Effects of rain intensity, slope gradient and particle size distribution on the relative contributions of splash and wash loads to rain-induced erosion

Abstract

Soil erosion during rainfall is a complex phenomenon resulting from detachment by raindrop impact and overland flow. The objective of this study is to investigate the transportation rate of splash load (SL) and that of wash load (WL) and their relative contributions to the rain-induced erosion rate affected by rain intensity, slope gradient and particle size distribution. A total of 60 simulation runs were carried out using a detachment tray under simulated rainfall with no inflow. The experiments were done on two soil samples (aggregate sizes finer than 2 and 4.75mm) from an agricultural land use at different rain intensities (57 and 80mmh−1) and varying slope gradients (0.5%, 2.5%, 5%, 10%, and 20%). Under unsteady state conditions, WL showed relatively high dependency on slope gradient, whereas approaching steady state conditions, nearly similar values of WL occurred at different slopes. SL and WL increased with increasing rain intensity and slope gradient, implying the importance of rain-induced erosion on bare agriculture lands especially at steeper slopes. The ratio of WL/SL decreased when slope gradient increased; however, WL increased more significantly than SL as soil aggregates became finer. The result indicates that at all the slope gradients, WL was much larger than SL, indicating that wash load significantly contributes to the rain-induced erosion in the agricultural soil. Furthermore, with increasing slope gradient, the contribution of wash load to rain-induced erosion decreased, while it was greater in the soil containing finer aggregates. Also, with increasing rain intensity at lower slope gradients (<10%), further increase in splashed materials than that in washed materials occurred, whereas the reverse order was found at steeper slopes (>10%). The finding of this study revealed that the rain-induced erosion was transport-limited at the gentler slopes, whereas at the steeper slopes, it shifted to detachment-limited conditions.