Experimental Study On Grass Regulation Of The Dynamics Process Of Sheet Erosion Of Loess Slope

Loess plateau suffers the severest soil erosion across the world.The grave soil erosion caused severe impact on and posed a threat to the sustainable development and the security of the banks of the downstream of Yellow River.Since 1999 with the implementation of the project of concede the land to forestry,the grass coverage rate of Loess plateau has gone up remarkably.Great changes have taken place in the process of soil erosion.The sand and soil in Yellow River has reduced dramatically at a historic low.Sheet erosion is one of the most important erosion processes and covers the largest area in the sloping field.The erosion mode that takes the upper hand at grass slopes also belongs to sheet erosion.Elaboration on the grass regulation of the dynamics process of sheet erosion of loess plateau could give an in-depth knowledge on the changing process of soil erosion driven by returning farmland to grassland of Loess plateau,advance the study on erosion process of grassland and soil erosion discipline,as well as providing important scientific proof for the management of soil erosion and ecological development of the slope surface of the Loess Plateau,revealing of the reason that the soil and sand reduces drastically in Yellow River and sound management of Yellow River.This paper adopts an experimental approach with simulated rainfall raining on two different fields,namely,one with grass and the other with no grass.Under the condition of same slope with different rainfall intensity and grass coverage(slope 15°,rainfall intensity 0.7 mm/min,1.0 mm/min,1.5 mm/min,2.0 mm/min,2.5 mm/min and grass coverage rate 0%,30%,40%,50%,60%and 70%in full combination)and the condition of same rainfall intensity with different slopes and grass coverage rate(rainfall intensity mm/min,slope 7°,10°,15°,20°,25°and grass coverage rate0%,30%,40%,50%,60%and 70%in full combination),an experimental study on grass regulation in the dynamics process of sheet erosion of loess slope is conducted.It studies the regulating effect of grass on the water depth reduction and sheet erosion modulus reduction,dynamics feature of sheet erosion and soil erodibility,as well as the mechanism of regulating to the sheet erosion process.Major conclusions are as follows:1.Studying the regulating effect of grass on the water depth reduction and sheet erosion modulus reduction at the slope in the sheet erosion process.(1)Under the condition of different rainfall intensity and slope,the depth of sheet flow of grassland decrease with the increase of the grass coverage density,which could be described with linear equation;with different rainfall intensity and slopes,coverage grows from30%to 70%,the water depth deduction effect of grass increases with the increase of grass coverage intensity,which could also be described well with logarithmic equation.Water reduction effect grows from 3.30%-21.69%to 23.73%-53.90%and from 9.03%-19.42%to 22.96%-40.57%respectively,and from 12.15%to 35.75%and12.14%to 30.77%on average,among which,average water depth reduction effect increases more obviously with the coverage ranging from 30%-40%and from60%-70%,growing 8.61%and 5.70%as well as 5.90%and 5.02%.(2)Under different rainfall intensity and slopes,the sheet erosion rate of loess slope is basically similar with that of bare slope under different rainfall intensity.Generally,both of them grow rapidly at the early stage,slow down gradually or tend to stability with the extension of rainfall time,which could be described with functional equation.Under different rainfall intensity and slopes,sheet erosion modulus of the grassland slope reduces with increase of grass coverage intensity,which could be described with linear equation.For the variation of rainfall intensity,slope and coverage intensity,they could be well described with ternary functional equation,having positive correlations with rainfall intensity and slope and negative correlation with coverage intensity;Under different rainfall intensity,grass coverage ranging between 30%-70%could realize the sheet erosion modulus reduction effect of 11.4%-72.91%,with an average rate of 37.13%.The sheet erosion modulus reduction effect of grass increases with the growth of grass coverage which could be well described with linear equation,with coverage increasing from 30%to 70%,the sheet erosion modulus reduction effect increases from 11.40%-23.42%to 41.44%-72.91%,and grow from 17.38%to57.11%on average.And the coverage of 30%-40%and 60%-70%,the average sheet erosion modulus reduction effect increases more obviously by 111.24%and 11.48%respectively.At different slopes,the sheet erosion modulus reduction effect of grass coverage ranging 30%-70%is 11.42%-61.26%,33.24%on average.The sheet erosion modulus reduction effect of grass grows with the increase of grass coverage,which could be described with linear equation.With the growing of coverage from30%-70%,the sheet erosion modulus reduction effect reduces from 11.42%-23.28%to 44.10%-61.26%,growing from 16.57%to 49.45%on average.The average sheet erosion modulus reduction effect increases more remarkably with coverage between30%-40%and 60%-70%,growing by 10.12%and 9.19%respectively.The sheet erosion modulus reduction effect of grass varying with the changes of rainfall intensity,slopes and coverage could be well described by the ternary logarithmic equation,having position correlation with coverage and slope and negative correlation with rainfall intensity.Under different rainfall intensity and slopes,grass coverage and management factor(C factor)reduces with the increase of grass coverage,which could be described with linear equation.Ternary logarithmic equation could be used to well describe the variation among coverage,rainfall intensity and slope,having positive correlation with coverage and slope and negative correlation with rainfall intensity.