Contributions of flow discharge, slope gradient, and scouring time on rill erosion: A quantitative study of exposed slopes in the loess region

Flow discharge, slope gradient, and erosion time are widely recognized as crucial factors in determining rill erosion and its morphological characteristics. However, the relative importance of these three factors needs to be further elaborated to refine the understanding of rill erosion. In the current study, scour experiments were done under various conditions, including five flow discharges (0.5, 1.0, 1.5, 2.0, and 2.5 mm/min), five slope gradients (5°, 10°, 15°, 20°, and 25°), and durations of 20, 40, and 60 min. The resulting rill erosion process and its related morphological characteristics have been documented and analyzed. The results demonstrate that the average soil loss rate increased significantly with the increase inflow discharge and slope gradient. The individual effect of flow discharge (38.35%) was more pronounced than that of slope gradient (18.38%). Increasing flow discharge, slope gradient, and scouring time intensified the occurrence of headward erosion. Over extended erosion durations, rill length, width, depth, and volume all experienced increases. Additionally, with higher flow discharge and steeper slope gradient, the rill width-depth ratio decreased, indicating that rills became narrower and deeper. The individual effect of flow discharge on all rill morphological characteristics was more pronounced than that of slope gradient and scouring time. Except for rill length, the slope gradient had a greater impact on rill morphological characteristics than scouring time. Importantly, a significant portion of the runoff's potential energy was channeled into soil erosion rather than kinetic energy in sediment-laden flow. Based on the principle of energy conservation, the occurrence of rills reduced the energy required for soil erosion from 83.84-598.93 J/kg to 2.22-37.53 J/kg. The current study deepens the understanding of rill erosion mechanisms on the Loess Plateau in China and provides a scientific foundation for soil erosion control.