Runoff generation and erosion processes at the rock–soil interface of outcrops with a concave surface in a rocky desertification area

Abstract

Rock surface flow, originating from outcrop surfaces during rainfall, represents a distinctive form of runoff influencing surface erosion and subsurface soil leakage in karst rocky desertification areas. Despite their significance, the processes governing runoff generation and erosion at the rock–soil interface (RSI) under the influence of rock surface flow remain poorly understood. This study aims to elucidate the generation and transformation of runoff at the RSI, along with their scouring erosion effects influenced by rock surface flow from concave-shaped outcrops. Artificial rainfall experiments were conducted on simulated rock–soil structural units featuring exposed rock surfaces and unexposed RSIs of outcrops with concave shapes. Two driving factors, namely rainfall intensity (R) and the inclination of the rock surface (IRS), were manipulated to investigate the formation and output processes of runoffs and sediments from the surface, RSI, and non-RSI (a site away from the RSI). The presence of an exposed rock surface altered the distribution pattern of rainfall–infiltration–runoff. An increase in the IRS led to a significant decrease in surface runoff (p < 0.05). When the IRS was close to vertical, the rainfall-runoff was primarily in the form of surface runoff; however, for inclinations less than 90°, the primary contributors to rainfall-runoff were the RSI and non-RSI runoffs. Surface soil erosion emerged as the predominant form of soil loss, with runoff yield demonstrating a linear positive correlation with sediment production. Both the IRS and R were identified as significant factors affecting surface runoff and soil erosion. Notably, when the IRS was less than 45°, the total soil loss reached its minimum. Therefore, runoff generation at the RSI led to a reduction in surface runoff and an increase in subsurface runoff, contributing to soil leakage at the RSI. Nevertheless, surface erosion remained the primary mechanism of soil loss.