| The Loess Plateau of China is the most severely soil erosion area in the world.The severe soil erosion in the the Loess Plateau has caused serious deterioration of local ecological environment,and caused the Yellow River becoming a world-famous "suspend river" which posed a serious threat to the downstream regions of the Yellow River.Since the China of Green Project was launched by the Chinese government in 1999,grassland has largely formed in the loess Loess Plateau has produced a widely ranged of grassland with different coverage,the Yellow River sediment has been reduced dramatically,the erosion intensity of the the Loess Plateau has been reduced significantly,and the processes of splash and sheet erosion have become the dominant erosion processes on grassland slope(sheet erosion was the absolutely predominant erosion process on the high-coverage grassland).The existence of large-area low-coverage grassland and the climatic characteristics of high precipitation variation rate in the Loess Plateau inevitably lead to a non-negligible occurrence of splash and sheet erosion on large areas.And the conflux of huge quantities of sheet flow with certain intensity,can cause serious erosion on the slopes and channels,contributing to the high content of sediment in the Yellow River.Therefore,this study aiming at the low-coverage grassland in the The Loess Plateau,the researches on the dynamic processes of splash and sheet erosion on the grassland slope in the loess region,which can deeply understand the mechanism of the splash and sheet erosion on grassland slope and the regulation effect of the grass on the processes of splash and sheet erosion.Simultaneously,it can lay an important foundation for establishing the model of splash and sheet erosion on grassland slope,promote further development of the study of grassland erosion process,as well as sheet erosion theory,and provide important information and scientific basis for soil and water conservation and ecological construction on the hillslope of the loess region,the analysis of the Yellow River sediment reduction and the Yellow River health management.Simulated rainfall experiments were conducted on grassland plots under the complete combinations of different rainfall intensities(0.7 mm min-1,1.0 mm min-1,1.5 mm min-1,2.0 mm min-1,2.5 mm min-1)and different slopes(7°,10°,15°,20°,25°),to study the dynamic processes of splash and sheet erosion on the grassland slope with low grass coverage(40%)in the loess region.Splash erosion process and its hydrodynamic mechanism,responses of sheet flow hydraulic properties toraindrop impact,and responses of sheet erosion to splash erosion,rainfall power and sheet flow powerwere investigated and analyzed.The main conclusions are as follows:1.Studying the splash erosion process on grassland slope in loess region.(1)Under different rainfall intensities,and slopes,the variations of splash erosion rate on grassland slopes with rainfall time are similar.It sharply decreases at the first few minutes,and then reaches a relative steady state with rainfall time,which can be described by logarithmic equations.(2)Under different rainfall intensities,the splash erosion rate on grassland slope increases with the increase of slope,which can be described by power function equation.Under different slopes,the splash erosion rate on grassland slopes also increases with the increase of rainfall intensity,which can be described by logarithmic equation.(3)The splash erosion equation on grassland slope in loess region is binary power function equation of splash erosion rate to rainfall intensity(I)and slope(S):Mr = 0.005I1.037S0.566.Under different rainfall intensities and slopes,the splash erosion rate on grassland slope was positively correlated with rainfall intensity and slope.The contribution rates of rainfall intensity and slope to the splash erosion rate were 69.61%and 25.20%,respectively.2.Clarifying the hydrodynamic mechanism of splash erosion process on grassland slope in loess region.(1)Under different slopes,the splash erosion increases gradually with the increase of raindrop kinetic energy,increases rapidly at first and then increases gradually with the increase of raindrop terminal velocity,and increases rapidly at first and then increases gradually or even reaches a relative stable state finally with the increase of raindrop median volume diameter,Which can be described by logarithmic equations.(2)Under different slopes and rainfall intensities,the splash erosion rate on grassland slope can be described by power function equation with raindrop kinetic energy,power function equation with raindrop terminal velocity and logarithmic equation with raindrop median volume diameter,respectively.To sum up,raindrop kinetic energy is the closest rainfall parameter that related to the splash erosion rate on grassland slope,and it is the best rainfall parameter to describe the splash erosion rate in this study.The best response equation of the splash erosion rate to rainfall parameters is the power function equation of the splash erosion rate(Mr)to the raindrop kinetic energy(KE):Mr = 7 × 10-5KE1.043.