Particle size characteristics of sediment by sheet erosion and their responses to related parameters on a Loess hillslope: a plot-scale study

Abstract

Abstract Particle size distribution (PSD) of sediment by sheet erosion not only indicates erosion behaviors, but also provides a basis for potential nutrient losses. PSDs are significantly influenced by topographic, rainfall, and associate runoff characteristics, such as rainfall intensity (I), rainfall kinetic energy (KE), slope gradient (S), runoff velocity (v), and stream power (Ω), while their effects have not been fully understood so far. To fill this research gap, in this study, simulated rainfall experiments were conducted to investigate the characteristics of effective and ultimate PSDs of sediments, as well as their responses to related parameters, under five levels of I and S. The results showed that (1) for effective PSDs, there was a significant enrichment of clay-sized and silt-sized fractions (P<2μm and P2–50μm) in sediment (percentage range: 12.11–20.64% and 57.09–65.94%), whereas there was a depletion of sand-sized fractions (P50–250μm and P250–2000μm). The ultimate PSDs of sediment were, however, similar to the soil matrix under experimental conditions. (2) With an increase in I and S, P<2μm and P2–50μm decreased exponentially or logarithmically, and P50–250μm and P>250μm increased correspondingly. The contribution rates (PCs) of the effect of I on clay-, silt-, fine-sand-, and coarse-sand-sized fractions in sediment were 23.44, 35.45, 29.09, and 56.82%, respectively, whereas the PCs of that of S were 56.54, 24.42, 59.89, and 31.47%, respectively, indicating that S plays a more pivotal role in influencing P<2μm and P50–250μm, whereas P2–50μm and P>250μm were more sensitive to I than to S. (3) The aggregation ratio (AR) for the fractions of different sizes indicated that clay-sized particles tend to be transported as aggregates, while silt-sized particles tend to be transported as primary particles. A higher I or steeper S leads to a larger proportion of particles being transported as aggregates. (4) KE, Ω, and unit stream power (U) were the most correlated parameters influencing effective PSDs. Equations were finally established using KE, Ω, and U to predict the effective PSDs of sediment by sheet erosion. This study can further the understanding of the sheet erosion process and provide a scientific basis for the establishment of a sheet erosion model.