Effects Of Wind And Water Agent Superposition Action On Hillslope Soil Erosion In Chinese Thin Mollisol Region

Farmland in Chinese Mollisol region is bare and dry,which suffers from serious water and wind erosion.Wind is strong in late spring and rainfall is concentrated in summer and autumn.The above background results in coupound soil erosion under the external erosion agencies of water and wind,which reflects in alternation in time and superposition in space.Soil erosion in this region is intensified.To date,studies on the effects of wind and water agent superposition on the impact of soil erosion on Mollisol hillslopes is rarely reported.In this study,based on the combination of simulated wind erosion experiment and simulated rainfall experiment,the effects of wind and water agent superposition on soil erosion on Mollisol hillslope was studied,and the mechanism of wind and water agent superposition on Mollisol erosion was analyzed.The results not only enrich the theory of compound soil erosion,but also provide an important scientific basis for the precise implementation of soil and water conservation measures in Mollisol region.The main research conclusions are as follows:（1）The action of wind-water agent superposition significantly increased the water erosion（P<0.05）.Compared with the rainfall experiment treatment without previous wind erosion,the the rainfall intensities of 50 and 100 mm h^-1 combined with previous soil wind erosion at the wind velocities of 9,12 and 15 m s^-1 increased the runoff,water erosion and runoff sediment concentration by 2.5%～17.9%,5.0%～80.3%and 11.5%～70.8%at 3°slope gradient;and by 1.6%～7.7%,2.1%～46.3%and 2.5%～31.1%at 7°slope gradient,respectively.The superposition of wind and water agent makes runoff,water erosion and sediment concentration increase with the increase of wind velocity of soil erosion.However,the increase of rainfall intensity weakend the effect of the previous wind erosion on water erosion.（2）The previous soil wind erosion has a significant positive interaction effect on the later water erosion,and the contribution of the previous soil wind erosion to the later water erosion is significantly related to the wind velocity of the previous soil wind erosion and the rainfall intensity of the later water erosion.The contribution of wind erosion to water erosion increased significantly with the increase of wind velocity of previous soil wind erosion,and decreased with the increase of rainfall intensity in the rainfall experiment.Under the rainfall intensities of 50 and 100 mm·h^-1,the contributions of previous wind erosion at wind velocities of 9,12and 15 m s^-1 to the water erosion amount were 5.0%～24.2%,18.3%～45.4%and 73.4～80.3%at 3°slope gradient;and were 2.1%～17.5%,19.5%～26.3%and 33.6%～46.3%at 7°slope gradient,respectively.The factor contributions of wind velocity,rainfall intensity and their interactions to the water erosion amount were 13.5%,67.1%,11.8%,14.2%,70.5%,and12.3%,respectively.The contribution to water erosion amount in descending order was:rainfall intensity>wind velocity>the interaction of wind velocity and rainfall intensity.（3）The action of water-wind agent superposition significantly reduced the soil wind erosion amount,and the reduction of soil wind erosion tends to increase with the increase of rainfall intensity caused by the previous surface water erosion.Compared with the wind-only treatment without early rainfall,for the rainfall intensities of 50 and 100 mm h^-1 and the slope gradients of 3°and 7°corresponding to the previous surface water erosion,the wind erosion amount of soil was reduced by 66.4%～96.7%and 77.2%～97.6%respectively under the action of water-wind agent superposition.It indicates that the previous surface water erosion has a significant negative interaction effect on later soil wind erosion,and the negative effect of the previous water erosion is more obvious with the increase of rainfall intensity and slope gradient of the previous water erosion.（4）The main reason for the increase of water erosion due to the superposition of wind and water forces is that,on the one hand,the previous soil wind erosion makes the micro-morphology dented,which changes the runoff path and connectivity in the process of water erosion,accelerates the runoff concentration,resulting in the increase of the erosion capacity of runoff,thus increasing the amount of water erosion.On the other hand,wind erosion at different wind velocities increased the surface roughness and wind erosion depth,and intensified the potential capacity of rainfall erosion.In addition,the accumulation of loose coarse particles on the hillslope surface caused by the previous wind erosion provides a material source for rainfall erosion in the later stage,which also leads to the increase of erosion amount on the slope.The smooth and dense soil crust formed by raindrops in the process of the previous surface water erosion inhibited the material source of later wind erosion,which is the main reason for the negative interaction effect of the previous surface water erosion on later soil wind erosion.（5）The mean velocity,Reynolds number and Froude number of runoff were increased by the action of wind-water agent superposition,while Darcy-weisbach resistance coefficient was reduced,indicating that the previous surface wind erosion increased the degree of runoff disorder of later hillslope water erosion,and enhanced the capacity of runoff erosion and transportation.At the same time,the previous soil wind erosion also increased the runoff shear stress,stream power and unit stream power in the later stage of water erosion,resulting in an increase in the amount of water erosion,which also explained the hydrodynamic mechanism of positive interaction effect between the wind erosion in early stage and water erosion in later stage.