Effects of rock outcrops on runoff and erosion from karst slopes under simulated rainfall

Abstract

Rock outcrops play a crucial role in affecting hydrological and erosion processes on karst hillslopes. However, due to various difficulties in constructing runoff plots on field rocky slopes, previous studies used small metal flumes to simulate outcropped slopes, which greatly differed from the natural rock outcropped slopes. In this study, the effects of rock outcrops on runoff and erosion were studied on karst slopes with natural soil–rock conditions at a larger plot scale (10 m in length × 2.2 m in width, 25° in gradient). Simulated rainfall experiments were conducted on the four runoff plots with different rock outcrop coverages (0%, 5%, 13%, and 42%) under five rainfall intensities (30, 60, 90, 120, and 150 mm·h−1). The study indicated that the rock outcrops complexed the water erosion processes by playing multiple and opposite roles simultaneously. It positively influences water erosion by serving as impermeable surfaces, providing preferential flow paths, and offering flow barriers to hillslopes. Conversely, it can exacerbate erosion by causing increased runoff, concentrating flow paths, and thus intensifying erosion in specific areas. Besides, the effects of these roles varied with the coverage of rock outcrops and rainfall intensity. The main results showed that (i) limited rock outcrop coverage (5% and 13%) increased runoff and erosion, but the opposite was observed on the severe rocky slope (42%). Both “infiltration‐excess” and “saturation‐excess” mechanisms coexisted by turns on karst slopes during rainfall; the break of “saturation balance” tended to occur on the severe rock outcrop slope with abundant soil–rock interface and underground fissures. (ii) Rock outcrops greatly increased the spatial variability and uncertainty of hydrological and erosion processes on karst slopes. The rock outcrops resulted in a larger range of variation in flow velocity and a greater difference among the three slope positions; it also brought more spatial heterogeneity in the development of rill erosion. (iii) Rock outcrops tended to increase flow velocity, and intensify flow turbulence, stream power, and flow energy. The severe rock outcrops significantly reduced the soil erosion induced by the limited soil detachment. This study contributes to a deeper understanding of the mechanisms behind how rock outcrops influence runoff and erosion and provides insights for improving soil erosion prediction in karst regions.