Quantitative analysis of rainwater redistribution and soil loss at the surface and belowground on karst slopes at the microplot scale

Abstract

On karst slopes, soil and bedrock form soil-rock structural units (SRSUs), leading to complicated soil erosion for which quantitative attribution can hardly be achieved through whole-slope tests. This study aimed to quantify the role of different SRSUs in the soil erosion process, including rock covering the soil (RCS), bedrock exposed at the surface (RES), and a thin soil layer covering bedrock (SCR). Surface flow (SF), subsurface flow (SBF), infiltrated rainwater entering underground bedrock fissures (UF) and transported sediments were measured in soil boxes representing these SRSUs on karst slopes (5° and 25°) under 50 mm/h rainfall. The results indicated that among the three SRSUs, SF accounted for < 1% (2.5%-4.2%) of the total rainfall on the 5° slope (25° slope); SBF of the SCR, RES and RCS SRSUs accounted for 12.9% (37.9%), 14.9% (46.9%) and 10.8% (35.9%), respectively, of the total rainfall on the 5° slope (25° slope), and UF accounted for 28.6% (6.5%), 25.7% (10.0%) and 32.3% (3.7%), respectively, of the total rainfall. The SBF values of the SCR and RES SRSUs were 23% higher than that of the RCS SRSU on average. The UF values of the SCR and RES SRSUs were 16% lower than that of the RCS SRSU on the 5° slope and were 125% higher than that of the RCS SRSU on the 25° slope on average. Soil loss mainly occurred through SF, and the amount of soil lost through SBF or UF was < 0.01 g/L. These results suggest that both the type of SRSU and slope could greatly influence rainwater redistribution in soil and that the rain-driven soil loss through underground bedrock fissures was low on karst slopes. This occurred because the infiltrated rainwater or rock surface flow reaching the rock-soil interface (RSI) underground became preferential flow (RSI-PF). Thereafter, part of RSI-PF entered bedrock fissures as UF, whereas the remaining RSI-PF likely seeped into the soil and contributed to SBF or SF. Therefore, on the whole slope, which comprises different SRSUs with high heterogeneity, the soil erosion process is very complex because of the interaction between the surface and subsurface.