Modifications of the Soil and Water . Assessment Tool (SWAT) for Application to Climate Change Studies

The Soil and Water Assessment Tool (SWAT) is a water quality model developed by the Agricultural Research Service (ARS) to simulate runoff from agricultural watersheds. In order to obtain the level of accuracy required for simulating the effect of climate change on water quality in the runoff from northern and forested watersheds, a number of modifications were made to the model. The crop growth submodel was adapted to better represent forests, the snowmelt submodel was altered to improve the prediction of spring runoff (without requiring more input data than the original snowmelt submodel), and the evapotranspiration submodel was altered to more accurately simulate the evaporation, or sublimation, of snow. An alternate submodel for calculating rates of runoff and an alternate submodel for calculating percolation were also examined. Furthermore, a number of bugs in the model code were corrected. The changes made and the alternate submodels examined are described in this report. The changes made to the model code, including the bug fixes, are listed in the appendix. Finally, the changes in the simulations of water quality and quantity achieved by the model modifications are compared to field data.