Roles of soil surface roughness in surface–subsurface flow regulation and sediment sorting

Abstract

Tillage-induced soil surface roughness (SSR) and rainfall interactions determine the process of water erosion. In this study, long-duration rainfall simulations were conducted to investigate the effect of SSR on the processes of surface–subsurface flow generation and soil loss, flow hydrodynamics, sediment sorting and transport on the steep red soil sloping farmland, southern China. Rainfalls with intensities of 30, 60, and 90 mm h−1 with duration of 180 min were applied to two soil surfaces (smooth and rough) with the slope gradient of 15°. Lesser surface flow and more subsurface flow were generated on the rough surface, which also accelerated the initial flow response and erosion stage advancement. The reduction effect of SSR on the sediment yield was significant under light rain or in the sheet erosion stage, while was limited when the rainfall intensity became stronger or entering the rill erosion stage. Differences in the spatial variability of soil crusts and the spatial distribution of rill networks were the main agents. Stream power took roughness factor into consideration well, thus was identified as the best hydraulic parameter for the prediction of soil loss rate. The investigation of particle size distribution of the eroded sediment showed the contents of sand, silt and clay changed from 15.9%, 64.9%, and 19.2% (average value of the original soil) to 22.9%, 53.1%, and 24% for the smooth surface, and to 37.8%, 42.1%, and 20.2% for the rough surface (average value of the eroded sediment). The selective and degrading effects of SSR on sediment particles were represented well. The rough surface was more inclined to transport clay particles than the smooth surface in the former period of erosion. Sand particles instead become more enriched with the extension of rainfall time. The results are beneficial for controlling the water erosion on agricultural sloping farmland.