Effects of rainfall pattern and soil surface roughness on surface–subsurface hydrological response and particle size distribution of red soil slope

Water erosion is strongly affected by rainfall pattern and soil surface roughness (SSR), the available information is inadequate to understand the influences of both on erosion processes and mechanisms. Four simulated rainfall patterns as constant (CR), increasing (IR), decreasing (DR), and rising-falling (RFR) with the same total rainfall amount (180 mm) and duration (180 min) were designed and applied to two soil surface conditions (smooth and rough) with the slope gradient of 15°. The response and generation of surface flow were positively correlated with the initial rainfall intensity of different rainfall patterns. The surface flow generation ranked as: DR > CR > RFR > IR, and the subsurface flow was the opposite. The CR pattern produced the lowest sediment yield, which was around 46.7% of the average soil loss for the varying-intensity rainfall patterns. SSR significantly promoted surface flow generation and sediment production under varying-intensity rainfall patterns, the surface flow coefficient and soil loss rate of the rough surface were 0.6%–13.1% and 3.20–11.27 g m−2 min−1 higher than that of the smooth surface. SSR inhibited the transport of sand particles and promoted the enrichment of clay particles. Under constant and variable rainfall patterns, the average content of clay particles of the rough surface was 2.8% and 0.9% more than that of the smooth surface, respectively. SSR could decrease fine particles and increase coarse particles in the eroded sediment, but only functioned under the rainfall patterns with lower initial rainfall intensity. Through comprehensively considering the effects of rainfall pattern and SSR, the stream power was the best hydraulic parameter for predicting the soil loss rate. The results are beneficial to the investigation of soil erosion in agricultural sloping farmland caused by natural rainfall events.