| Soil Water Assessment Tool: Joint USDA-NASA Applications
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SWAT (Soil Water Assessment Tool) is a public domain model and decision support tool developed and actively supported by the USDA Agricultural Research Service (ARS). It is a river basin model developed to predict the impact of land management practices on daily water, sediment, and agricultural chemical yields in watersheds with varying soils, land use, and management conditions (Arnold et al., 1998).
SWAT uses a geographic information system (GIS) based interface and GIS coverages of digital elevation models to divide a drainage basin into similar-sized subbasins that are further divided on the basis of soil type and land cover, into areas of like hydrologic characteristics, called hydrologic response units (HRUs). HRUs are portions of a subbasin that possess unique combinations of land use, management, and soil attributes. The GIS module also estimates the stream length, stream slope and geometrical dimensions, accumulation area, and aspect.
Applied research with SWAT at NASA Ames Research Center is directed toward innovative uses of satellite remote sensing imagery and data products to enhance and update the characterization of HRUs in selected watersheds of the western United States. In addition, SWAT model output data are being summarized and visualized in new internet-based applications that can be easily displayed from any web page browser.
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| Watershed Simulation with the SWAT Model |
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| Major Features of the USDA-SWAT Model |
- Predicts the impact of land management practices on daily water, sediment, and agricultural chemical yields in large watersheds;
- ArcGIS-based and computationally efficient, readily accepts updated remotely sensed layers, land cover, climate and soil file inputs;
- Defines hydrologic response units (HRUs) as portions of a sub-basin that possess unique combinations of land use, vegetation cover, and soil attributes. Land management settings can be customized for local practices;
- Predicts transport of constituents into and out of all sub-basins and river channels: sediment (metric tons), organic nitrogen, nitrate, and ammonium (kg N), organic and mineral phosphorus (kg P), chlorophyll-a, algal biomass, carbonaceous biochemical oxygen demand, dissolved oxygen, soluble and sorbed pesticide, and number of persistent bacteria.
- Determines sediment yield used for in-stream transport from the Modified Universal Soil Loss Equation (MUSLE). For sediment routing, deposition calculation is based on fall velocities of various sediment sizes.
- Estimates channel erodibility as a function of properties of the bed or bank materials and a channel (vegetation) cover factor. Rates of channel degradation are determined from Bagnold's stream power equation.
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