Soil physical degradation and rill detachment by raindrop impact in semi-arid region

Tillage along slopes creates furrows similar to rills which exposes the soil surface directly to raindrop impact (RI) especially in semi-arid regions. In order to quantify the effect of RI on soil physical degradation and rill detachment we determined the change of soil aggregate stability, bulk density, surface resistance and rill detachment in semi-arid soils. Toward these some dominant soils including silt, silty clay, clay, sandy clay and loamy sand were exposed under rainfall impact at four slope gradients (5, 10, 15 and 20%). The laboratory experiments were set up at two soil surface conditions: with and without RI in a flume under simulated rainfalls with an intensity of 90 mm h 1. Four rills with parabolic cross sections, 10 cm width and 4 m length were installed along the flume. Results indicated that the soils are physically degraded (aggregate breakdown, soil compaction and surface resistance) by RI. The rate of physical degradation processes was significantly influenced by the soil type and slope gradient. The soils with higher water-stable aggregates such as sandy clay showed lower physical degradation symptoms. The susceptibility of these soils to physical degradation processes increased with increasing slope gradient. Soil detachment through concentrated flow under RI was affected by the soil physical degradation rate, which in turn was controlled by the two variables i.e. soil type and slope gradient. However the direct effect of RI act in rill detachment (rainfall-driven force) increased rill detachment. As a conclusion, the rill detachment showed higher dependency to RI than soil physical degradation processes. This study revealed that the importance of maintaining the soil surface cover for controlling the negative effects of RI in the soil physical degradation as well as rill detachment especially in semi-arid soils with lower aggregate stability and under steep slopes.