Comparing erosion and rill development processes by simulated upslope inflow in two red soils from subtropical China

Rill erosion is an important component of soil erosion, however, the process and mechanism of rill erosion on red soil slopes are not clear enough. In this paper, runoff scouring experiments were conducted with five varying inflow rates (8.5, 10, 11.5, 13, 14.5 L min−1 m−1) and one slope (8°) using two typical red soils in subtropical China: Ultisols derived from shale (UDS) and Ultisols derived from quaternary red clay (UDQ). The results showed that for the two red soils, runoff rate, sediment yield rate, and rill erosion amount were all positively correlated with inflow rates in linear relationships (p < 0.05). The high sand and low clay contents in eroded sediments were closely related to the original soil texture (dominated by clay and silt) and rill flow energy. Stream power and unit stream power were the optimal hydraulic parameters to characterize rill erosion mechanisms for UDS and UDQ, respectively. For UDQ with higher clay content and bulk density, the sediment yield process was more stable and the sand content in the eroded sediments was significantly higher than that of UDS, while the silt content was significantly lower. The rill flow of UDQ had slower velocity and larger Darcy–Weisbach resistance coefficient than that of UDS. For UDS with lower clay and higher sand content, the average sediment yield rate and rill depth were 2.26–4.71 and 2.79–6.53 times of UDQ, respectively, and the rill erodibility parameter was 1.84 times that of UDQ. The results are helpful for understanding the rill erosion mechanism and preventing soil loss in subtropical areas.