Upslope inflow-driven variations in microtopography and size fractions effect on rill erosion in purple soil hillslopes

Abstract

Upslope drainage area contributing inflow to downslope rill erosion can be affected by the dynamics of eroded sediment and soil surface microtopography. However, limited information has previously not recognized the variation of the sediment particles and microtopography on hillslopes where upslope inflow perpetually exists. A series of scouring flume experiments were performed on a 15° slope under upslope inflow rates (4, 6 and 8 L min−1) to investigate the inflow-driven microtopography and sediment particle-size distribution (PSD) during the rill erosion process. The results indicated that the surface elevation decay mostly occurred in the later stage of rill erosion. The spatial variability of microtopography increased gradually with increasing inflow rates, and obvious microtopography sharpness intensified with the development of rill erosion. The surface runoff under the 8 L min−1 inflow rate increased by 15.03 % and 90.14 % for 6 and 4 L min−1, respectively, and the sediment yield reached a maximum at the rill stabilization stage (RS). The overall particle-size distribution was mainly composed of silt-sized particles, accounting for approximately 40.97 %−50.15 % of the total eroded sediment. Considerable depletion of clay fractions occurred in the rill erosion process. High inflow rate significantly enhanced variation amplitude of critical hydrodynamic parameters (e.g., flow velocity, flow shear stress and Reynolds number). The spatial heterogeneity of microtopography was correlated with the coarser fraction of sediment PSD in rill erosion, suggesting that erosion processes were affected by the interaction between eroded sediment distribution and roughness. These findings highlight the importance of the PSD of eroded sediment and the development of soil surface structure for an in-depth understanding of the purple soil erosion mechanism at the hillslope scale.