An experimental study of rill sediment delivery in purple soil, using the volume-replacement method

Quantifying the contributions of soil surface microtopography and sediment concentration to rill erosion

Soil surface microtopography plays a significant role in rill erosion. In addition, upslope inflow has a large effect on downslope soil erosion processes. Experiments including four different upslope filling areas (0 m², 0.15 m², 0.30 m², and 0.45 m²) with a upslope inflow rate (6 L min^-1) were conducted in two 1 m × 2 m boxes on a 15° slope to examine the effects of microtopography and sediment concentration on rill erosion processes. The upslope filling areas were used to simulate different areas of earthen dike terraces. The results showed that the minimum values of soil surface elevation increased from -120 mm to -110 mm as the upslope filling area increased. The values of the simple fractal dimension (f(α)_max), the singular index span (Δα) and the difference of multifractal spectrum (Δf(α)) reached minimum values in the 0.45 m² upslope filling area. With the development of rill erosion, the soil surface microtopography tended to sharpen, and the relative elevation changed greatly. The runoff and soil loss associated with rill erosion gradually decreased as the upslope filling area increased. We identified the temporal evolution of rill erosion using Morlet wavelet analysis. The main period of temporal fluctuation of rill erosion was 28 min under different upslope filling areas. Multi-scale periods of temporal fluctuation of rill erosion emerged with the increase in upslope filling area. The Δα significantly affected the runoff and soil loss. The proportional contributions of Δα to the runoff and sediment yield were 80.95% and 77.34%, respectively. While the contributions of sediment concentration to runoff and sediment yield were 17.05% and 20.66%, respectively. The findings are important significance for better understanding rill erosion mechanisms of purple soil.