Sediment transport over steep hillslope; importance of splash-wash erosion and interpretation of processes

Soil erosion over steep forested hillslopes due to decrease of vegetation is one of the major environmental concerns in steep mountainous watershed. On interr-rill area of steep slopes, soil detachment and transport by rain splash and overland runoff are processes of soil erosion. We examined the contribution of erosion by rain splash in total sediment transport. We did 2 sets of experiments over a steep slope of a Japanese beech forest: 1) Three erosion plots (2 m wide × 5 m long) with average understory coverage of 1%, 45%, and 94% to examine contribution of rain splash in total transported sediment. 2) Measurement of detachment rate and rain splash distance distributions. Rain splash and wash erosion were sampled separately. For the plots with sparse, moderate, and high understory coverage, the average proportions of splashed soil to total soil erosion during monitoring period were 12%, 39%, and 22%, respectively. For sparse ground cover rain splash ratio in total sediment movement was in range of 4% ~ 61%. The amount of wash erosion was significantly correlated with the amount of splashed soil under the condition of sparse ground cover. This relationship was more pronounced during high precipitation events and the rainy season. Results suggest that splash detachment was not only an essential process in soil transport by overland flow, but also that splashing contributed to the sediment transport mechanism of soil erosion at the forested hillslope. Using results of splash distance experiment, down slope splash transport was modeled by detachment rate and splash transport distribution function along maximum splash distance. Model was applied for data of sparse covered plot. There were good agreements between estimated and measured splash rate of plot data. Transportation of sediment by overland flow was dominant movement mechanism. Splash transport itself was contributed in sediment transportation considerably.