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All rights reserved. No part of this periodical may be reproduced or transmitted in any form or by any means, electronic or mechanical, including photocopying, recording, or any information storage and retrieval system, without permission in writing from the publisher. Permission for printing and for reprinting the material contained herein has been obtained by the publisher. Effects of Mulch Cover Rate on Interrill Erosion Processes and the Size Selectivity of Eroded Sediment on Steep Slopes Soil & Water Management & Conservation S oil erosion by water is not only associated with on-site land degradation but also greatly contributes to negative downstream off-site impacts such as flooding, pollution, and siltation of water bodies. Soil erosion by water involves the detachment, transport, and deposition of soil materials due to the erosive forces of raindrops and runoff, and these processes are commonly divided into rill and interrill components depending on the source of eroded sediment (Meyer and Wischmeier, 1969). In regions of the world where rainfall intensities are not high, rates of interrill erosion can be considerable; even where rainfall intensities are high, interrill areas occupy a pivotal position in the erosion system, acting as links between incident rainfall and those areas of concentrated flow (rills and gullies) where most erosion occurs (Issa et al., 2006). Interrill soil erosion processes tend to be size selective, and the particle-size distribution (PSD) of eroded sediment can provide basic information regarding erosion processes (

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 interrill 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°). 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.

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