Assessment of the size selectivity of eroded sediment in a partially saturated sandy loam soil using scouring experiments

Infiltrating water accumulates in the plough pan because of the low permeability of this layer, which results in saturation or partial saturation (soil water contents between the field water capacity and saturated water content) of the cultivated layer during continuous rainfall. Even in sloping farmlands, soil infiltration via large pores created by roots, wormholes and straw rot continuously increases, and the infiltrating water forms subsurface flow above the plough pan, causing the soil in the cultivated layer to become partially saturated. The processes and characteristics of soil particle sorting and transport in partially saturated soil were investigated in the laboratory using scouring experiments. The results showed that the stability of the aggregates was destroyed when the soil was in a soaking state and tended to be saturated, which produced a unimodal distribution of eroded sediment finer than 0.021 mm, and the main transport mechanism along the partially saturated soil was suspension/saltation. The eroded sediment in the erosion process was mainly composed of silt-sized particles, accounting for approximately 44-60% of the sediment load. Destruction of aggregates during erosion can easily enrich the soil in fine particles, especially clay-sized particles. Most fine and coarse sand-sized particles were predominantly aggregates of other finer particles; the aggregation of fine sand-sized particles was relatively poor, and a certain number of fine sand-sized particles were transported as single particles. The results reported in this study have important implications for an in-depth understanding of sediment transport in partially saturated soil.