Hydrologic Response of a Small Watershed Model to Generated Precipitation

Watershed models such as SWAT (Soil and Water Assessment Tool) simulate water quality impacts of land andwater resource management alternatives. To simulate these impacts, long-term daily rainfall data are necessary. In theabsence of measured rainfall data, watershed models use weather generators to simulate rainfall events. The objective ofthis study is to examine several daily precipitation generators in terms of the hydrologic response of SWAT. SWAT isgenerally applied to large river basins but has been validated and applied on the small watershed scale as well. Dailyrainfall inputs included a 60-year measured rainfall record from 1939 to 1998 for Riesel, Texas, in the heart of theBlackland Prairie, and data generated with the precipitation components of three weather generation programs: WGEN,WXGEN, and USCLIMATE. Measured and generated rainfall were input into SWAT and run for a 53 ha watershed nearRiesel, Texas. Rainfall totals, extreme rainfall events, and the resulting hydrologic responses of runoff volume and peakflows were then examined. For this study scenario, WXGEN was able to more closely match observed rainfall thanWGEN and USCLIMATE. In terms of resulting SWAT hydrologic response, WXGEN rainfall best reproduced runoffvolumes simulated with measured rainfall, and USCLIMATE performed better in reproducing peak runoff rates. These areimportant results as probabilities of exceeding runoff volume or peak flow thresholds are often questions of interest inwatershed projects.