Background Soil detachment is the initial phase of soil erosion. Quantifying soil detachment capacity is crucial to establish the process-based soil erosion model. There is obvious difference in soil detachment mechanisms between gentle slope and steep slope, but the mechanism of slope effect is still unclear. Thus, the objective of this study is to reveal the slope effect of soil detachment capacity.

Methods The soil samples from abandoned cropland on yellow soil slope in the upper and middle Yangtze River were selected as research object. The effect of hydrodynamic parameters (i.e., flow rate, flow depth, flow velocity, Darcy-Weisbach resistance coefficient, flow shear stress, stream power, unit stream power and unit energy of water-carrying section and so on) on soil detachment capacity were analyzed by Pearson correlation analysis and regression analysis under different slope gradients (1.75%-48.77%) and flow rates (0.21-2.63 L/s).

Results 1) The stream power, which represented the flow energy, was the best parameter to describe the soil detachment capacity, followed by the flow shear stress associated with forces. However, the water depth and Darcy-Weisbach resistance coefficient were the worst to describe the soil detachment capacity. 2) Based on the calculation equation of soil detachment capacity of WEPP, the response of soil erodibility and critical shear stress to slope was relatively sensitive and not constant value. The soil erodibility decreased firstly and then increased as the slope gradient increased, and the critical shear stress changed from power function to linear function as the slope gradient increased. This result showed that the constant values of soil erodibility and critical shear stress for previous studies can be extended to different slopes has great limitations. 3) There was obvious slope effect in using the power function of flow shear stress or stream power to characterize soil detachment capacity. The coefficient a₁ of flow shear stress changed from increased firstly and then decreased as a quadratic function to decreased as an exponential function with increasing slope gradient, and the maximum value reached at 17.63% of slope. The exponent b₁ of flow shear stress showed a quadratic function relationship, which decreased firstly and then increased with the increase of slope gradient, and the slope reached the minimum value at 26.71% of slope. However, the coefficient a₂ of stream power decreased as a power function with increasing slope gradient. The exponent b₂ of stream power decreased firstly and then increased as a quadratic function, and it reached the minimum value when the slope was 28.10%.

Conclusions This study illustrated that the results of gentle slopes cannot be directly generalized to steep slopes and reveal the slope effects and its thresholds of soil detachment capacity. The results of the study provide new insights into understanding soil detachment mechanism.