Size Selectivity of Eroded Sediment Associated with Soil Texture on Steep Slopes

Sediment selectivity during transport may provide basic information for evaluating on-site and off-site impacts of soil erosion. Rainfall simulation experiments were performed to investigate the effects of soil texture and aggregation on sediment particle size distributions (PSDs) in rill and inter-rill erosion material. Four soils with decreasing clay content were selected and subjected to simulated rainfall with an intensity of 120 mm h(-1) on three steep slopes (15, 20, and 25 degrees). A comparison of the sediment effective PSD (undispersed) and ultimate PSD (dispersed) for the four soils revealed that clay-sized particles were prone to transport as aggregates. Moreover, the degree of sediment aggregation decreased with decreasing clay content in the original soils, as reflected in the effective/ultimate PSD ratio close to 1. Temporal fluctuations in the effective/ultimate PSD ratio indicated that aggregates were disintegrated by raindrop impact and/or runoff energy during the erosion process. The sediment after rill development was coarser than that before rill development owing to the large flow depth and runoff energy of the rill flow. Soils that contained more heavy particles, including large aggregates and coarse primary soil particles, more easily rolled on steep slopes than soils containing fewer heavy particles due to the strong gravity and inertial forces in the direction of the slopes, which may increase the relative contribution of the bed-load transport mechanism. The results of this study are expected to improve the accuracy of soil loss prediction and provide appropriate conversion measurements under steep slope conditions by considering soil texture, aggregation, and rill development.