Size characteristics of sediments eroded from three soils in China under natural rainfall

The particle-size distribution of runoff sediment is important in understanding, characterizing and modeling the transport behavior of sediment and sediment-associated chemicals. The objective of this study was to investigate the particle-size distribution of sediments eroded from three soils in China under natural rainfall. Each of the three soils was packed to a depth of 30 cm in a 20 x 2.1 m runoff plot. Sediments yielded in nine natural rainfall events were analyzed for their particle-size distribution prior to and following dispersion. The sediment size measured in the undispersed condition was always larger than the one determined after chemical dispersion, indicating that part of the sediment was eroded in aggregated form. The degree of sediment aggregation depended on the clay content and the organic matter content of the sources. The mean sediment size quantified by mean weight diameter linearly increased with sediment yield for the two soils with relatively high clay content. The rate of increase was greater in the undispersed condition than that in the dispersed condition for these two soils. Comparing sediments to the corresponding source soil, the results of mean weight diameter and enrichment ratio both revealed that aggregate-size distribution was more sensitive to soil erosion than the primary particle-size distribution. Small aggregates, rather than the primary particles, were selectively eroded in the rainfall events. These findings support the use of both dispersed and undispersed sediment-size distributions for the characterization of sediment transport and the associated sediment-bound nutrients and contaminants.