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Title: HYDROLOGIC SIMULATION ON AGRICULTURAL WATERSHEDS: CHOOSING BETWEEN TWO MODELS

Author
item Van Liew, Michael
item Arnold, Jeffrey
item Garbrecht, Jurgen

Submitted to: Transactions of the ASAE
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 8/1/2003
Publication Date: 10/1/2003
Citation: VAN LIEW, M.W., ARNOLD, J.G., GARBRECHT, J.D. HYDROLOGIC SIMULATION ON AGRICULTURAL WATERSHEDS: CHOOSING BETWEEN TWO MODELS. TRANSACTIONS OF THE AMERICAN SOCIETY OF AGRICULTURAL ENGINEERS. 2003. v. 46(6). p. 1539-1551.

Interpretive Summary: Hydrologic simulation models are important tools for water resources managers. However, more and more simulation models are becoming available with little guidance to help managers choose between models for a particular application in water resources. Improved methods are therefore needed to identify the most suitable model for a given application. A study was conducted to compare how well two hydrologic simulation models predicted streamflow on agricultural watersheds with differing sizes, soils, and land uses under varying climatic conditions. The study involved calibrating and validating the Soil and Water Assessment Tool (SWAT) and the Hydrologic Simulation Program-Fortran (HSPF) simulation models on eight agricultural subwatersheds within the Little Washita River Experimental Watershed (LWREW) and two agricultural watersheds adjacent to the LWREW within the Washita River Basin in Southwestern, OK. Calibration is the process of assigning specific values to the simulation model so that it accurately simulates runoff from the watershed. Validation is used to verify that the model has been properly calibrated by applying it to different sets of data than were used for calibration. Two subwatersheds within the LWREW were first used to calibrate parameters in both models for a period of record when rainfall on the watershed was higher than normal. This period was referred to as a "wetter than average" climatic condition. Both models were then applied to six other subwatersheds within the LWREW and the two adjacent watersheds, under varying climatic conditions. Five evaluation criteria provided valuable information for better understanding of how well the two models simulated the overall water balance, the overall fit of the monthly and daily hydrographs, and the range in magnitude of the daily flows for various soil, land use, and climatic conditions. A comparison of model performance showed that while HSPF performed better on the watersheds used for calibration, SWAT gave better results on the validation watersheds. Since SWAT performed better than HSPF in estimating streamflow for agricultural watersheds under various climatic conditions, it may be better suited for investigating the long-term impacts of climate variability on water resources.

Technical Abstract: Strengths and limitations of hydrologic simulation models are used as criteria for selecting a particular model for a given water resources application. The performance of the Soil and Water Assessment Tool (SWAT) and the Hydrologic Simulation Program-Fortran (HSPF) simulation models was compared on eight nested agricultural watersheds within the Little Washita River Experimental Watershed (LWREW) and two agricultural watersheds adjacent to the LWREW within the Washita River Basin in Southwestern, OK. Two subwatersheds within the LWREW were first used to calibrate parameters in both models for a "wetter than average" period of record. Both models were then applied to six other subwatersheds within the LWREW and the two adjacent watersheds, under varying climatic conditions. Three quantitative and 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 better on the watersheds used for calibration, SWAT gave better results on the validation watersheds. On one of the validation subwatersheds adjacent to the LWREW, values of deviation of streamflow volume were -38.7%, -13.3%, and -1.3% for SWAT and -64.3%, -81.1%, and -8.2% for HSPF under "much dryer than average," "dryer than average," and "near average" climatic conditions, respectively. Differences in model performance were mainly attributed to the runoff production mechanism in the two models. Results of this study showed that SWAT exhibited an element of robustness in that it gave more consistent results than HSPF in estimating streamflow for agricultural watersheds under various climatic conditions. SWAT may therefore be better suited for investigating the long-term impacts of climate variability on surface-water resources.