Sensitivity Analysis of the Soil and Water Assessment Tool (SWAT) for Simulation of Climate Change Effects

A sensitivity analysis of the input parameters of the Soil and Water Assessment Tool (SWAT), developed by the USDA Agricultural Research Service, was conducted. The effects of input parameters describing watershed characteristics and land management practices on SWAT results were analyzed using individual parameter perturbation. The model outputs were monthly streamflow (mm) and monthly averages for sediment yield (t/day), total phosphorus yield (kg/day), ammonia/organic N yield (kg/day), and nitrate/nitrite yield (kg/day). The results were reported qualitatively, not quantitatively, because of the large number of input parameters required for SWAT. Streamflow was most sensitive to the SCS runoff curve number, sediment yield to sediment routing parameters, total P and ammonia/organic N yields to land management practices and to P and N concentrations in the top soil layer, and nitrate/nitrate yield to concentrations in the top soil layer. The effects of input parameters describing climate. conditions were analyzed using error analysis. The climate parameters were monthly precipitation; monthly averages for maximum, minimum, and mean daily air temperature; and monthly averages for relative humidity, solar radiation, and wind speed. SWAT was run using historical weather data and modified historical weather data representing a doubling of atmospheric CO2 concentration. The percent of the variations in the output variables that were explained by the variations in the climate parameters were calculated. Variations in precipitation and, when snowmelt was a significant part of the hydrologic budget, in temperature explained the variations in the model output the most.