SWAT Literature Database for Peer-Reviewed Journal Articles

Title:Unit source area data: Can it make a difference in calibrating the hydrologic reponse for watershed-scale modeling? 
Authors:Van Liew, M.W., C.H. Green, and P.I. Starks 
Journal:Journal of Soil and Water Conservation 
Volume (Issue):62(3) 
Article ID: 
URL (non-DOI journals):http://www.jswconline.org/content/62/3/162.short 
Broad Application Category:hydrologic only 
Primary Application Category:calibration, sensitivity, and/or uncertainty analysis 
Secondary Application Category:hydrologic assessment 
Watershed Description:four 0.6 to 4 ha unit source area watersheds and 4.3, 33.3, and 160 km^2 subwatersheds all in the Little Washita River Experimental Watershed in southwestern Oklahoma 
Calibration Summary: 
Validation Summary: 
General Comments: 
Abstract: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 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, and the usefulness of such parameters in calibrating the runoff response for a watershed scale model such as SWAT is not well documented. The objective of this study was to determine if model parameters that govern the surface runoff response in SWAT that were calibrated from rain-fed unit source area watersheds could be scaled up to provide accurate runoff simulations at a watershed scale. Model testing was conducted on four unit source area watersheds that consisted of homogeneous Bermuda grass, pasture, and winter wheat land cover types and three larger subwatersheds of the Little Washita River Experimental Watershed in southwestern Oklahoma. Data from the unit source area watersheds were used to calibrate parameters in SWAT that govern only the surface runoff output from the model. These parameter values were extended to the larger, 160 km^2 (61.9 mi2) subwatershed 526, and model simulations were then evaluated by examining both the surface runoff and total water yield response of the model. Simulation results from the unit source area watersheds suggest that the soil evaporation compensation factor (ESCO) in SWAT not only reflects soil field conditions for which it was intended to describe, but the impact of land management conditions on surface runoff response as well. Findings from this research indicate that if a value of 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 a surface runoff to total runoff fraction that is more than 15% too high. 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 infiltration functions or verifying values of the runoff curve number for watershed simulations. 
Keywords:calibration, hydrology, modeling, simulation, SWAT