HYDROLOGIC SIMULATION ON AGRICULTURAL WATERSHEDS: CHOOSING BETWEEN TWO MODELS

Abstract

Strengths and limitations of hydrologic simulation models are used as criteria for selecting a particular modelfor a given water resources application. The performance of the Soil and Water Assessment Tool (SWAT) and the HydrologicSimulation Program-Fortran (HSPF) continuous simulation models was compared on eight nested agricultural watershedswithin the Little Washita River Experimental Watershed (LWREW) and two agricultural watersheds adjacent to the LWREWwithin the Washita River Basin in southwestern Oklahoma. Two subwatersheds within the LWREW were first used to calibrateparameters in both models for a wetter than average period of record. Both models were then applied to six othersubwatersheds within the LWREW and the two adjacent watersheds, under varying climatic conditions. Three quantitativeand two qualitative evaluation criteria were used to assess streamflow simulated by SWAT and HSPF: computation of(1) deviation of streamflow volume, (2) coefficient of efficiency, and (3) prediction efficiency and visual inspection of(4) hydrographs and (5) flow duration curves. A comparison of model performance showed that while HSPF performed betteron the watersheds used for calibration, SWAT gave better results on the validation watersheds. On one of the validationsubwatersheds adjacent to the LWREW, values of deviation of streamflow volume were -38.7%, -13.3%, and -1.3% for SWATand -64.3%, -81.1%, and -8.2% for HSPF under much dryer than average, dryer than average, and near averageclimatic conditions, respectively. Differences in model performance were mainly attributed to the runoff productionmechanism in the two models. Results of this study showed that SWAT exhibited an element of robustness in that it gave moreconsistent results than HSPF in estimating streamflow for agricultural watersheds under various climatic conditions. SWATmay therefore be better suited for investigating the long-term impacts of climate variability on surface-water resources.