Temporal and spatial evolution of soil surface roughness on stony plots

Soil surface roughness (SSR) is widely recognized as an important factor influencing water erosion processes. On semiarid hillslopes with stony soils, rock fragments accumulate as the result of preferential erosion of fine materials, often creating a rough, rocky surface. A series of rainfall events were simulated on a stony plot (2 × 6.1 m) at three slope gradients (5 %, 12 %, and 20 %) and rock cover was measured. Surface elevations were sampled by terrestrial LiDAR at high resolutions. Roughness indices, including random roughness (RR), fractal dimension, crossover length, and generalized fractal dimension, were calculated from LiDAR points directly. Results showed: 1) SSR displayed an increasing trend as the rainfall simulation proceeded for all three slope treatments; 2) the steeper slope developed greater surface roughness; and 3) both the increase of surficial exposed rocks and the formation of erosional features, e.g., rills and depressions, contributed to the spatiotemporal variations of SSR. Results also showed that the fractal dimension was not a good indicator of soil surface roughness, but rather was an index of the form of the surface. Crossover length was a measure of roughness at a scale of a few millimeters, while random roughness was a measure of elevation variations on the scale of the length of the transect measured, and thus encompassed larger morphological features including rills. We also established a new method for multifractal analysis that characterized the heterogeneity of soil surface roughness. These results improve our understanding of the evolution of semiarid stony hillslopes and the dynamic feedback mechanism between erosion, surface morphology and hydraulics.