Background Soil erosion on the red soil area derived from granite in southern China leads to serious soil quality degradation and environmental problems. Rill erosion is the beginning of increasing soil erosion and demonstrates significant contributions to slope soil erosion of runoff and sediment yields. The aim of this study is to explore the hydraulic characteristics and dynamic mechanisms at the threshold moment of rill erosion occurring on the slope of granite red soil by simulating rainfall.

Methods Topsoil of typical granite red soil from western Fujian was selected as an experimental object. A steel channel soil could be filled in the slope was subjected to indoor simulated rainfall experiment under two rainfall intensities (70 and 100 mm/h) and three slope gradients (10, 15 and 20°). During rainfall, time of slope runoff, yielding time and position of falling were recorded. The runoff flow velocity on the slope was measured by the dyeing method. Within a set time interval, runoff and sediment yield were calculated by a water bottle used to collect runoff samples from the bottom of the tank. Reynolds number, Froude number, and other related hydrodynamic parameters were calculated accordingly.

Results 1) When the rainfall intensity is relatively large, the red soil area derived from granite is easier from surface erosion to gully erosion. The velocity range while rill erosion occurring is 0.074-0.103 m/s. 2) When rill erosion occurs, most of the flow pattern of slope runoff present "mixed flow-slow flow". The average velocity, shear stress, Darcy-Weisbach resistance, stream power, Reynolds number, and unit energy of water-carrying section increase with the increase of rain intensity and slope. However, the variation law of unit water power is not obvious. Meanwhile, from the perspective of runoff or sediment, the Reynolds number and the composite hydrodynamic characteristic parameter μ (the geometric square root of the Reynolds number and the Froude number) composed of the Reynolds number and the Froude number can be used as the hydrodynamic parameters to characterize the erosion of the red soil slope developed by granite in the west of Fujian province under rainfall conditions when rill erosion occurs.

Conclusions The experimental results show that at the moment of rill erosion occurring, rainfall intensity has a greater impact on it, and the hydrodynamic parameters can well represent the erosion characteristics of the red soil area derived from granite. The results of the study have great significance for elucidating the hydrodynamic characteristics of soil erosion, obtaining the evolution process of erosion patterns, controlling soil erosion and restoring the ecological environment in the south of China.