Effects Of Herbaceous Vegetation Pattern On Erosion And Sediment Yield On Typical Slope In Loess Region Of Western Shanxi

Soil erosion is one of the most important ecological and environmental problems in the world.It not only causes land degradation and ecosystem destruction,but also causes sediment to block river channels,raise river beds,reduce river flood carrying capacity and promote the occurrence of flood disasters.Vegetation is an important factor to control soil erosion on slope.Appropriate vegetation coverage,location and patch pattern are of great significance to control soil erosion and water and soil loss on slope.In this study,artificial rainfall was used to study the runoff and sediment yield process of vegetation coverage,vegetation placement position and vegetation patch pattern under different rainfall intensity and slope,analyzed the influence of slope vegetation(coverage,location and patch pattern)on soil erosion.Based on the hydrodynamic characteristics of runoff,determined the optimal vegetation coverage on the slope,the optimal distribution position of vegetation,and the pattern of vegetation patches,established a water-sediment model to evaluate the vegetation patch pattern,which could provide theoretical support for the selection of soil erosion control measures in the loess region of western Shanxi Province.The main research results are as follows:(1)Rainfall intensity and slope had significant effects on initial runoff time,steady infiltration rate,average runoff rate,cumulative runoff yield,sediment yield rate,cumulative sediment yield and runoff mud content.With the increase of vegetation coverage,runoff yield rate and cumulative runoff yield decreased,and the time to reach stable runoff yield increased.With the increase of vegetation coverage,both Reynolds number and Froude number of slope runoff decreased significantly,while slope resistance increased significantly with the increase of vegetation coverage.(2)When the vegetation relative distance(the ratio of bare land length at the lower edge of vegetation belt to slope length minus the difference of vegetation length along slope)is 0.2,the slope runoff yield is the minimum.The cumulative runoff yield,Reynolds number and Froude number of runoff with relative distance of 0,0.2 and 0.4 were significantly lower than those of slope with relative distance of 0.6,0.8and 1,but the smaller the relative distance of vegetation was,the greater the resistance to runoff was.Under the condition of vegetation,the runoff resistance increased by 1.49?9.61 times.(3)The order of Reynolds number,Froude number and runoff rate of different vegetation patterns were as follows: S road patch pattern > horizontal strip patch pattern > field type patch pattern > random patch pattern;The order of drag coefficient was as follows: random patch pattern > field grid patch pattern >horizontal strip patch pattern > S road patch pattern;The runoff power and unit runoff power showed a random pattern of bare land > S road pattern > horizontal strip pattern > field grid pattern.Rainfall intensity and slope can increase Reynolds number and Froude number,and rainfall intensity has the greatest effect on slope hydrodynamic parameters.(4)Slope runoff shear force,runoff power and unit runoff power all decreased with the increase of vegetation coverage,but increased with the increase of rainfall intensity and slope.There is a linear correlation between runoff shear force and sediment yield rate when vegetation coverage is less than 60%.On the whole,slope runoff power and unit runoff power are proportional to the relative distance of vegetation.(5)With the increase of runoff power and unit runoff power,the slope sediment yield rate,runoff power and unit runoff power increased significantly.When the coverage increases from 0% to 90%,the critical runoff power of slope sediment yield decreases from 0.023 to 0.016.When the relative distance of vegetation changes from 0 to 1,the critical runoff power of slope sediment yield firstly decreases and then increases,and the critical runoff power of slope sediment yield is the minimum when the relative distance of vegetation is 0.2.(6)The correlation between vegetation pattern index and runoff yield and sediment yield was significant.There was a significant correlation between patch density(PD),landscape connectivity(COHESION)and runoff rate and sediment yield rate.The landscape shape index(LSI)and the runoff rate and sediment yield rate were significantly correlated.PD,LSI and COHESION were negatively correlated with flow rate and sediment rate.(7)Spilt,IJI and Division had significant positive correlation with runoff and sediment yield(p<0.05).COHESION rate could be determined by landscape connectivity degree(COHESION).The runoff effect evaluation model of vegetation pattern on slope was as follows: runoff yield rate =10.437-0.099×COHESION;The modulus of erosion could be characterized by landscape connectivity degree(COHESION),and the evaluation model of sediment transport effect of slope vegetation landscape pattern was as follows: sediment yield rate = 161.911-1.564×COHESIO.(8)Based on the response surface optimization(RSM)method,the optimal vegetation coverage calculation model for soil erosion control on slope surface was proposed.The model could be used to calculate the optimal vegetation coverage required by different rainfall intensity conditions and different slopes.If the slope coverage of 20 degrees must reach 90%,the amount of erosion caused by 60mm/h rain intensity can be controlled to be less than the allowable amount of erosion.(9)When the relative distance of slope vegetation is 0?0.22,the effect of reducing runoff and sediment is the best.It is suggested that when restoring slope vegetation in areas with limited precipitation resources,priority should be given to restoring vegetation on the middle and lower slopes.At the same time,the connectivity of vegetation patches should be enhanced,the patch density should be increased,and the patch shape should be complicated and the patch distribution should be random as far as possible.