Study On The Rill Detachment Rate Of Purple Soil Under The Action Of Subsurface Water Flow

Purple soil formed on purple sand shale is one of the main soils in southern China.But its soil-forming soil structure is loose and has high infiltration performance,while large pore preferential flow channels such as root holes and worm holes in sloping cultivated land further increase the infiltration of water into the soil,and the infiltrated water forms subsurface water flow above the subsurface weakly permeable layer.Subsurface water flow will weaken the cementing capacity of the soil,weaken the erosion resistance of the soil,and intensify soil erosion.The interaction of subsurface water flow and surface water flow can cause strong soil erosion.In this paper,we experimented with purple soil on sloping land.We simulate scouring test by limiting rill,supply water from above the weakly permeable layer of soil,and control the head height to make the subsurface water flow of the test soil reach the expected saturation depth.Then measure the rill detachment rate under the following conditions(three subsurface water flow saturation depths 5,10 and 15 cm,three slopes 5°,10°and 15°and three discharges 2,4 and 8 L·min^-1).By this method,we studied the trend of rill detachment rate under different flow,slope and saturation depth of subsurface water flow conditions and elaborated the variation of rill detachment rate with saturation depth of subsurface water flow.The model was established to quantify the effect of discharge,slope and saturation depth of subsurface water flow on rill detachment rate.The analytical method was also used to calculate the rill detachment rate to verify the correctness of the test method.The numerical improvement method was used to calculate the rill detachment rate to verify the applicability of this method in calculating the rill detachment rate of purple soil under subsurface water flow conditions.This study can help to understand the rill detachment pattern under the interaction of subsurface water flow and surface runoff in purple soils,thus providing a basis and model parameters for soil erosion forecasting under subsurface water flow in purple soil slopes,and providing a new idea for the calculation of rill detachment rate in purple soils.The main findings of this study are as follows.（1）Under the action of subsurface water flow,the rill detachment rate of purple soil was exponentially correlated with the rill length,and the decreasing trend was exponential with the increase of the rill length,and this decreasing trend was more obvious when the slope was steeper and the flow was higher.The rill detachment of purple soil was linearly correlated with the sand content,and the decreasing trend was linear with the increase of the sand content,and this decreasing trend was more obvious when the slope was steeper and the flow was higher.The maximum detachment rate of purple soil,ranged from 0.005 to 0.281 kg·m^-2·s^-1 at different discharges,slopes and saturation depths of subsurface water flow.The detachment rate increased with increasing slope or discharge under the same subsurface water flow conditions;the greater the saturation depth of subsurface water flow,the greater the rill detachment rate at the same discharge and slope length.（2）The multivariate nonlinear regression equations of slope,discharge and saturation depth of subsurface water flow with rill detachment rate indicate that saturation depth of subsurface water flow is positively correlated with fine trench detachment rate,and the discharge has the most significant effect on fine trench detachment rate,followed by slope and finally saturation depth of subsurface water flow.In the case of flat slope and low discharge,the effect of subsurface water flow on the detachment rate of the fine channel is not obvious.The results of the linear fit of the detachment rate under subsurface water flow conditions to the detachment rate of saturated soil with a zero intercept.It showed that the detachment rate of saturated soil was greater than the detachment rate under subsurface water flow;the detachment rate of saturated condition was 127%,68%and 34%greater than the detachment rate of saturated soil with subsurface water flow at 5 cm,10 cm and 15 cm depths,respectively.This indicates that the higher the saturation depth of subsurface water flow,the greater the rill detachment rate and the closer to the rill detachment rate of saturated soil,but all are smaller than the rill detachment rate of saturated soil.（3）Under different conditions,the rill detachment calculated by the analytical method showed an exponential decreasing trend with the length of fine channel and a linear decreasing trend with the sand content of water flow.The test values are distributed around the fitted curves of the analytical value and the length of the fine channel and the fitted straight line of the analytical value and the sand content,which indicates that the agreement between the test values and the analytical values is good.The contrast between the test value and the analyzed value is distributed near the 1:1line,the fitted line R² is larger than 0.92.The K value of the fitted line ranges from0.929 to 2.058,the test value is generally larger than the analyzed value.The average value of K value is 1.138,which means that the test value is 13.8%larger than the analyzed value on average.With the increase of slope,flow and saturation depth of subsurface water flow,the data points are closer to the 1:1.The correlation increases with the increase of slope gradient,discharge,and saturation depth of subsurface water flow.（4）Under different conditions,the numerical improvement method calculates rill detachment rate with the length of the groove is exponentially decreasing trend,and the improved value is closer to the analyzed value with the length of the curve;the slope of the linear fit K value with zero intercept between the improved value and the analyzed value ranges from 0.827 to 1.882,R² is larger than 0.97,the improved value is closer to 1:1 compared with the analyzed value.The test value is generally greater than the analyzed value.The average value of K is 1.032,which means that the improved value is 3.2%larger than the analyzed value on average.The numerical improvement method reduces the calculation error and improves the calculation accuracy compared with the improved value.The numerical improvement method is well applied to the calculation of fine groove spalling rate.