Soil and Water Assessment Tool (SWAT):

From the Blacklands Research Center, TAES

SWAT was developed to predict the effect of alternative management decisions on water, sediment, and chemical yields with reasonable accuracy for ungaged rural basins. The model was developed by modifying the SWRRB (Arnold et al., 1990) model for application to large, complex rural basins. Major changes involved (a) expanding the model to allow simultaneous computations on several hundred subwatersheds and (b) adding components to simulate lateral flow, ground water flow, reach routing transmission losses, and sediment and chemical movement through ponds, reservoirs, streams and valleys. SWAT operates on a daily time step and is capable of simulating 100 years or more. Major components of the model include, hydrology, weather, sedimentation, soil temperature, crop growth, nutrients, pesticides, ground water and lateral flow, and agricultural management.

The SWAT model boasts significant advantages over the combined SWRRB/ROTO (Arnold, 1990) model. SWAT offers distributed parameter and continuous time, flexible watershed configuration, irrigation and water transfer, lateral flow, ground water, and detailed lake water quality components. The distributed parameter, continuous time feature was achieved by adding a new routing structure to the SWRRB/ROTO model. The input data structures (Arnold et al., 1993) have been changed for the SWAT model. In SWRRB, channels are routed directly from the subbasin outlets to the basin outlet, while the SWAT reach routing structure routes and adds flow down through the basin reaches and reservoirs, allowing flexible basin configuration. In general, the basin is divided into subbasins by natural flow paths, boundaries, and channels required for realistic routing of water, sediment and chemicals, thus preserving watershed configuration. However, most of the models are one-dimensional and require lumping at the subbasin level. Four strategies for parameterizing the subbasins in the SWAT model (Arnold et al., 1993) include: 1. a three-dimensional grid; 2. two- dimensional hillslope; 3. multiple one-dimensional; and 4. lumped one-dimensional. Due to its flexible basin and subbasin configuration and routing structures, the SWAT model can read in measured stream flow and can be used to model areas where input data collection is impossible. Measured average monthly stream flow data can be directly input to one of the subbasins.