Varying particle size selectivity of soil erosion along a cultivated catena

Sheet erosion is a complex multi-factor-dependent process with high spatial heterogeneity on hillslopes. Although the individual factors have been well studied, their aggregated effect on size-selective erosional processes is highly uncertain. Therefore, this study concentrates on the aggregate size distribution and effective particle size distribution (PSD) of the aggregates in the soil loss, collected from different simulated hillslope positions and surface conditions. These simulated hillslope positions combine moisture content from the extremely dry to the saturated with related slope positions of 2, 5, and 12% steepness and different surface roughness (tilled and crusted surfaces) modelled in a laboratory rainfall simulator. Using hierarchical cluster analysis, the PSD of the aggregates was separated into three groups based on the differences in the 59-116 mu m range of the PSD histograms, namely, macro-aggregates, 50-250 mu m sized micro-aggregates, and <50 mu m sized fractions were classified into distinct groups, although some micro-aggregate samples were classified into the macro-aggregate group. PSDs from the 50-250 mu m aggregate size fraction were clustered into a group of macro-aggregates if the PSD changed with time (during the rainfall event), notably on rough surfaces. The role of the specified size range in the classification is believed to be due to the parallel presence of aggregates and single particles in this range. As aggregates have a lower density than mineral particles, they tend to be enriched in soil loss under low-energy runoff conditions. Moreover, all samples in the <50 mu m fraction clustered into the macro-aggregate group were eroded from the smooth/crusted surface, probably due to the presence of larger particles. The results indicate that the combined effect of erosional factors is not apparent, and the impact of the crust and extreme moisture content on the selectivity and size distribution of the sediment requires further investigation.