A new RUSLE slope length factor and its application to soil erosion assessment in a Loess Plateau watershed

The Universal Soil Loss Equation and Revised Universal Soil Loss Equation have been widely used in watershed scale soil erosion assessments. However, the influences of the coupled effects of upslope topography and vegetation cover on flow accumulation and the downslope soil erosion are not fully considered in the current methods. In this study, a new calculating method for slope length factor (L), named the modified grid formula based on upslope contributing areas (LM), was established by adding a new parameter, the effective contributing area ratio, to the existing L calculating method, the grid formula based on upslope contributing areas (LG). A small watershed on the Loess Plateau was selected to evaluate the effects of the LM on watershed soil erosion assessment. The soil erosion was estimated via the Revised Universal Soil Loss Equation using with two L calculating methods: LM and LG. Then, the soil loss of various land use types estimated from the two L methods were compared with the corresponding observed values from the nearby runoff plots, and the sediment yields based on the two L methods were estimated using the Sediment Delivery Distributed model and compared with measured values at the hydrologic station in the outlet of the watershed. The results indicated that, compared with LG, 1) the L values obtained via the LM were lower, contributing to a substantial decrease in soil erosion estimated at the watershed scale; 2) the average annual soil erosion estimated via the LM for slope farmland, forestland and grassland in the watershed were closer to the observed values from local runoff plots; 3) the average annual sediment yield of the watershed estimated via the LM was only ±5% relative to the measured values, implying that the values estimated via LM was more accurate than the current methods. Overall, the LM has accounted for the effect of vegetation hydrologic process and erosion process, which can improve the estimation accuracy of soil erosion assessments at the watershed scales and limit the risk of overestimating.