(3)With different rainfall intensity and slopes,sand content of grass sheet flow reduces with the increase of grass coverage and the regulating effect to sand content of grass coverage grows with the increase of grass coverage respectively,which could be described with linear equation and exponential equation,indicating that with the increase of grass coverage,sheet erosion modulus reduction effect is greater than water depth reduction effect.2.Revealing the regulating effect of grass coverage on sheet erosion dynamics feature of sloping surface.(1)With different rainfall intensity and slopes,bare slope and sheet erosion modulus of sloping surface with grass coverage of 30%,40%,50%,60%,70%are arranged in descending order with average water flow shear stress,Stream power,Unit stream power,coefficient of determination of per-unit energy coupling relation of cross section,average value of coefficient of determination,as follows,Stream power(0.956;0.9568)>Shear stress(0.921;0.9136)>Unit energy of cross section(0.839;0.8976)>Unit stream power(0.486;0.5876).Stream power a hydrodynamic parameter,which best in sheet erosion coupling relationship and mostly closely connected with different slope surfaces under test condition,indicating that it will not change the source of driving force of sheet erosion after grass growing out from the bare slope.(2)The stream power of grass slope reduces with the increase of grass coverage and both linear equation and power function equation can be used to describe this respectively.Under the condition of different rainfall intensity and slopes,with grass coverage increasing from 30%to 70%,the regulating effect of grass to stream power is growing with the increase of coverage,which could be well described with logarithmic equation,with regulating effect increasing from 3.48%-20.28%and9.68%-20.80%to 21.91%-49.90%and 20.56%-39.15%,and increasing from 12.30%and 12.67%to 32.90%and 28.27%respectively.The average regulating effect increases more prominently with coverage of 30%-40%and 60%-70%,growing by8.11%and 5.68%,as well as 5.22%and 3.74%.3.Analyzing the regulating effect of grass to the soil erodibility of the slope surface.(1)The coefficient of soil erodibility at different grass slopes reduces with the increase of grass coverage,which could be described with linear equation,with the coverage increasing from 30%to 70%,the coefficient of soil erodibility of different grass slopes of 7°,10°,15°,20°and 25°is 0.0121-0.0156s~2/m~2,0.0111-0.0144s~2/m~2,0.0100-0.0140 s~2/m~2,0.0110-0.0133 s~2/m~2,0.0156-0.0170s~2/m~2.The coverage ranging 30%-40%and 60%-70%,the coefficient of soil erosion reducing relatively obviously occurs at a large number of slopes.(2)The influence of grass coverage and slope on the coefficient of soil erosion of grass slope can be described with binary exponential equation.This coefficient has negative correlation with coverage and positive correlation with slope,which shows the role that the grass coverage and slope play in regulating the soil erodibility of grass slope.Coverage coefficient(0.534)is far greater than the slope coefficient(0.007),indicating that the influence of grass coverage is much greater than that of slopes.(3)Under the condition of rainfall intensity 1.5mm/min,slope 7°-25°below,coverage 30%-70%,the regulating effect of grass to soil erodibility coefficient should be 0.27%-33.20%,average regulating effect 14.63%.The regulating effect of soil erodibility with grass increases with the increase of grass coverage,and can be well described with linear equation.With coverage increasing from 30%to 70%,the regulating effect growing from0.27%-13.30%to 8.32%-33.20%,average regulating effect from 6.10%to 23.99%.The average regulating effect of coverage between 60%and 70%increases most obviously,by 5.4%.The regulating effect of grass to soil erodibility coefficient varies with different coverage and slopes,which could be described with binary linear equation,having positive correlation with coverage and negative correlation with the slope.Grass coverage coefficient(0.439)is far greater than that(0.009)of the slope,indicating that the influence of grass is obviously greater than slopes.4.Illustrating the mechanism of regulating effect of the grass to the sheet erosion process.(1)With different rainfall intensity and slopes,the regulating effect of grass to the stream power of sheet erosion varies with the changes of grass coverage and basal cover,which could be described with binary logarithmic equation.With different rainfall intensity and slopes,the contribution ratio of regulating effect of grass coverage and basal cover to The stream power of sheet erosion is 82.86%-97.51%and1.48%-14.82%,as well as 86.36%-97.51%and 1.48%-20.44%respectively,indicating that the regulating effect of grass to The stream power of sheet erosion is realized through the regulation of grass coverage.(2)At different slopes,the regulating effect of grass to soil erodibility varies with the difference of grass root volume and soil bulk density,which could be described with binary logarithmic equation.The contribution ratio of the regulating effect of grass root volume and soil bulk density to soil erodibility at different slopes is 73.61%-97.94%and 0.04%-0.22%,indicating that the regulating effect of grass to soil erodibility is realized through the regulation of grass root volume and improving the soil bulk density has little regulating effect to soil erodibility.(3)With different rainfall intensity,different slopes and different rainfall intensity and slopes,the relationship of the regulating effect(sheet erosion modulus reduction effect with grass)of grass to sheet erosion with that to stream power of sheet erosion and that to soil erodibility can be well described with binary linear equation,with the regulating effect of grass to stream power of sheet erosion having positive correlation with that of soil erodibility.With different rainfall intensity,different slopes and different rainfall intensity and slopes,the regulating effect of grass to stream power of sheet erosion and that to soil erodibility both have contributed to the regulating effect of grass to sheet erosion,with contribution ratio as follows,70.17%and 23.27%,45.41%and 52.29%,61.02%and 33.55%respectively.