Study On The Influence Of Vegetation Restoration On The Surface Runoff Generation Mechanisms On Loess Plateau

With the development of various ecological protection measures on Loess Plateau,the vegetation is gradually restored in recent years.The changes of underlying surface characteristics and surface runoff generation mechanisms are the key issues related to soil and water loss control during the vegetation restoration process.However,the existing researches lack physically based exploration and judgment on whether the surface runoff generation mechanism on Loess Plateau is likely to change.Based on the field sampling and monitoring,combined with the numerical simulations,this study explores the possible change of the surface runoff generation mechanism during the vegetation restoration on Loess Plateau according to the research path of 'pre simulation—sampling and measurements—re simulation'.Main contents are as follows:(1)During the 'pre simulation' stage,the sensitivity analyses are conducted on Shejiagou,a small catchment with the typical topographic characteristics of Loess Plateau.The results show that,during the surface runoff generation stage,when the Ks increases to be larger than rainfall intensity,the surface runoff generation mechanism changes from infiltration excess overland flow occurring at the whole basin to the saturation excess overland flow mainly at the areas where the initial groundwater table is shallow;in addition,during the confluence stage,the increase of surface roughness would trap the water and soil along the flow path by slowing down the surface runoff velocity and prolonging the confluence time.Moreover,the decrease of erodibility is not only beneficial to the reduction of total erosion,but also can effectively slow down the fragmentation of landscape on Loess Plateau.(2)During the 'sampling and measurements' stage,the results show that the vegetation restoration is beneficial to the improvement of saturated hydraulic conductivities of each soil layer and the increase of vertical heterogeneity.From grassland to shrub land,and then the forest land,the significant increase of K in the near surface layer and the non significant difference of K in the deep layers cause the vertical difference to become more and more prominent.In addition,the soil saturated hydraulic conductivity in north is greater than that in the south.The comparison results show that the revegetation can affect the soil hydrological response and runoff pathes.With the recovery of vegetation,the proportion of water that becomes'infiltration excess Horton overland flow' gradually decreases or even disappears;correspondingly,the propotion of water that infiltrates to deep layer or accumulates above the perched water table increases.(3)During the 're simulation' stage,through the systematic simulation analyses,the occurrences of different surface runoff generation mechanisms in typical slope areas of the Loess Plateau under the different rainfall and soil saturated hydraulic conductivity conditions are explored.The possibility of 'saturation excess overland flow' caused by perched water on Loess Plateau is vertified.and this kind of surface runoff generation mechanism occurs from the top to the slope.Next,the research preliminarily summarizes the factors that affect the occurrence of different surface runoff genenration mechanisms on Loess Plateau.And then,the three-dimensional coordinate system is construced based on these factors,and the occurrence conditions of different slope surface runoff generation mechanisms are quantified preliminarily At last,the research discusses the impact of vegetation restoration on the changing trend of surface runoff generation mechanism.The simulation results show that with the vegetation restoration,the proportion of 'saturation excess overland flow generation mechanism increases from 5%to 27.5%This study discusses the possibility of the change of surface runoff generation mechanisms on Loess Plateau during the vegetation restoration stage.It puts forward innovatively that vegetation restoration is not only conducive to the occurrence of'saturation excess overland flow' near the river,but also to the transformation from the 'infiltration excess overland flow' to the 'saturation excess overland flow' caused by perched water table on slopes.It provides a new idea or a new direction for the study of surface runoff generation on Loess Plateau,and provides meaningful references for ecological planning work,such as soil and water conservation and vegetation restoration.