SWAT is a free river basin model from USDA-ARS and Texas A&M that simulates water, sediment, nutrients and crops under different land uses.
SWAT, the Soil and Water Assessment Tool, is a river basin model developed by the USDA Agricultural Research Service (USDA-ARS) and Texas A&M AgriLife Research. It is free and widely used to study how land use, farming practice and climate affect water, sediment and nutrients over long periods in large catchments. SWAT is semi-distributed and runs on a daily time step. It divides a basin into subbasins, then splits each subbasin into hydrologic response units (HRUs). An HRU is a unique mix of land use, soil and slope. For each HRU the model keeps a soil water balance: rainfall in, minus surface runoff, evapotranspiration, deep seepage and return flow. Surface runoff is usually found with the SCS curve number method, and sediment with the Modified Universal Soil Loss Equation. The model also grows crops, applies fertiliser and irrigation, and tracks nitrogen, phosphorus and pesticides. Flows from the subbasins are then routed down the stream network. Inputs are a digital elevation model, land use and soil maps, daily weather (rainfall, temperature and others) and management details. Outputs include streamflow, sediment load, nutrient loads, crop yield and the water balance for each HRU, subbasin and reach. For example, a team can set up SWAT for an agricultural basin, calibrate it to measured flow and nitrate at a gauge, then test scenarios such as buffer strips, reduced fertiliser or a drier climate to see the change in nutrient loading at the outlet. SWAT handles many processes in one model and has a large user base and free data tools. Its limits: it has many parameters, so calibration is needed and equifinality is common. Classic HRUs have no fixed location in the subbasin, so it cannot say where in a field runoff starts. Daily steps suit long-term studies more than single flood peaks.