Regulation And Critical Threshold Of Grass Cover On Slope Runoff Under Artificial Rainfall

The rainfall in the Loess Hilly and gully area is concentrated and mostly rainstorm.The runoff on the slope caused by rainstorm is the main cause of serious soil and water loss in the loess hilly area,and the grassland vegetation has a good inhibitory effect on the runoff on the slope.Grassland vegetation can delay the generation of slope runoff by changing the underlying surface conditions and soil infiltration rate.Therefore,it is of great significance to reveal the regulation of grassland vegetation on the process of slope runoff for controlling soil erosion in this area.In this study,the Typical Grassland Slope in the Loess Hilly and gully area is taken as the research object.Through the field artificial rainfall,the response law of rainfall runoff process on the slope to different grass coverage is explored.The main conclusions are as follows:(1)The time for slope flow to reach stable runoff generation is longer with the increase of grassland vegetation coverage,and the time for stable runoff generation decreases with the increase of rainfall intensity.The time for runoff generation is negatively correlated with rainfall intensity,and positively correlated with grassland vegetation coverage.Under the rainfall intensity of60mm/h,the runoff generation time of 80% coverage slope is 467 s,while under the rainfall intensity of 120mm/h,the runoff generation time of bare slope is 56S;under the rainfall intensity of60mm/h,the runoff generation time of each coverage slope is 89s-467s;under the rainfall intensity of 90mm/h,the runoff generation time is 74s-267s;under the rainfall intensity of 120mm/h,the runoff generation time is 56s-183 s.(2)The infiltration rate of slope soil decreased gradually with the experiment,and finally tended to be stable.Under the condition of high vegetation coverage and low rainfall intensity,the change of soil infiltration rate is not significant,the change of infiltration rate on bare slope isobvious,and the influence of vegetation coverage on soil infiltration rate is significant.(3)Slope runoff is significantly affected by the vegetation coverage,when the vegetation coverage is higher than 60%,the slope runoff will be significantly reduced.Under the rainfall intensity of 60mm/h,80% of the slope coverage can inhibit 95.7% of the runoff compared with the bare slope,while under the rainfall intensity of 90mm/h,the slope coverage can inhibit 72.2% of the runoff,while under the rainfall intensity of 120mm/h,the inhibition rate is only 54.8%.For the intensity of heavy rainfall,the inhibition effect of grassland vegetation coverage on runoff is not significant,so many measures should be taken to consider soil and water conservation.(4)Slope flow velocity is significantly affected by vegetation coverage and rainfall intensity.The greater the rainfall intensity is,the faster the slope flow velocity is.This is because the rainfall intensity will increase the carrying capacity of rainfall on the slope,increase the energy of raindrops splashing on the surface,and make the soil surface more easily skinned,making the slope more smooth,which is beneficial to increase the slope flow velocity.The grassland vegetation coverage can effectively inhibit the generation of slope flow,and also reduce the flow velocity of slope flow.(5)The Reynolds number and Froude number of slope flow are 40.07-695.22 and 0.33-1.56,respectively,and the resistance coefficient is 1.42-43.53.The larger the rainfall intensity is,the less the influence of vegetation coverage on these parameters is.The time for Reynolds number to reach stability increases with the increase of grassland vegetation coverage,because vegetation effectively improves soil infiltration rate,and more vegetation intercepts rainwater.The Froude number changes more obviously under low rainfall intensity,and the range is smaller under 120mm/ h rainfall intensity.However,the change of resistance coefficient shows a slow growth in the early stage and a relatively rapid growth in the later stage,which is due to the change of underlying surface conditions and the shape of rill flow with the progress of the test,so that the resistance coefficient can grow more rapidly in the later stage of the test.(6)For the Loess Hilly and gully area,75.31% of the grassland vegetation coverage is the threshold of the grassland vegetation coverage,When the rainfall intensity is not higher than60mm/h,it can effectively inhibit 80% of the runoff,and when the rainfall intensity is not greater than 90mm/h,it can effectively inhibit 66.68% of the runoff,and it can control the water and soil loss in the region with the greatest benefit.