Jump to main content.


Model Report


Last Revision Date: 08/25/2009 View as PDF
General Information Back to Top
Model Abbreviated Name:

Model Extended Name:

Model Overview/Abstract:
The Soil and Water Assessment Tool (SWAT), is a river basin, or watershed, scale model developed to predict the impact of land management practices on water, sediment and agricultural chemical yields in large complex watersheds with varying soils, land use and management conditions over long periods of time. It is a continuous time model, i.e. a long-term yield model and is not designed to simulate detailed, single-event flood routing.To satisfy these objectives, the model is physically based. Rather than incorporating regression equations to describe the relationship between input and output variables, SWAT requires specific information about weather, soil properties, topography, vegetation, and land management practices occurring in the watershed. The physical processes associated with water movement, sediment movement, crop growth, nutrient cycling, etc. are directly modeled by SWAT using this input data. Benefits of this approach are:
  • Watersheds with no monitoring data (e.g. stream gage data) can be modeled
  • The relative impact of alternative input data (e.g., changes in management practices, climate, vegetation, etc.) on water quality or other variables of interest can be quantified.
Some additional attributes of SWAT are that it:
  • Uses readily available inputs. While SWAT can be used to study more specialized processes such as bacteria transport, the minimum data required to make a run are commonly available from government agencies.
  • Is computationally efficient. Simulation of very large basins or a variety of management strategies can be performed without excessive investment of time or money.
  • Enables users to study long-term impacts. Many of the problems currently addressed by users involve the gradual buildup of pollutants and the impact on downstream water bodies. To study these types of problems, results are needed from runs with output spanning several decades.

SWAT2000 was developed by the USDA, ARS, and the Texas A&M Spatial Sciences Laboratory with funding from EPA and is incorporated into EPAs BASINS 3.0 water quality modeling system.

Model Technical Contact Information:
Jeff Arnold, Agricultural Engineer
USDA, ARS, Grassland Soil and Water Research Laboratory

Raghavan Srinivasan, Associate Professor and Director of Spatial Sciences Lab
Texas A&M University

David Wells, Hydrologist
USEPA, Office of Water, Office of Science and Technology

Developer's Webpage

Model Homepage: http://www.epa.gov/waterscience/basins/

User Information Back to Top
Technical Requirements
Computer Hardware
Minimum -Pentium166 MHz or above ( 400 MHz preferred), CD reader
Compatible Operating Systems
Windows 95, 98, NT, 2000, XP
Other Software Required to Run the Model
ESRI ArcView 3.x and Spatial Analyst 1.1 or 2.0 software
Download Information
The model is publicly available from the U.S. EPA, Office of Water, as part of the BASINS water quality modeling system.
Using the Model
Basic Model Inputs
The first step in setting up a simulation in SWAT is to partition the watershed into subunits. The first level of subdivision is the subbasin. Subbasins are further subdivided into hydrologic response units (HRUs). HRUs are portions of a subbasin that posses unique land use, management, and soil attributes that are input into the model.
Basic Model Outputs
SWAT generates several output files that provide information on the loadings from HRUs to the streams, water quality in stream reaches and ponds.
User Support
User's Guide Available?
The SWAT2000 Users' Manual is available.

Model Science Back to Top
Problem Identification
SWAT incorporates features of several USDA ARS models and is a direct outgrowth of the SWRRB model (Simulator for Water Resources in Rural Basins) (Williams et al., 1985; Arnold et al., 1990). Specific models that contributed significantly to the development of SWAT were CREAMS (Chemicals, Runoff, and Erosion from Agricultural Management Systems) (Knisel, 1980), GLEAMS (Groundwater Loading Effects on Agricultural Management Systems) (Leonard et al., 1987), and EPIC (Erosion-Productivity Impact Calculator) (Williams et al., 1984).

Read more about the Development of SWAT. exit EPA

Summary of Model Structure and Methods
See Users' Manual.
Model Evaluation
List of Peer Reviewed Publications exit EPA

List of applications using SWAT exit EPA

Local Navigation

Jump to main content.