4.3. Particle size distribution and composition of eroded sediment
Regarding the particle size distribution of eroded sediments, Dx (10), Dx (50), and Dx (90) were mainly made up of silt, coarse powder grains and very fine sand, and very fine sand and fine sand, respectively, with the main enriched particle size increasing. Under the influence of overland runoff, the structurally unstable fine particles were most easily eroded, leading to different levels of fine particle enrichment (Berger et al., 2010). Moreover, the erosion sediment gradually showed a trend of coarse particles with longer scouring time (Schiettecatte et al., 2008). These results were similar to those obtained by Jiang et al. (2014). The enriched sediment of different particle sizes showed a trend of first increasing and then decreasing with increasing slope gradient, consistent with the results of Zhang et al. (2015). Under different vegetation covers, the difference of sediment particle size was obvious on slopes less than 15° but not slopes greater than 15°, suggesting that the flow velocity, which was influenced by slope gradient, can affect the movement of soil particles differently. When the slope was less than 15°, the larger sediment particles were not easily eroded and transported, meanwhile vegetation patches acted as sinks of sediment (Cantón et al., 2012; Muñoz-Robles et al., 2013), to effectively intercept and restrain sediment, thus the coarse particles were reduced and the average particle size became smaller (Schiettecatte et al., 2008; Zhu et al., 2016). However, when slope was greater than 15°, the runoff eroding power was increased, and soil corrosion resistance and anti-scourability were reduced (Berger et al., 2010; Zhu et al., 2016). Meanwhile, the effect of vegetation on interception and sorting of eroded sediment particles was gradually decreased, thus Dx (10), Dx (50), and Dx (90) gradually stabilized under different vegetation covers.