Development of rill erosion on bare sloping farmland under natural rainfall conditions

Rill erosion plays an important role in soil erosion, and studying this process can provide a basis for controlling soil loss on sloping farmland. The objectives of this study were to observe runoff and sediment transport processes during two continuous and two intermittent natural rainfall events and to monitor the changing morphological characteristics of rills within a standard runoff plot of bare soil (20 m length, 5 m width, and slope of 10°) at five successive observation times. We found that the processes of runoff and sediment transport presented a pattern with multiple peaks during continuous rainfall events and with a single or two peaks during intermittent rainfall events. The peak runoff and sediment yield rates exhibited a time-lag phenomenon of 1‒12 min compared with instantaneous rainfall intensity. Rills occurred as strip-shapes, V-shapes, and with a tree-branch-like distribution; their widths were mainly 5–20 cm and their depths 0–10 cm. Compared to our initial rill observation, the mean rill length, width, and depth increased by 227%, 26%, and 6%, respectively, after four subsequent rill observations over a period of almost one month. Side-wall collapse erosion was greater than downcutting erosion on the middle slope (section II; 6.67–13.34 m), while rill depth reached a minimum value on the lower slope (section III; 13.34–20 m). These findings help us to understand bare slope runoff and erosion mechanisms and provide a scientific basis for soil erosion modeling of sloping farmland.