Unit source area data: Can it make a difference in calibrating the hydrolgoic response for watershed scale modeling?

Authors: Van Liew, Michael; Rossi, Colleen; Starks, Patrick

Submitted to: Journal of Soil and Water Conservation
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 3/16/2007
Publication Date: 5/1/2007
Citation: Van Liew, M.W., Green, C.H., Starks, P.J. 2007. Unit source area data: Can it make a difference in calibrating the hydrolgoic response for watershed scale modeling? Journal of Soil and Water Conservation. 62(3):162-170.

Interpretive Summary: Watershed computer models such as the Soil and Water Assessment Tool (SWAT) contain parameters that describe watershed properties such as vegetative cover, soil characteristics, or landscape features. For investigations such as USDA’s Conservation Effects Assessment Project that involve changes in land cover or land management on agricultural lands, proper adjustment of these parameters is important not only for runoff estimation, but also for the simulation of sediment, nutrients, and other pollutants. However, these parameters may only be known for a few small, homogeneous areas (2 to 12 acres in size) such as unit source area watersheds, and the usefulness of such parameters in calibrating the runoff response for a watershed scale model such as SWAT is unknown. The objective of this study was to determine how unit source area watershed data from the Little Washita River Experimental Watershed (LWREW) in Southwestern Oklahoma could be used for calibrating parameters that control the surface runoff response in SWAT. The advantages and disadvantages associated with applying values of the parameters calibrated from the unit source area watersheds to a larger, 62 square mile subwatershed on the LWREW were evaluated by examining both the surface and subsurface runoff response of the model. Results obtained from the calibration performed on the unit source area watersheds suggest that a winter wheat land cover type produces about 5 to 8 times as much surface runoff as does a pasture cover type. Test results also indicate that if a value of the soil evaporation compensation factor (ESCO) that was calibrated from the unit source area watershed data for winter wheat was applied at the watershed scale, it would lead to model simulations that give unrealistically high values of surface runoff. Due to uncertainties in relating ESCO to soil and land management properties, results of this study suggest that runoff data from unit source area watersheds may be best suited for calibrating parameters like the runoff curve number in SWAT that estimates the amount of surface runoff for a given rainstorm.

Technical Abstract: The use of homogeneous, unit source area watersheds (USAWs) holds promise for calibrating watershed scale, hydrologic simulation models such as the Soil and Water Assessment Tool (SWAT). The objective of this study was to determine how data from USAWs in the Little Washita River Experimental Watershed (LWREW) in Southwestern Oklahoma could be used to calibrate parameters that govern the surface runoff output from SWAT. The strengths and weaknesses associated with upscaling values of the parameters calibrated from the USAWs to the larger, 160 km2 subwatershed 526 on the LWREW were evaluated by examining both the surface runoff and total water yield response of the model. Results obtained from the calibration performed on the USAWs suggest that a winter wheat land cover type produces about 5 to 8 times as much surface runoff as does a pasture cover type. Test results also indicate that if a value of the soil evaporation compensation factor (ESCO) that was calibrated from the unit source area watershed data for winter wheat was applied at the watershed scale, it would lead to model simulations that give unrealistically high values of surface runoff. Due to uncertainties in relating ESCO to soil and land management properties, results of this study suggest that runoff data from USAWs may be best suited for calibrating infiltration functions or verifying values of the runoff curve number for watershed simulations. For investigations such as USDA’s Conservation Effects Assessment Project that involve changes in land cover or land management on agricultural lands, proper adjustment of these parameters is critical not only for runoff estimation, but also for the simulation of sediment, nutrients, and other water quality constituents.