The destruction of soil aggregates upon transport by overland flow may produce a significant effect on sediment transport capacity and general intensity of erosion. The particle size distribution of destructed soil aggregates has a close relation to the surface runoff and permeability of soils. The objective of this study is to quantify the effects of transport distance and flow discharge of overland flow on the destruction of aggregates of Ultisols in a 3.8 m long flume with a fixed bed. A series of experiments were carried out at a slope of 17.6%, including six transport distances （9-108 m） and eight discharges （0.4-1.2 L/s）. The results indicate that （1） the extent of the destruction of aggregates became weaker with the decrease in size over the same transport distances or at the same discharges; （2） the aggregates derived from Shale were rapidly abraded and had more serious destruction as compared to the aggregates from Quaternary red clay during the transport process, which was relevant to the stability difference of the two parent materials： （3） two stages of aggregate breakdown could be identified in terms of the coefficient c~ during transport, that is, the aggregates were rapidly abraded and became round and were predominantly broken down into smaller fragments at the first stage, while the smaller fragments and the round aggregates were weakly abraded with reduction in weight and their shape became regular; and （4） the extent of the destruction decreased with increasing discharge, which was due to the changes in the hydraulic properties （flow depth and friction factor） and in movement modes during the transport process. The analysis of the characteristics on aggregate destruction by overland flow can contribute to the development of soil erosion models.
Land use and land cover change is a key driver of environmental change. To investigate the runoff and erosion responses to frequent land use change on the steep lands in the Three Gorges area, China, a rainfall simulation experiment was conducted in plots randomly selected at a Sloping Land Conversion Program site with three soil surface conditions: existing vegetation cover, vegetation removal, and freshly hoed. Simulated rainfall was applied at intensities of 60 (low), 90 (medium), and 120 mm h 1 (high) in each plot. The results indicated that vegetation removal and hoeing significantly changed runoff generation. The proportion of subsurface runoff in the total runoff decreased from 30.3% to 6.2% after vegetation removal. In the hoed plots, the subsurface runoff comprised 29.1% of the total runoff under low-intensity rainfall simulation and the proportion rapidly decreased with increasing rainfall intensity. Vegetation removal and tillage also significantly increased soil erosion. The average soil erosion rates from the vegetation removal and hoed plots were 3.0 and 10.2 times larger than that in the existing vegetation cover plots, respectively. These identified that both the runoff generation mechanism and soil erosion changed as a consequence of altering land use on steep lands. Thus, conservation practices with maximum vegetation cover and minimum tillage should be used to reduce surface runoff and soil erosion on steep lands.
