Effects of erosion degree and rainfall intensity on erosion processes for Ultisols derived from quaternary red clay

Soil erosion poses a major threat to the sustainability of soil and water resources. Soil horizons differ in their properties due to pedogenic differentiation. Currently, limited research attempts have been made to assess the impact of the removal of topsoil horizons on the subsequent erosion process. The main objective in this study was to investigate the effects of erosion degree and rainfall intensity on erosion process and sediment transport mechanism. Field plot experiments were conducted on pre-wetted bare fallow Ultisols (derived from quaternary red clay) under four erosion degrees (no (E0), moderate (E1), severe (E2), and very severe (E3)) and two rainfall intensities (60 and 120 mm h−1). The erosion degrees were judged according to the outcrop of eluvium, illuvium (B1, B2) and parent material horizons. The simulated rainfall lasted one hour after runoff generation, and runoff and sediment were sampled at 3-min intervals to determine the runoff coefficient, sediment concentration, soil detachment rate and sediment effective size distribution. Runoff coefficient was negatively correlated with bulk density (r = −0.76, p < 0.05). Sediment concentration and soil detachment rate decreased with an increased erosion degree at the low rainfall intensity while they decreased in the order of E1 > E0 > E2 > E3 at the high rainfall intensity. Rainfall intensity and clay content played the negative and positive roles in the erosion process, respectively (Adj-R2 > 0.80, p < 0.01). For E3, sediment transport was dominated by suspension/saltation (<0.1 mm) while for the other erosion degrees it was dominated by mechanisms of both suspension/saltation and rolling (2-0.5 mm). The average sediment size was positively correlated with silt content (r = 0.82, p < 0.05). The high variation of erosion process with erosion degree was intrinsically attributed to the difference in soil horizon nature (mainly soil texture and bulk density) and also influenced by rainfall intensity to varying extent, which should be considered in future erosion prediction.