Estimation of soil detachment capacity on steep slopes in permanent gullies under wetting-drying cycles

Abstract

Permanent gullies, mainly distributed on granite weathering mantles in tropical and subtropical areas, are the major threat to the sustainable development for economy and ecosystem of southern China. Collapsing wall, with its stability influenced by the frequent wetting-drying cycles under the hot and rainy climatic conditions, is the most active part of permanent gully. The free surface along the collapsing wall provides a favorable condition for soil detachment by runoff scouring, however, little attention has been given to the detachment capacity of granite soils under the influence of wetting-drying cycles in permanent gullies. To quantify the impacts of wetting-drying cycles on soil erosion resistance in permanent gullies, the detachment capacity of granite soils (surface soil layer A; lateritic layer B; sandy layer C1; and detritus layer C2) that were collected from the collapsing wall was investigated under 0, 2, 5, and 10 cycles of wetting-drying. The impacts of wetting-drying cycles could be well quantified by soil pore size distribution that was calculated by soil-water characteristic curve (p < 0.01). The content of macro-porosity (>30 μm) increased constantly with increasing W-D cycles, while micro-porosity (<0.2 μm) decreased, especially for soils in the surface and lateritic layers (A and B). Soil erosion resistance of granite soils under the W-D cycles was determined by the integrated effects of soil porosity, and the tested soil detachment capacity soil detachment capacity in a range of 0.008–12.488 kg.m-2 s−1, increased with the increase of W-D cycles. In addition, the rill erodibility (Kr) was significantly influenced by soil bulk density, macro-porosity, and micro-porosity (R2 = 0.930, Nash-Sutcliffe efficiency = 0.910), demonstrating a power function relationship. Moreover, the critical shear stress (τc) had a significant and positive correlation with soil cohesion (R2 = 0.576, p < 0.010). Furthermore, soil detachment capacity could be well predicted by these measurable parameters including bulk density, macro-porosity, micro-porosity, soil cohesion, and shear stress of flowing water (R2 = 0.950, NSE = 0.980), under the theoretical foundation of WEPP model. The obtained results would facilitate the understanding of the formation mechanisms of the permanent gullies, as well as the development of soil conservation measures in southern China.