Nonlinear response of hydrodynamic and soil erosive behaviors to biocrust coverage in drylands

Abstract In dryland ecosystems, soil erosion poses severe threats to ecosystem productivity and human well-being. Biological soil crusts (biocrusts) are ubiquitous living covers in these ecosystems and play an important role in soil stabilization and erosion prevention. However, the quantitative relationship between biocrust coverage and soil water erosion as well as the driving mechanism have not yet been fully discussed, which hinders the development of soil erosion models that consider the effect of biocrusts. Accordingly, we set up 19 biocrust plots (10 m × 2.1 m) with coverage ranging from 9.3% to 80.0% on the Loess Plateau of China and conducted a series of simulated rainfall experiments. The quantitative relationship between runoff and sediment yield and biocrust coverage was assessed, and further complex correlations among these variables were analyzed using structural equation modeling (SEM). We found that (1) the runoff rates and sediment yield were significantly related to biocrust coverage. The runoff rates and sediment yield decreased with increasing biocrust coverage via a logarithmic function and an exponential function, respectively. The runoff rate and sediment yield rate from the plot with 9.3% biocrust coverage were 3.2 and 51.5 times higher, respectively, than those from the plot with 78.4% biocrust coverage. (2) The flow pattern and regime of overland flow were significantly affected by biocrust coverage. The overland flow was shifted from tranquil flow to supercritical flow when the biocrust coverage declined to 12.3%. The flow velocity, stream power, Darcy-Weisbach resistance coefficient, Froude number, and Reynolds number were strongly correlated with biocrust coverage (R2 = 0.548–0.830). (3) Biocrust coverage both directly and indirectly affected sediment yield. The anchoring effect of biocrusts directly protected the surface soil from raindrop impacts and runoff scouring (direct effect = −0.59). Additionally, biocrust cover had an indirect negative effect (−0.38) on sediment yield by decreasing the flow velocity and runoff rate with path coefficients of −0.81 and −0.78, respectively. Our results confirm that soil hydrological and erosive behavior have nonlinear relationships with biocrust coverage at the hill-slope scale, and they also reveal that variation in biocrust coverage is one of the main driving factors affecting soil hydrological and erosive behaviors. These results would improve soil water erosion estimation on biocrust-covered slopes and provide guidance for revising soil erosion models to include the effectiveness of biocrusts.