Soil and Water Assessment Tool (SWAT)


SWAT was developed to predict the impact of land management practice on water, sediment and agricultural chemical yields in large complex watersheds with varying soils, land use and management conditions over long periods of time.


Soil and Water Assessment Tool (SWAT) is a watershed scale model originally developed by Dr Jeff Arnold for the USDA Agricultural Research Services. It was designed to predict the impact of proposed or alternative land management practices on water, sediment and agricultural chemical yields within complex watersheds of varying soils, land use and management conditions over extended periods of time.

The model operates on a daily time step, it is process based, computationally efficient, and capable of continuous simulation over long time periods. SWAT has the capability of incorporating a number of differing physical processes (major model components) in water shed modelling, these include;

Land Phase of the Hydrologic Cycle

  • Climate
  • Hydrology
  • Soil temperature and properties
  • Land cover/plant growth
  • Erosion
  • Nutrients
  • Pesticides
  • Bacteria and Pathogens
  • Land Management

Also considered is the routing phase of the hydrologic cycle;

  • Routing in the main channel or reach i.e. flood routing, sediment routing, nutrient routing channel pesticide routing.
  • Routing in the reservoir i.e. reservoir outflow, sediment routing, reservoir nutrients, reservoir pesticides

SWAT flow calibration2

SWAT - Flow Calibration


SWAT is a public domain model actively supported by the USDA Agricultural Research Service at the Grassland, Soil and Water Research Laboratory in Temple, Texas, USA. In modelling a watershed may be partitioned into a number of sub-watersheds which are then further subdivided into hydrologic response units (HRUs) that consist of homogeneous land use, management, topographical, and soil characteristics. The HRUs are represented as a percentage of the subwatershed area and may not be contiguous or spatially identified within a SWAT simulation. Alternatively, a watershed can be subdivided into only subwatersheds that are characterized by dominant land use, soil type, and management.  

This catchment model is able to quantify the impact of land management practices on flow and water quality. As a result the model has widespread use internationally, including New Zealand, in informing land use planning, water quality guide lines and water allocation limits. The only limitation of SWAT then being the quality and amount of input data required to set up modelling on a given catchment study area.


Latest Version SWAT2012 re.664 released December 2016
State of Development Stable Release
Current Development Activity Active development by USDA and various research organasations.

Development Contact

Jeff Arnold - Hydraulic Engineer
+1 254 770-6502
Grassland, Soil & Water Research Laboratory, USDA-ARS
808 East Blackland Road
Temple, Texas 76502 USA


Intended End Users Specialist modellers supporting land and water managers
Spatial Extents Unknown
Spatial Dimensions Unknown
Temporal Resolutions Days
Temporal Extents Years, Decades
Steady State or Dynamic Dynamic

Input & Output Data


Open/Closed Source Unknown
Licence Type

User Information

Operating Systems MS Windows
Software Needed Unknown
User Interface Please Select
Ease of Use Moderate
Use in Policy Process Plan (Policy Formulation), Do (Policy Implementation)

SWAT Documentation:


Support No formal support. Training workshops are run regularly and there is an annual SWAT conference.
Users Forum Forum available

Technical Considerations

Programming Language Fortran for the main engine code. ArcSWAT is written in a .net language
Methods included for calibration and validation Calibration tools are provided in the SWATCUP tool from Eawag
Analytical Techniques Input/output
Model Structure

SWAT Water movement

SWAT Water Movement

Keywords soil, water, sediment, chemical, land use, land management

SWAT Model Home Page -

SWAT: Soil and Water Assessment Tool

SWAT publications:

An OpenMI compliant version is being developed, see

HydroModeller version available. SWAT home page -


SWAT theory:

SWAT Calibration/Validation Publications > SWAT: MODEL USE, CALIBRATION, AND VALIDATION.


Key References

SWAT Publications:

New Zealand SWAT References:

Ekanayake, J.C.; Davie, T. (2005). The SWAT model applied to predicting nitrate fluxes in the Motueka catchment. Motueka Integrated Catchment Management Report Series. Landcare ICM Report No. 2004-2005/04. Landcare Research. Lincoln, New Zealand. -

University of Waikato: LERNZ - Catchment modelling with SWAT-

Associated Case Studies

IDEAS - ICM Motueka Catchment

ICM's aim was to integrate research and management to address multiple issues; there were 5 key themes and areas of research associated with the Motueka project.


SWAT - Motueka River Watershed

The Motueka River basin drains an area of 2075km2 providing ~65% of the major freshwater flow into Tasman bay.