(3)Under different slopes and rainfall intensities,the comprehensive response of the splash erosion rate on grassland slope to raindrop kinetic energy and sheet flow depth,raindrop kinetic energy and sheet flow velocity,raindrop kinetic energy and sheet flow depth and flow velocity can be described by binary power function equations,binary power function equations and ternary power function equation,respectively.The combination of raindrop kinetic energy,sheet flow depth and velocity is the closest rainfall and hydraulic combined parameters that related to splash erosion rate on grassland slope,and is the closest hydrodynamic indicators for splash erosion rate on grassland slope.In summary,hydrodynamic equation of the splash erosion process on grassland slope in loess region is the ternary power function equation of splash erosion rate to raindrop kinetic energy,sheet flow depth and velocity:Mr =3 × 10-7KE1 309h-0 541V0.677.(4)Under different slopes and rainfall intensities,the contribution rates of raindrop kinetic energy,sheet flow depth and flow velocity to the splash erosion rate were 64.95%,11.33%and 17.73%,respectively,indicating that raindrop kinetic energyhas the greatest effect on splash erosion rate,followed by sheet flow velocityand sheet flow depth has the lowest effect on splash erosion rate in this study.3.Analyzing the response of hydraulic properties of sheet flow to raindrop impact ongrassland slope in the loess region.(1)Under different slopes and rainfall intensities,most hydraulic parameters of sheet flow(velocity,depth,Reynolds number,resistance coefficient,shear stress and stream power)are affected by the rainfall parameters(raindrop kinetic energy,raindrop terminal velocity.Among the seven hydraulic parameters,the most affected by rainfall parameters is the Reynolds number,followed by shear stress,again sheet flow depth and stream power,velocity and resistance coefficient are less affected.Raindrop kinetic energy is the closest rainfall parameter that related to these hydraulic parameters,and is the main rainfall characteristic parameter that affects hydraulic parameters.The relationship ofthe Froude number(characterizing sheet flow state)and unit stream power to rainfall parameters were extremely poor,indicating that the two hydraulic parameters are little influenced bythe raindrop impact.(2)Under different slopes and rainfall intensities,most hydraulic parameters of sheet flow(velocity,depth,Reynolds number,resistance coefficient,shear stress and stream power)increase with the increase of the rainfall parameters(raindrop kinetic energy,raindrop terminal velocity and raindrop median volume diameter).It shows that the increasing of values of the rainfall parameters can accelerate the sheet flow velocity,increase the sheet flow depth,flow turbulence,resistance,shear stress,and stream power.(3)Under different slopes and rainfall intensities,the variations of flow velocity,sheet flow depth,Reynolds number,resistance coefficient,shear stress and stream power with raindrop kinetic energy can be described by the logarithmic equation,power function equation,power function equation,exponential equation and power function equation,respectively.4.Revealing the response of sheet erosion to splash erosion,rainfall power and sheet flow power on grassland slope in the loess region.(1)Under different slopes or rainfall intensities,the sheet erosion rate on grassland slope increases with the increase of splash erosion rate,which can be described by power function equation or logarithm function equation.Under different slopes and rainfall intensities,the sheet erosion rate on grassland slope increases with the increase of splash erosion rate,and the response equation of sheet erosion to splash erosion on grassland slope is the power function equation of sheet erosion rate(Di)to splash erosion rate(Mr):Di = 1.508Mr0.121.(2)Under different slopes and rainfall intensities,the response relationships of sheet erosion rate to shear stress and splash erosion rate,stream power and splash erosion rate,unit stream power and splash erosion rate,respectively,can be described by binary power function equations.Stream power together with splash erosion rate is the best combination for describing the relationship of sheet erosion rate to the hydraulic parameters of sheet erosion and splash erosion.The best response equation of sheer erosion to the hydraulic parameters and splash erosion is the binary power function equation of sheet erosion rate(Di)to stream power(?)and splash erosion rate(Mr):Di=1.514?0.296Mr0.899.(3)Under different rainfall intensities and slopes,the variations of sheet erosion rate on grassland slope with stream power and raindrop kinetic energy and splash erosion rate,stream power and raindrop terminal velocity and splash erosion rate,stream power and raindrop median volume diameter and splash erosion rate,respectively,can be described by the ternary power function equations.Furthermore,the sheet erosion rate on grassland slope is positively correlated with the stream power,rainfall parameters and splash erosion rate.Stream power,together with raindrop kinetic energy and splash erosion rate is the best combination fordescribing the relationship of sheet erosion rate on grassland slope and to the hydraulic parameters of sheet flow,rainfall parameters and splash erosion.The sheet erosion equation on grassland slope in the loess region is the ternary power function equation of the comprehensive response of sheet erosion rate(Di)to stream power(?),raindrop kinetic energy(KE)and the splash erosion rate(Mr):Di=0.297?0.308KE0.197Mr0.753. |
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