HYDROLOGIC CALIBRATION OF THE SWAT MODEL IN A WATERSHED CONTAINING FRAGIPAN SOILS

The Soil and Water Assessment Tool (SWAT) model, designed for use on r ural ungaged basins and incorporating a GRASS GIS interface, was used to model the hydrologic response of the Ariel Creek watershed of north eastern Pennsylvania. Model evaluation of daily flow prior to calibrat ion revealed a deviation of runoff volumes (D-v) of 68.3 percent and a Nash-Sutcliffe coefficient of -0.03. Model performance was affected b y unusually large observed snowmelt events and the inability of the mo del to accurately simulate baseflow, which was influenced by the prese nce of fragipans. Seventy-five percent of the soils in the watershed c ontain fragipans. Model calibration yielded a D-v of 39.9 percent and a Nash-Sutcliffe coefficient of 0.04, when compared on a daily basis. Monthly comparisons yielded a Nash-Sutcliffe coefficient of 0.14. Snow melt events in the springs of 1993 and 1994, which were unusually seve re, were not adequately simulated. Neglecting these severe events, whi ch produced the largest and third largest measured flows for the perio d of record, a D-v of 4.1 percent and Nash-Sutcliffe coefficient of 0. 20 were calculated on a daily comparison, while on a monthly basis the Nash-Sutcliffe coefficient was 0.55. These results suggest that the S WAT model is better suited to longer period simulations of hydrologic yields. Baseflow volumes were accurately simulated after calibration ( D-v = -0.2 percent). Refinements made to the algorithms controlling su bsurface hydrology and snowmelt, to better represent the presence of f ragipans and snowmelt events, would likely improve model performance.