Impacts Of Erosive Force Factors On Hillslope Water Erosion Process In Typical Mollisol Region Of Northeast China

Hillslope water erosion process in the typical Mollisol region of northeast China was affected by several erosive force factors such as rainfall,inflow,soil pipe flow,and seepage flow.However,mechanisms of these erosive force factors and their interaction on hillslope water erosion process are unclear at present.Therefore,this study designed three inflow rates of 30,40 and 50 L·min^-1 and different near-surface hydrological conditions（without soil pipe,with soil pipe and without soil pipe flow,with soil pipe flow,and seepage flow）to analyze impacts of erosive force factors（inflow,soil pipe flow,seepage flow）on hillslope water erosion process,to separate the contributions of inflow,soil pipe flow,and seepage flow to slope soil erosion,and to reveal mechanisms of inflow,soil pipe flow,and seepage flow on hillslope water erosion.The results can provide an important scientific basis for slope erosion control in the typical Mollisol region.The main research results are as follows:（1）The influence of inflow rates on the hillslope water erosion process was explored.When the inflow rate increased from 30 L·min^-1 to 40 and 50 L·min^-1,hillslope soil erosion intensity and rill erosion rates increased by 100%,214.8%,and 106.3%,241.3%,respectively.The average rill width and depth increased by 4.5%,16.7%and 62.8%,81.4%,respectively.An increase of inflow rates caused that overland flow velocity increased by 17.1%-31.4%and stream power increased by 17.7%and 37.9%,respectively,while the Darcy-Weisbach friction coefficient decreased by 23.1%and 34.1%,respectively.Therefore,increasing inflow rates enhanced slope runoff detachment capacity and sediment transport capacity,promoted rill development process,and further increased hillslope soil erosion rates.（2）The influence of soil pipe generation and pipe flow occurrence on hillslope water erosion was analyzed.Under three inflow rates of 30,40 and 50 L·min^-1,the hillslope erosion rates with soil pipe but without soil pipe flow（PF₀）and PF₁（with soil pipe flow）treatments increased 36.4%-60.3%and 68.2%-107.9%,compared with without soil pipe（NP）treatment,respectively,and the rill erosion rates under PF₀ and PF₁ treatments increased 38.1%-66%and93.7%-128.4%,compared with NP treatment,respectively.The average rill width and depth under PF₀ and PF₁ treatments were 1.0-1.4 times,1.3-1.6 times,and 1.0-1.5 times,1.4-1.6times of those under NP treatment,respectively.The soil pipe generation and pipe flow occurrence reduced soil cohesion and soil shear strength,and further increased overland flow velocity,shear stress and stream power,while declined the Darcy-Weisbach friction coefficient,which enhanced slope rill erosion process and increased slope soil erosion rates.Meanwhile,soil pipe collapse also promoted rill undercut erosion and sidewall failure erosion,and further aggravated the hillslope erosion process.（3）The effects of different erosive force factors on hillslope erosion process were clarified.Under the superposition of inflow,soil pipe flow,and seepage flow,the hillslope erosion and rill erosion rates increased by 141.9%-371.6%and 164.2%-368.9%with seepage water head height 20 cm above the soil surface,respectively,compared with the treatment without seepage flow.Compared with seepage water head height 20 cm below the soil surface,hillslope erosion and rill erosion rates with seepage water head height 20 cm above the soil surface increased by 4.7%-104.1%and 9.2%-112.6%respectively.On the one hand,the occurrence of seepage flow weakened the cohesive force of soil particles,dispersed soil particles,reduced the friction coefficient of sediment transport,and increased the flow velocity and Flow shearstress,which enhanced slope runoff detachment capacity.On the other hand,seepage flow promoted soil pipe erosion,and the time of soil pipe collapse was significantly advanced,which greatly enhanced the rill undercut erosion and sidewall failure,and further intensified the hillslope water erosion process.（4）The contributions of inflow,soil pipe flow,seepage flow and their interaction to hillslope erosion were separated.The contribution rate of inflow factor was 36.8%,followed by the seepage flow（23.5%）,and then the soil pipe flow（16%）.The contribution rates of interaction among inflow,soil pipe flow,and seepage flow was 10.7%.The results indicated that the inflow was the main erosive force factor in the process of hillslope water erosion under the experimental treatments,and the influence of seepage flow on slope water erosion was greater than that of soil pipe flow.（5）Mechanisms of different erosion forces impacts on slope soil erosion were discussed.Equations between slope soil erosion rates and overland flow shear stress,stream power were fitted.It is found that slope erosion rate was significantly positively correlated with overland flow shear stress and stream power,and the stream power was the most suitable hydrodynamic parameter to characterize hillslope soil erosion rate under different erosive force factors of inflow,soil pipe flow and seepage flow.