Study On Water Transfer Process And Erosion Characteristics Of Soil-bedrock Composite Slope In Bare Pisha Sandstone Area

The control of soil erosion in the Yellow River Basin is an important content and fundamental measure for ecological protection and high-quality development of the Yellow River Basin.The study of the spatial characteristics,water transport and hydraulic erosion of soil-bedrock composite slope in the exposed Pisha sandstone area is the main source area of coarse sand in the middle reaches of the Yellow River,and it is of great significance to study the spatial characteristics,water transport and hydraulic erosion of soil-bedrock composite slope in this area.In this paper,a typical small watershed in the exposed Pisha sandstone area was selected as the research area,and the spatial distribution characteristics and influencing factors of soil thickness were studied at the slope scale combined with geostatistical theories and methods.Through the one-dimensional vertical infiltration test of soil moisture,the infiltration characteristics of overlying Pisha sandstone in the exposed Pisha sandstone area were clarified,and the water transport characteristics of the rainfall-infiltration-production flow process were simulated and studied.Based on runoff community and field simulation erosion tests,the hydraulic erosion process and hydrodynamic characteristics of bedrock-soil composite slope in exposed Pisha sandstone area were clarified,and the soil erosion model of composite slope in exposed Pisha sandstone area was preliminarily established.The main findings of the study are presented as follows:（1）With the increase of the average slope of the exposed Pisha sandstone area,the landscape of bedrock and soil staggered layered distribution became more obvious.The spatial variation of soil thickness showed the characteristics of moderate intensity variation to high intensity variability.Moreover,the optimal semivariance theory model presents the transformation of linear model-exponential model-spherical model.The results of ranking the important values obtained by the random forest empirical model showed that slope,soil bulk density,soil sand content and vegetation cover were the main factors affecting soil thickness,and based on the above factors,the slope of the exposed Pisha sandstone area was divided into two categories:the homogeneous soil slope in the bare Pisha sandstone area（average slope<14.2°,the average soil bulk density of the 10cm topsoil layer<1.53g/cm³,the average sand content of soil mechanical composition<81.178%,and the vegetation coverage>37.2%）.Bedrock-soil layered composite slope in exposed Pisha sandstone area（average slope value is between 14.2～45°,average soil bulk density of topsoil layer is between 1.53～1.67g/cm³,average sand content of soil mechanical composition is between 81.178～91.38%,and vegetation coverage is between 4.42～37.2%）.（2）The infiltration characteristics such as cumulative infiltration amount,stable infiltration rate and vertical migration distance of wet front all reflected the characteristics of poor water infiltration capacity of overlying Pisha sandstone.The increase of water head shortens the time for the wet front to reach the soil-rock transition face;Among them,the loess-feldspar sandstone-argillaceous sandstone structure of overlying Pisha sandstone has obvious impermeability reduction effect on water infiltration.Based on the inversion of the Hydrus model,the optimal hydraulic parameters of various types of overburden Pisha sandstone were obtained,and the water transport model of the rainfall-infiltration-production flow process was established,and the accuracy of the model prediction was high（R²>0.9481,RE<0.1948）.（3）Compared with the homogeneous slope of loess,the composite slope has stronger production capacity,higher slope flow rate and erosion dynamics,and smaller resistance.Compared with Pisha sandstone,the white homogeneous slope has the opposite regularity.When the erosion intensity is 40～200L/h,the flow state of the soil-bedrock composite slope changes from jet to slow flow,and the greater the erosion intensity,the longer the slope flow state maintains the jet flow state.The average flow velocity of feldspar sandstone-argillaceous sandstone-loess in the composite slope is the largest,the flow mode maintains the jet flow for a long time,the erosion dynamic parameters are high,and the slope flow resistance of argillaceous sandstone-loess-feldspar sandstone is large.The runoff rate is a polynomial function of the flow rate,flow state and erosion dynamic parameters.It has an exponential function relationship with the characteristic parameters of slope flow resistance.Slope flow rate and Fred number in hydrodynamic parameters have a significant effect on runoff rate（Mantel’s p<0.01）.（4）The composite slope has stronger sand production capacity than the homogeneous slope of loess soil,and weaker than the white homogeneous slope of Pisha sandstone.The soil erosion of the composite slope had a good linear relationship with rainfall,I₃₀,rainfall erosion force and runoff（R²>0.5042）.The soil loss rate was related to the slope flow velocity and resistance characteristic parameters as a polynomial function（R²>0.5415）,the power function relationship with the slope flow characteristic parameters（R²>0.5702）,and the slope flow flow shear force as an exponential function（R²>0.5596）.The soil loss rate was significantly correlated with slope flow velocity,Fred number,Darcy-Weisbach drag coefficient,Manning roughness coefficient and water flow shear（Mantel’s p<0.01）.Under the 40L/h erosion condition,the periodic change of composite slope runoff was the strongest in the 660～720s time scale,and the periodic change of soil erosion was the strongest in the 300～660s time scale.The erosion intensity,runoff rate and hydrodynamic parameters were selected as independent variables,and the multiple regression model was established for the four composite slopes with soil loss rate as the dependent variable,and the model accuracy was good.