Understanding the interaction between raindrop splash and concentrated flow on soil detachment capacity across different slope gradients

The interaction between raindrop splash and concentrated flow affecting soil detachment capacity (D_c) on gentle-to-steep slopes has rarely been studied. Three series of experiments (upstream inflow, raindrop splash, and upstream inflow with rainfall) were conducted on clay loam soil across different slope gradients. Results indicated that D_c was in the 0.0043 to 0.0058 kg m⁻² s⁻¹ range under raindrop splash conditions and in the 0.011 to 4.81 kg m⁻² s⁻¹ range under upstream inflow conditions. Compared with raindrop splash, concentrated flow scouring more significantly affected D_c. The presence of raindrop splash increased D_c by 6.45 %–85.64 % under lower unit width inflow discharge conditions and predominantly on gentler slopes under a limited number of experimental conditions. This result was attributed to the dominant effect of the additional flow discharge from rainfall-induced runoff, increasing the flow energy. Conversely, under most experimental conditions with higher inflow discharges and steeper slopes, raindrop splash reduced D_c by 10.44 %–81.91 %, which was attributed to the intensified turbulent motion and flow resistance, decreasing the flow energy. The relationships between D_c and slope gradient varied for gentle and steep slopes. A critical D_c threshold of 10°–12° slope gradients distinguished between gentle and steep slopes across all experimental treatments. The contribution of raindrop-impacted concentrated flow to D_c increased rapidly at lower flow discharges and on gentler slopes. Still, it increased more slowly at higher flow discharges and on steeper slopes. The contribution of raindrop-impacted concentrated flow to D_c ranged from 49.84 % to 99.78 %. The D_c reduction rates for this interaction effect ranged from –33.36 % to 82.02 %. This study revealed the interaction between raindrops and concentrated flow on D_c and established a slope threshold for D_c. These findings elucidate the mechanisms underlying soil detachment driven by raindrop-impacted concentrated flow and facilitate the development of process-based soil erosion models.