Comparative study of soil erodibility and critical shear stress between loess and purple soils

Loess and purple soils are two very important cultivated soils, with the former in the loess region and the latter in southern sub-tropical region of China, featured with high-risks of erosion, considerable differences of soil structures due to differences in mineral and nutrient compositions. Study on soil erodibility (Kr) and critical shear stress (τc) of these two soils is beneficial to predict soil erosion with such models as WEPP. In this study, rill erosion experimental data sets of the two soils are used for estimating their Kr and τc before they are compared to understand their differences of rill erosion behaviors. The maximum detachment rates of the loess and purple soils are calculated under different hydrodynamic conditions (flow rates: 2, 4, 8 L/min; slope gradients: 5°, 10°, 15°, 20°, 25°) through analytical and numerical methods respectively. Analytical method used the derivative of the function between sediment concentration and rill length to estimate potential detachment rates, at the rill beginning. Numerical method estimated potential detachment rates with the experimental data, at the rill beginning and 0.5 m location. The Kr and τc of these two soils are determined by the linear equation based on experimental data. Results show that the methods could well estimate the Kr and τc of these two soils as they remain basically unchanged under different hydrodynamic conditions. The Kr value of loess soil is about twice of the purple soil, whereas the τc is about half of that. The numerical results have good correlations with the analytical values. These results can be useful in modeling rill erosion processes of loess and purple soils.