Effect And Dynamic Mechanism Of Biocrusts With Different Coverage On Runoff And Sediment In The Hilly Loess Plateau

Loess Plateau is one of the most serious soil erosion regions in China.Serious soil and water loss leads to the degradation of ecological environment,and seriously restricts the sustainable development of local economies.In recent years,with the implementation of the“Grain for Green Project”on the Loess Plateau,biocrusts have been widely developed in the region,with an average coverage of 60%-70%,which significantly affects soil and water loss on the slope.To date,numerous researches have focused on the effect of biocrusts on soil and water loss,however,few studies focus on the impact of biocrusts with different coverage on slope runoff and sediment yield.The influence mechanism of biocrust coverage on soil and water loss on slopes is still unclear,and the relationship between biocrusts coverage and sediment and runoff yield on slope cannot be quantified,which is a weak link in the study of soil and water conservation function of biocrusts,affecting the revision of soil erosion model considering biocrusts and also hindering the accurate evaluation of soil and water conservation benefit of the“Grain for Green Project”on the Loess Plateau.Therefore,taking the biocrust slope with different coverage in the Hilly Loess Plateau region as the research object,this study investigated the effect of biocrust slope with different coverage on the generation process of runoff and sediment,quantified the relationship between biocrust coverage,runoff and sediment yield and hydrodynamic parameters,analysed the influence of biocrusts on the process of sediment detachment,transport,and deposition,clarified the influence of biocrusts on the sediment transport process,revealed the dynamic mechanism of biocrust coverage on sediment generation,by using simulated rainfall,natural rainfall monitoring combined with rare-earth element tracing technology.The main conclusions are as follows.（1）Biocrust coverage affected the slope runoff generation process by influencing the initial runoff generation time and the stable time of runoff generation.The initial runoff generation time extremely significantly decreased with the increase of biocrust coverage,while the stable time of runoff generation increased firstly and then decreased with the increase of biocrust coverage.The average initial runoff generation time of bare soil slope was 7.0 min,while it was 2.5 min on the biocrust slope with 79%coverage.The initial runoff generation time of bare soil slope was 2.8 times that of biocrust slope.The runoff yield of the biocrust slope firstly increased and then tended to be stable.The runoff generation of the biocrust slope with 79%coverage reached the stable state fastest,and tended to be stable at about 8 min.The runoff generation of the biocrust plot with44.5%coverage reached the stable state slowest,and tended to be stable at 40 min.（2）The relationship between runoff rate and biocrust coverage was a logarithmic function:y=-a ln（x）+b.Under simulated rainfall,when biocrust coverage increased from9.3%to 78.4%,the average runoff rate decreased by 68.7%.The relationship between runoff rate and biocrust coverage was expressed as:y=-0.185ln（x）+1.0808（R₂=0.752,P<0.001,n=19）.Under natural rainfall,when biocrust coverage increased from 19.6%to 42%,the runoff rate decreased by 44.9%.The relationship between runoff rate and biocrust coverage was expressed as:y=-1.108ln（x）+8.6794（R₂=0.274,P<0.001,n=12）.（3）The variation in biocrust coverage affected the hydrodynamic characteristics of runoff.The difference of fragmentation degree and Splitting index of biocrust patches caused by biocrust coverage variation is the main factor affecting the hydrodynamic characteristics of runoff.With the increase of biocrust coverage,the overland flow was shifted from supercritical flow to tranquil flow,but the flow pattern is always laminar flow;both runoff velocity and runoff power decreased logarithmically;and the Darcy-Weisbach resistance coefficient increased in a power function trend.The flow velocity,Reynolds number and runoff power,all increasing with the increasing Splitting index,were most closely correlated with Splitting index.Runoff depth,Froude number,shear stress and resistance coefficient had the strongest correlation with Patch density.Concretely,with the increase of Patch density,the runoff depth,shear stress and resistance coefficient decreased,while Froude number increased.（4）The relationship between sediment yield rate and biocrust coverage could be expressed as:y=ae^-bx+c.Under simulated rainfall,sediment rate decreased by 98.1%when biocrust coverage increased from 9.3%to 78.4%,and the relationship between sediment yield rate and biocrust coverage was y=21.665e^-0.05x+1.0808（R₂=0.936,P<0.001,n=19）.Under natural rainfall,when biocrust coverage increased from 11.6%to54.8%,sediment yield decreased by 69.0,and the relationship between sediment yield rate and biocrust coverage was y=20.755e^-0.012x（R₂=0.3152,P<0.001,n=12）.（5）Biocrust coverage affected the process of sediment delivery.Biocrusts reduced sediment yield and sediment delivery by the effects of covering and intercepting.Under the experimental conditions of this study,only 6.1%of sediment yield generated from the area covered by biocrusts,and the sediment yield generated from bare soil area was 15 times that in the area covered by biocrusts.The sediment deposition on biocrust slope was 5 times that of bare soil slope.The sediment delivery ratios of biocrust and bare soil slopes were0.46 and 0.93,respectively.The amounts of sediment intercepted by biocrusts decreased gradually with the increasing distance from erosion source.（6）The distribution pattern of biocrust with different coverage affected runoff dynamics and slope resistance,and further affected slope sediment yield.Under the rainfall intensity of 90 mm h^-1,according to the magnitude of influence factors on sediment yield,the order was biocrust coverage（-0.949）,Patch density（0.782）,runoff power（0.596）,resistance coefficient（-0.280）,Landscape shape index（0.173）,and Splitting index（0.167）.The results showed that biocrust cover was the main factor to control soil loss.In conclusion,this study clarified the characteristics of sediment yield and runoff on biocrust slope with different coverage,quantified the relationship between biocrust coverage and sediment yield,runoff,clarified the contribution of biocrust to the process of soil detachment,transportation and deposition on slope,and revealed the dynamic mechanism of biocrust coverage affecting soil and water loss on slope.The results lay a scientific foundation for revising the soil erosion model considering biocrusts and accurately evaluating the benefits of soil and water conservation in the“Grain for Green Project”on the loess Plateau.