The Processes And Mechanisms Of Sloping Soil Erosion Driven By Freeze-thaw,Wind And Water Agents In Typical Chinese Mollisol Region

At present,basic theoretical researches on compound soil erosion in Mollisol region are still relatively scarce,especially researches regarding compound soil erosion driven by freezethaw,wind and water agents are rarely reported,which seriously affects the decision making for controlling soil erosion in Chinese black soil region.Thus,this thesis,using indoor simulated freeze-thaw test,wind tunnel test and the artificial simulated rainfall experiment,combined with analyzing soil property,monitoring soil surface morphology and measuring surface flow hydraulics and hydrodynamics,analyzed effects of freeze-thaw,wind and water agent superposition on sloping soil erosion in Chinese Mollisol region and clarified processes and mechanisms of sloping soil erosion driven by freeze-thaw,wind and water agents.The results enrich the basic theory of compound soil erosion,and provide a scientific basis for preventing compound soil erosion in Mollisol region.The main research results are as follows:(1)The effects of freeze-thaw,wind and water erosive agents on sloping soil erosion were clarified.Compared with the corresponding experimental treatment without prior soil freezethaw action,after soil experienced one freeze-thaw cycle,soil wind erosion intensity and sediment delivery rate were increased by 0.2-4.0 times and 0.31-11.63 times(P<0.05),respectively.Meanwhile,the prior freeze-thaw action significantly increased sloping water erosion intensity by 10.3%-17.5%(P<0.05).Compared with the water erosion only experimental treatment without previous wind erosion,the previous wind erosion action significantly increased the sloping water erosion intensity by 2.9%-23.1%(P<0.05);and the superposition of freeze-thaw,wind and water agents significantly increased the sloping water erosion intensity by 16.5%-36.1%(P<0.05).(2)The positive cumulative effects of multi-forces superposition on sloping soil erosion intensity were analyzed.At the experimental conditions,the cumulative effect of freeze-thaw and water agent superposition on sloping soil erosion intensity ranged from 10.3% to 13.3%;the cumulative effects of freeze-thaw,wind and water forces superposition on sloping soil erosion intensity varied from 16.5% to 36.0%.Moreover,the cumulative effect of wind and water agents on sloping soil erosion intensity changed from 2.9% to 8.1%.(3)The contribution rates of freeze-thaw,wind and water forces on sloping soil erosion were quantified,and interactions among affecting factors were also clarified.The contribution rates of freeze-thaw action to soil wind erosion and sloping water erosion were 23.5%-404.2%and 10.3%-17.5% respectively,the contribution rate of previous surface wind erosion action to later sloping water erosion was 2.91%-23.07%;meanwhile,the contribution rate of freezethaw and wind forces superposition to sloping water erosion was 16.49%-36.07%.For the experimental treatment that without the prior soil freeze-thaw action,the contribution rates of initial soil water content,wind speed of wind erosion and their interaction to soil wind erosion intensity were 5.0%,74.6% and 12.7%(P<0.01),respectively.For the experimental treatment with prior soil freeze-thaw action,the contribution rates of initial soil water content,wind speed of wind erosion and their interactions to soil wind erosion intensity were 26.1%,34.8%and 36.7%(P<0.01),respectively.For the water erosion only experimental treatment without the prior soil freeze-thaw action,the contribution rates of initial soil water content and slope gradient to water erosion intensity were 2.53% and 97.16%(P<0.01),respectively.Moreover,for the water erosion treatment with prior soil freeze-thaw action,the contribution rates of initial soil water content and slope gradient to water erosion intensity were 1.78% and 98.11%,respectively(P<0.01).In addition,for the experimental treatment of wind and water agent superposition,the contribution rates of initial soil water content,wind speed and their interaction to sloping water erosion intensity were 27.3%,70.2% and 1.0%,respectively(P<0.01).(4)The affecting mechanisms of freeze-thaw,wind and water agent superposition on sloping soil erosion were revealed.The prior soil freeze-thaw action significantly weakened the indicators of soil anti-erosion ability,thus it increased both soil wind erosion and sloping water erosion intensities.For example,the prior soil freeze-thaw action caused that soil hardness reduced by 24.4%-53.3% and soil shear strength decreased by 48.9%-54.5%(P<0.05).Since the previous wind erosion action produced a large number of wind erosion stripes and large area of wind erosion dents on soil surface,thus,previous wind erosion action intensified confluences of sloping runoff during the later sloping water erosion process,which caused that flow velocity increased,while Darcy-Weisbach resistance coefficient reduced,and then runoff erosion capacity increased,and resulted in an increase of water erosion intensity.Moreover,it is that coupling action of previous freeze-thaw reducing soil anti-erosion ability and surface wind erosion increasing runoff erosion capacity,therefore,wind and water agent superposition significantly increased late sloping water erosion intensity.