Assessing applicability of the WEPP hillslope model to steep landscapes in the northern Loess Plateau of China

Abstract The Water Erosion Prediction Project (WEPP) model has been extensively evaluated at lower slope gradients, but its applicability to steep slopes is not yet known. The objectives of this study were to evaluate the WEPP model’s ability to predict runoff and soil erosion on steep slopes in the hilly-gully region of the Loess Plateau of China, and to provide insights for adapting it for this region. Field runoff and soil loss data collected from six bare slope gradient plots, four bare slope length plots, and four cropped plots at the Ansai experimental station during 1985–1992 were used to calibrate, validate, and evaluate the WEPP hillslope model. Measured rainfall intensity (rainfall break point data) was directly used to minimize climate-induced prediction errors. Overall, the calibrated WEPP model predicted event, annual, and average annual runoff and soil loss for different slope gradients, slope lengths, and cropped conditions reasonably well with model efficiencies (ENS) being greater than 0.5 for most cases, but there is still room for improvement. Specifically, for the slope gradient plots, simulated runoff was somewhat insensitive to slope increases at large slope gradients while measured runoff tended to increase with slope steepness due to a decrease in surface storage capacity; however, simulated average annual soil loss, though satisfactory (ENS = 0.83), was slightly oversensitive to slope increases at large slope gradients. For the slope length plots, simulated average annual soil loss was overly responsive to slope length increases at steep slopes, indicating a potential deficiency in representing slope length influence in the model for large slope gradients. The use of the default 1-m rill spacing might have also affected the sensitivity of WEPP-predicted soil loss to slope gradients and lengths, because rill spacing varies with slope steepness and lengths and WEPP –predicted erosion is sensitive to rill spacing. For the cropped plots, WEPP tended to underpredict runoff and soil loss, indicating that the internal model adjustments of hydraulic conductivity and soil erodibility for the effects of factors like roots and residue need to be reexamined for use on steep slopes.