Predicting the interrill erosion rate on hillslopes incorporating soil aggregate stability on the Loess Plateau of China

The Water Erosion Prediction Project (WEPP) model is a physical process-based model that can predict both interrill and rill erosion on hillslopes. The interrill erodibility (Ki) is quantified by the soil particle size distribution, which is a key parameter in WEPP model. However, the important effects of soil structure on soil erodibility are ignored, which seriously affects the prediction accuracy of the model. In this study, simulated rainfall experiments (60, 90, 120 mm h-1) were performed to improve the prediction equation of interrill erosion incorporating the aggregate instability index (As). Rare earth elements (REEs) were employed as tracers to distinguish interrill and rill erosion on slopes in a box (2 m long, 0.5 m wide, 0.4 m deep) with gradients of 10 degrees, 20 degrees and 30 degrees. The Le Bissonnais method was used to determine the soil aggregate stability. The results showed that aggregate stability played a crucial role in interrill erosion under similar runoff rates. The improved equation contains four parameters, As, runoff rate (q), rainfall intensity (I) and slope gradient (Sf), with a coefficient of determination (R2) of 0.81 and model efficiency (ME) of 0.79. This equation exhibits greatly improved efficiency in estimating the interrill erosion rate compared to the WEPP model. Furthermore, a new equation incorporating the As, flow velocity (v), flow depth (h) and hydraulic gradient (J) was developed, with R2 of 0.90 and ME of 0.89. This study provides a deeper understanding of the mechanism of interrill erosion and improves upon the physical process-based soil erosion model.