Effects of the distribution of biological soil crust on the hydrodynamic characteristics of surface runoff.

The distribution pattern of biological soil crusts (biocrusts) is one of the main factors affecting runoff and sediment yield. The relationship between runoff and sediment yield and biocrusts' distribution pattern is not clear, which hinders understanding the mechanism underlying the effects of biocrusts on runoff and sediment from slopes. To fill the knowledge gap, we investigated the relationship between the landscape indices of three biocrusts' distribution patterns, i.e. zonation, chessboard and random, and the hydraulic parameters, using of simulated rainfall experiments and landscape ecology methods. The results showed that biocrust significantly affected the erosion force of slopes and that its distribution pattern could affect slope erosion dynamics. Compared to bare soil, the presence of biocrusts significantly reduced the runoff velocity (54.6%) and Froude number (67.0%), increased the runoff depth (86.2%) and Darcy-Weisbach resistance coefficient (10.68 times), but did not affect the Reynolds number and runoff power. Expect for the runoff depth, there were significant differences in the hydraulic parameters of the three biocrusts' distribution patterns, with the random pattern having the strongest impacts on the dynamics of slope erosion. Based on factor analysis and cluster analysis, five indices of percentage of patch to landscape area, patch density, landscape shape index, patch cohesion and splitting could be used as the indicators for the distribution characteristics of biocrust patches. The patch cohesion and splitting of biocrust patches were the main distribution pattern indices of the hydrodynamics of surface runoff. As the patches patch cohesion decreased, the splitting increased, which caused the surface runoff velocity increase, the resistance decrease, and the slope erosion became more severe.