EVALUATION OF WEPP RUNOFF AND SOIL LOSS PREDICTIONS USING NATURAL RUNOFF PLOT DATA

Model testing and evaluation are critical to the acceptance of any new prediction tool. This study was conducted to evaluate the overall per formance of the Water Erosion Prediction Project (WEPP) hillslope mode l in predicting runoff and soil loss under cropped conditions. Natural runoff plot data, including 4,124 selected events, 556 plot years, an d 34 cropping scenarios, from eight locations were selected The averag e length of record for the cropping scenarios was about nine years. Se veral common crops and tillage systems were included. The WEPP input f iles for soil, slope, climate, and crop management were compiled based on measured data. The coefficient of determination (r(2)) between mod el-predicted and measured-runoff volumes for optimized Green and Ampt hydraulic conductivity (K-b) was 0.77 for selected events, 0.76 for an nual values, and 0.87 for average annual values; the r(2) between pred icted and measured soil losses (excluding fallow and corn plots at Bet hany, Mo,) was 0.36, 0.60, and 0.85, respectively. Similar predictions of runoff and soil loss were also obtained with WEPP internally estim ated K-b values. Runoff and soil loss were slightly overpredicted for small storms and for years with low runoff and soil loss rates, and we re underpredicted for large storms and for years with high runoff and soil loss rates. However, average runoff and soil loss rates for diffe rent cropping and management systems were adequately predicted. The ac curacy and reliability of the predictions were shown to improve from a n event to annual to average annual basis. Results of this study show that the WEPP model is a useful tool for predicting runoff and soil lo ss rates under cropped conditions.