Processes And Mechanisms Of Interrill Erosion Under Different Near-surface Soil Hydrological Conditions On The Yellow Soil Slope

Soil and water loss is one of the important ecological environment problems in the Yangtze River Basin.However,the near-surface soil hydrological conditions are neglected in the study of soil and water loss.Interrill erosion areas occupy an important position in soil erosion process.The impact of near-surface soil hydrological condition on the interrill erosion is still obscure.In view of the problem of hillslope soil erosion under different near-surface soil hydrological conditions,this study selected eroded yellow soil sloping farmland of the upper and middle the Yangtze River as the research object,and a method of combining simulated rainfall experiment and rare earth element tracer method was conducted to investigate the processes and mechanisms of interrill erosion under the different near-surface soil hydrological conditions.The characteristics of the interrill erosion and its key influence factors were analyzed under different near-surface soil hydrological conditions;the dominant erosion process and the detachment mechanism of the interrill erosion were anatomized.The characteristics of erosion-deposition distribution in the interrill erosion process were revealed under the near-surface soil hydrological condition;the dynamic mechanism of interrill erosion of a coupled near-surface soil hydrological condition was clarified;the interrill erosion prediction models of a coupled near-surface soil hydrological condition were also established.These main conclusions were as follows:(1)The characteristics of the interrill erosion under different near-surface soil hydrological conditions were analyzed.Runoff rate was increased at the beginning and then remained stable for all treatments.Sediment concentration for saturation and seepage conditions decreased gradually at the beginning and then remained relatively low value in middle and latter period of rainfall.Sediment concentrations for drainage conditions were increased firstly and then decreased or obviously fluctuated in majority of experiments.The runoff rates and sediment concentrations for seepage conditions were 1.02 to 4.78 times and 0.57 to 7.02 times obviously greater than that for drainage conditions respectively.(2)The key influence factors of the interrill erosion were analyzed under different near-surface soil hydrological conditions.The contributions of the near-surface soil hydrological conditions to runoff and erosion were 9.55%to 59.79%and 15.75%to 68.91%under the same rainfall intensity,respectively.The contributions of the near-surface soil hydrological conditions to runoff and erosion were 49.03%to 98.94%and 32.35%to 78.73%under the the same slope gradient,respectively Overall,the contributions of near-surface soil hydrological conditions and rainfall intensities to runoff and erosion were greater than those slope gradients in this study.There were significant correlations between the interrill erosion rate and near-surface soil hydrological condition,rainfall intensity,runoff rate,slope gradient and coupling effects of factors,indicating that the near-surface soil hydrological condition was the key influence factor in the interrill erosion process in addition to rainfall intensity,runoff,and slope gradient(3)The dominant erosion process and the detachment mechanism of the interrill erosion were anatomized under different near-surface soil hydrological conditions.The detachment rates by raindrop impact for seepage conditions were 1.12 to 4.60 times greater than that for drainage conditions.The thin overland flow does not have sufficient capacity to detach soil particles within the underlying surface without raindrop impact.There was a transition from transport-limited to detachment-limited as the near-surface soil hydrological condition shifted from drainage conditions to seepage conditions.This study also found that the raindrop detachment and raindrop-affected flow detachment with flow transport system(RD-RAFD-FT)existed in seepage-affected interrill erosion process.The dominant detachment mechanism involving the combined action of raindrop and raindrop-impact overland flow existed in the RD-RAFD-FT system(4)The characteristics of erosion-deposition distribution in the interrill erosion process were revealed under different near-surface soil hydrological conditions.The trend of soil loss per unit area calculated from each tracer segment as spatial and temporal distribution was complex in interrill erosion process.The contribution to total amount of sediment deposition above tracer segments was 7.04%obviously less than that those below the tracer segments.The greater sediment deposition rates usually occurred at the downslope position of 0-20 cm below tracer segments.The rates of downslope deposition to sediment yield were 42.77%to 135.39%in the drainage-saturation conditions,and the rates of downslope deposition to sediment yield were 23.51%to 29.70%in the seepage conditions.The transport by raindrop-induced and overland flow-driven rolling was dominant transport mode.In transport-limited process,raindrop-induced transport was more efficient than raindrop impact overland flow-driven transport.However,the raindrop impact overland flow-driven transport was more efficient than raindrop-induced transport in detachment-limited process.(5)The dynamic mechanism of interrill erosion of a coupled near-surface soil hydrological condition was clarified.The correlation analysis showed that flow velocity was the best hydraulic parameter for the interrill erosion.The interrill erosion rate was simulated well by a simple power function of the flow velocity.Considering the integrated effect of the exogenic erosional dynamics on the interrill erosion,a power function that included the physical description of the hydrodynamic parameters,rainfall intensity and hydrostatic pressure head was used to predict the interrill erosion rate.Furthermore,we also found that the interrill erosion rate was better correlated to a power function of the stream power,rainfall intensity,and hydrostatic pressure head than to functions of the flow shear stress,rainfall intensity,and hydrostatic pressure head or the unit stream power,rainfall intensity,and hydrostatic pressure head.(6)The interrill erosion prediction models of a coupled near-surface soil hydrological condition were also established.An assessment for existing empirical models was conducted under different near-surface soil hydrological conditions.The WEPP-M was a modified version of the WEPP model(Water Erosion Prediction Project)based on the factor of the near-surface soil hydrological condition.The R2 and NSE values from WEPP-M were 22.36%to 210.00%higher than those of existing empirical models.The mechanism models of different dominant erosion processes of the interrill erosion were further established(WEPP-MM)when the slope length factor was considered.The sediment transport capacity and soil detachment rate could be simulated using the WEPP-MM.The average absolute values of the relative error(RME)derived from WEPP-MM decreased by 38.03%to 87.21%compared with the existing interrill erosion empirical models.