SWAT

SWAT (Soil & Water Assessment Tool)

Criterion	Explanation
General Description	SWAT (Soil & Water Assessment Tool) is a river basin scale model developed to quantify the impact of land management practices in large, complex watersheds. SWAT is public domain software actively supported by the USDA Agricultural Research. It is a hydrology model with the following components: weather, surface runoff, return flow, percolation, evapotranspiration, transmission losses, pond and reservoir storage, crop growth and irrigation, groundwater flow, reach routing, nutrient and pesticide loading, and water transfer.
Model Domain	The model domain is flexible, as specified by user.
Developer	USDA Agricultural Research Service (ARS) and Texas A&M AgriLife Research
Hardware computing requirements	Windows; 2GB free space if using 32bit system
Code language	FORTRAN for scientific code; Python and Javascript for user interface
Original application	The original application of the SWAT model was for agriculture. SWAT was developed to predict impacts of land management practices on water, sediment and pollutant yields from watersheds. The model simulates physical processes associated with water movement, sediment erosion, crop growth and nutrient cycling.
Public/proprietary and cost	The model is in the public domain; no cost.
Physically or empirically based	The model is physically based.
Mathematical methods used How are pesticide and nutrient loading calculated?	The following mathematical methods are used by SWAT: Hydrologic cycle simulated based on the water balance equation. Runoff rate is estimated by a modified rational method or the NRCS TR-55 method. Infiltration is estimated by the Green Ampt method. Evapotranspiration by Penman-Monteith, Hargreaves, or Priestly-Taylor. FLow and average velocity calculations in channels by Manning's equation. Reservoirs by Muskingum routing method. Sediment erosion is modeled through the modified universal soil loss equation (MUSLE) and Bagnold's equation to predict degradation of stream linings.
Input data requirements	Watershed input files with routing and land parameters defined, precipitation, temperature, solar radiation, wind speed, relative humidity, potential evapotranspiration, weather forecast, land cover/plant growth, pesticide, fertilizer, urban pollutant build-up/washoff, septic, subbasin, pond/wetland, water use, soil chemical and physical characteristics, and main water channel parameters files. The model also requires water quality files associated with QUAL2E transformations in main channels and streams.
Outputs	Flow (surface runoff, lateral flow contribution to streams, groundwater, water percolation, drainage tile, stored soil water, actual and potential evapotranspiration, water yield) and water quality (sediment yield, nitrate loadings, plant uptake of N, soluble and organic phosphorus loadings, ammonia distributions in flow and solids and changes in bacterial loadings) by sub-catchbasin; There are several graphical user interfaces (GUIs) available, both GIS based (ArcGIS extension) and non-GIS based.
Pre-processing and post-processing tools	Several pre- and post-processing tools are available, both in the public domain and for purchase.
Representation of uncertainty	Offers automated method for uncertainty analysis/auto-calibration. Use SWAT-CUP as a calibration/uncertainty or sensitivity program.
Prevalence	The model is widely used in the research and commercial fields.
Ease of use for public entities	The model requires training to use; however, there is much support available, through training and active user groups. The hardware/software requirements are easily obtained.
Ease of obtaining information and availability of technical support	Product information available online. User support available from user groups and development team.
Source code availability	Yes, the source code is available for download.
Status of model development	The SWAT model has undergone active development. A completely revised version of the SWAT model called SWAT+ has been released in 2018. SWAT+ provides a more flexible spatial representation of interactions and processes within a watershed.
Challenges in integrationWhat are the obstacles to adapting this model in the Delta? Has it been used in the Delta?	Challenges to integration are few, given the open source code and active user community. Spatial variation is specified on the hydrologic response unit (HRU) level. The temporal scale is an hourly time step.