Response of flow hydraulic parameters to different rock fragment coverages and sizes under simulated rainfall

Rock fragments widely occur on slopes and play a vital role in flow hydraulics, which in turn greatly affects soil erosion. This study aimed to explore the effects of the rock fragment coverage and size on flow hydraulic parameters and determine the optimal flow hydraulic predictor of soil loss rate. A series of simulated rainfall experiments (intensity: 60 mm h-1) were conducted in a 2 x 1 x 0.3 m flume with a gradient of 15 degrees. The rock fragment treatments included a combination of eight sizes (0.7, 1.1, 3.0, 5.5, 12.0, 18.8, 26.4, 36.0 cm) and four coverages (15, 30, 45, 60%). Bare land was used as a control. The results indicated that compared with bare land, rock fragments remaining on the soil surface decreased the flow velocity (V) by 1-51% but increased the flow depth by 8-315%. Laminar flow occurred in all experiments (Reynolds number < 500). Only the overland flow under rock fragment sizes <= 3 cm was subcritical flow. Resistance (f) exponentially increased with rock fragment coverage at each size, while grain resistance percentage (f'/f) exponentially decreased with rock fragment coverage. Rock fragment size impacted positively on f'/f, grain shear stress (tau'), stream power (omega), grain stream power (omega'), but negatively on f. tau' and omega' decreased with rock fragment coverage in exponential equations, and the parameters in the equations logarithmically increased with rock fragment size. The soil loss rate increased exponentially with V and tau', and increased linearly with omega'. omega' was the optimal hydraulic parameter for soil loss prediction (R2 = 0.88; NSE = 0.80). Our findings can help to deepen the understanding of the mechanisms of the soil erosion process and provide insights into establishing soil loss prediction models on stony slopes.