The Soil and Water Assessment Tool (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. The main purpose of the model is to predict the effects of management decisions on water, nutrients, sediments and pesticide yield with reasonable accuracy on large, ungaged river basins. It is a distributed model that simulates all previously mentioned constituents on a daily time step. SWAT defines hydrology by a specific list of parameters including interception, evapotranspiration, surface runoff, lateral flow, soil percolation and ground water flow as well as river routing processes.
Please visit http://swatmodel.tamu.edu for further details about SWAT and a free software download.