on, primarily, agricultural watersheds. It is useful for assessing nonpoint source effects of various land use, conservation practice, and climatic change scenarios on downstream water bodies. It is calibrated when possible.


Dr. Jeff G. Arnold, USDA-ARS,

Contact information

Presenter: Dr. Tamie L. Veith, Agricultural Engineer,

USDA - Agricultural Research Service, Pasture Systems and Watershed Mgmt Research Unit, Building 3702 Curtin Road, University Park, PA 16802


SWAT can be used with or without the ArcGIS interface. Necessary inputs: DEM, soils, landuse, watershed outlet location. Helpful: land management, desired subwatershed size or stream network resolution and hydrologic mass balance, daily air temperature and streamflow, soil-nutrient levels. Optional: field boundaries, additional climatic and water quality constituents.


Daily, monthly, or yearly precipitation, streamflow and water quality loads at the stream, subbasin, and hydrologic response unit (HRU) levels; plant stress, biomass, evapotranspiration, and other values useful to the modeler for troubleshooting and calibrating the SWAT project.


SWAT is used worldwide, typically by researchers familiar with ArcGIS and basic watershed hydrology to answer stakeholder and policy-maker questions at the community and broader levels. SWAT theory and requirements are well-documented. Workshops and conferences are held annually. The website contains current information on these and links to input data sources.

Scales of Relevance

SWAT has been used at the field, sub-watershed, watershed, and basin levels. The smallest unit appropriate in a given project depends on landuse and soil resolution. Processes within 1m2 HRUs can be simulated as can 8-digit HUC watersheds. However, routing is only at the sub-watershed and larger levels.


SWAT is free and publically available. However, the ArcSWAT interface, which is the most supported way to create input files, requires ESRI’s ArcGIS and Spatial Analyst Extension ( ). These software are a moderate one-time cost. Other interfacing options are on the SWAT website.


USDA-ARS Grassland Soil and Water Research Laboratory, Temple TX,; Texas A&M Agrilife Blackland Research and Extension Center,; US EPA Office of Science & Technology, Mention of trade names or commercial products in this publication is solely for providing specific information and does not imply recommendation or endorsement by the U.S. Department of Agriculture. USDA is an equal opportunity provider and employer.