SOIL AND WATER ASSESSMENT TOOL (SWAT)
Soil and Water Assessment Tool (SWAT) is the continuation of a long-term effort of nonpoint source pollution modeling with the US Department of Agriculture -Agricultural Research Service (ARS). SWAT is a continuous time model that operates on a daily time step. The objective in model development was to predict the impact of management on water, sediment and agricultural chemical yields in large ungaged basins. To satisfy the objective, the model (a) is physically based (calibration is not possible on ungaged basins); (b) uses readily available inputs; (c) is computationally efficient to operate on large basins in a reasonable time, and (d) is continuous time and capable of simulating long periods for computing the effects of management changes. SWAT uses a command structure for routing runoff and chemicals through a watershed similar to the structure of HYMO (Williams and Hann, 1973). Commands are included for routing flows through streams and reservoirs, adding flows, and inputting measured data or point sources. Using a routing command language, the model can simulate a basin subdivided into grid cells or subwatersheds. Additional commands have been developed to allow measured and point source data to be input to the model and routed with simulated flows. Also, output data from other simulation models can be input to SWAT. Using the transfer command, water can be transferred from any reach or reservoir to any other reach or reservoir within the basin. The user can specify the fraction of flow to divert, the minimum flow remaining in the channel or reservoir, or a daily amount to divert. The user can also apply water directly to a subbasin for irrigation. Although the model operates on a daily time step and is efficient enough to run for many years, it is intended as a long term yield model and is not capable of detailed, single-event, flood routing.