The Erosion Evolution Process From Sheet Erosion To Rill Erosion On A Saturated Purple Soil Slope

Sheet erosion and rill erosion are the main erosion stages in the erosion evolution process of a saturated purple soil slope,quantifying runoff masses,erosion masses and sediment concentrations and analyzing the change of sheet erosion and rill erosion in the erosion evolution process along a saturated soil slope are of great significance to the deeply understand the mechanism of slope erosion process and to control soil and water loss.Purple soil is prone to rainwater infiltration.Soil infiltration of farmlands further increases due to the large pores caused by roots and straw rot.During a heavy rain event,infiltrated water accumulates on top of a low-infiltration purple soil layer with the Weak permeable layer exhibiting low permeability,which causes the cultivated layer to remain saturated.Compared with unsaturated purple soil,the cohesion of soil particles and the critical shear stress of saturated purple soil decrease sharply,thereby significantly affecting the interaction between sheet erosion and rill erosion and changing the soil erosion process.Nevertheless,most of related studies focus on runoff masses and erosion masses of the single erosion pattern either sheet erosion or rill erosion on unsatured soil slope,and lack of the systematic analysis about the erosion evolution process on saturated soil slope.In this study,based on the saturated water supply device,the experimental flume for distinguishing the sheet erosion and rill erosion and indoor simulated rainfall experiments,the critical condition from sheet erosion to rill erosion on saturated purple soil slope was determined,the quantitative methods of sheet erosion and rill erosion were established,the runoff masses,erosion masses and sediment concentrations and those percentage contributions were quantified,their differences and influencing mechanism were analyzed systematically,and their prediction models were established.The main conclusions are as follows:（1）With increasing rainfall intensities and slope gradients,the critical slope lengths from the end of sheet erosion to the beginning of rill erosion decreased from 8.00 to 0.94 m.The contributions of the slope gradient and rainfall intensity to critical slope length were 58.92%and 44.18%,respectively,indicating that the slope gradient has greater effect on critical slope length than rainfall intensity.The cumulative runoff masses and cumulative erosion masses in sheet erosion and rill erosion stages could be accurately calculated by the modeling method and the analytical method.Comparing the cumulative runoff masses and cumulative erosion masses in sheet erosion stage calculated by the measured methods and modeling method,the linear slopes of the cumulative runoff masses and cumulative erosion masses in sheet erosion stage by two mehods were 0.98 and 1.02,respectively.The determination coefficients（R²）were 0.98 and 0.98,respectively.Comparing the cumulative runoff masses and cumulative erosion masses in rill erosion stage calculated by the numerical method and the analytical method,the linear slopes the cumulative runoff masses and cumulative erosion masses in rill erosion stage by two mehods were 0.94and 0.96,respectively.The determination coefficients（R²）were 0.92 and0.98,respectively.Those results presented that quantitative methods of sheet erosion and rill erosion were accurated.（2）The cumulative runoff masses and cumulative erosion masses in sheet erosion and rill erosion stages decreased and increased with increasing slope gradients,rainfall intensities and critical slope lengths.The runoff contribution rates and erosion contribution rates in sheet erosion stage decreased from 100%to 7.45%and from 100%to 2.34%,respectively,whereas the runoff contribution rates and erosion contribution rates in rill erosion stage increased from 0%to 92.55%and from 0 to97.66%,respectively.Runoff masses per unit area,erosion masses per unit area and sediment concentrations in the rill erosion stage were 0.96-1.65,2.32-5.24 and 1.18-3.65 times of those in sheet erosion stage,respectively.The relationship among the rainfall intensity,slope gradient,the runoff mass per unit area,erosion mass per unit area and sediment concentration in sheet erosion and rill erosion stages could be expressed by a linear function,power function and logarithmic function.（3）The runoff masses,erosion masses and sediment concentrtions in from sheet erosion to rill erosion on saturated soil slope were 4.43-54.96 L,1.24-54.44 kg and 0.13-0.94 kg L^-1,respectively.The runoff masses,erosion masses and sediment concentrtions increased with the increasing slope lengths as linear functions,power functions and exponential functions.Slope gradients and rainfall intensities have positive effects on runoff masses,erosion masses and sediment concentrtions.The increase ranges of runoff masses,erosion masses and sediment concentrations at steep slope gradients and heavy rainfall intensities were larger than those at gentle slope gradients and low rainfall intensities.When the sediment concentration reached the maximum value,the increase rate of sediment concentrations tended to be stable.（4）The relationship of cumulative runoff masses and cumulative erosion masses in sheet erosion and rill erosion stages with different rainfall intensities,slope gradients and critical slope lengths could be well predicted by power function.The cumulative runoff masses and cumulative erosion masses in the sheet erosion stages were positive correlated with slope gradients,rainfall intensities and critical slope lengths.The cumulative runoff masses and erosion masses in the rill erosion stages were positively correlated with slope gradients and rainfall intensities,but were negatively correlated with critical slope length.The relationships of runoff masses,erosion masses and sediment consentrations with different rainfall intensities and slope gradients could be predicted by multivariate nonlinear regression models.Runoff masses,erosion masses and sediment concentrations simulated by the modeling methods were fitted with the measured data with the straight line slope of 1.03,1.08 and 0.92,respectively,which showed that multivariate nonlinear regression models have good accuracy.The results provide theoretical basises and practical guidances for revealing the mechanism of soil erosion,effectively preventing and controlling soil erosion from sheet erosion to rill erosion on slope farmland.