Response Of Soil Hydrological Physical Properties And Changes Of Runoff Yield And Confluence To Vegetation Restoration In The Loess Region

Soil and the aboveground vegetation are important interfaces that affect rainfall infiltration,evaporation and runoff yield and confluence.In order to control soil and water loss,vegetation restoration has been paid much attention for the Loess Plateau region,especially the implementation of the measures of returning farmland to forest and grass since 1999.The regional vegetation coverage increased from 28.8%in the 1980s to 43.8%in 2012.The runoff and sediment discharge at Huayuankou station of the Yellow River decreased from 44.6 billion m³ and 1.28 billion tons before 1980 to 25.1 billion m³ and 100million tons after 2000.Large-scale vegetation construction and restoration effectively reduced soil erosion and changed soil hydrological physical properties in the Loess Plateau area,thus exerting significant effects on runoff and sediment yield on the hillslope and the mechanism of runoff yield and confluence in the watershed.How to evaluate the intensity and mechanism of vegetation restoration in the change of runoff and sediment discharge of the Yellow River has attracted great attention,which is also one of the keys to scientifically explain the sediment reduction of the Yellow River.Taking the Yan River basin and a small watershed in the middle reaches of the Yellow River basin,in which underwent long-term vegetation restoration as the study area.Three land uses（Robinia pseudoacacia forestland,korshinsk peashrub shrubland and secondary grassland）with different restoration periods were seclected and a farmland was used as a control.The field and laboratory test were used to investigate the effects of vegetation restoration on soil hydrological physical property,the runoff and sediment yield on the hillslope and the characteristics of runoff yield and confluence in watershed.The main conclusions are as follows:（1）Vegetation restoration significantly reduces soil bulk density,increases aggregate content and macroporosity,improves soil water holding capacity and water supply capacity,and increases soil saturated water conductivity.Compared with the farmland,soil bulk density of the forestland,shrubland,and grassland were decreased by 4.8%,8.2%,and7.4%,respectively.The>0.25 mm soil aggregate stability content in the sampling plots with long-term vegetation increased by 73.1%reltive to the farmland.In addition to forestland,the soil macroporosity of forestland and grassland increased by 39.7%and35.1%,respectively.The soil water holding and supply capacity of the vegetation restoration area were both higher than those of the farmland under low pressure water head（<100 cm H₂O）,and the order of soil water holding and supply capacity of the vegetation restoration area is shrubland>grassland>forest.The soil saturated water conductivity of the sampling plots ranged from 10.7 mm h^-1 to 18.6mm h^-1.The soil saturated water conductivity of the forestland,scrubland and grassland are 1.2,1.6 and 1.3 times of the farmland,respectively,and increased with the vegetation recovey periods.（2）Vegetation restoration significantly improved soil infiltration capacity.The initial soil infiltration rate,stable soil infiltration rate and average soil infiltration rate of vegetation restoration plots were 1.5,2.1 and 2.5 times of the farmland plots,respectively.The characteristic values of soil infiltration in different vegetation types in the order of shrubland>grassland>forest,and increases with the increases of the vegetation restoration years.Under the same conditions,the average infiltration depth of the surface soil layer of the vegetation restoration sample was 1.22 times of the farmland,and the soil layer below 20 cm was 1.16times of the farmland.The comprehensive evaluation of soil infiltration ability based on principal component analysis showed that the order of soil infiltration ability under topsoil was:40-year shrubland>40-year grassland>20-year grassland>15-year shrubland>25-year forestland>35-year forestland.The order of soil infiltration ability under 20 cm was:40-year shrubland>40-year grassland>15-year shrubland>25-year forestland>35-year forestland>20-year grassland.The fitting goodness of the soil infiltration model are in the order of Jiang Dingsheng model>Horton model>Philip model>Kostiakov model.（3）Vegetation restoration plays a important role in prolonging runoff generation time,increasing infiltration,and reducing sediment yield on the hillslopes,and soil bulk density and>0.25mm aggregate content are the main soil hydrological and physical parameters that affect the benefits of vegetation runoff and sediment reduction.Vegetation restoration can prolong runoff-start time,increase infiltration and reduce runoff and sediment yield.Compared with the farmland slopes,the part of vegetation aboveground prolong runoff-start time by 140%,reduce runoff by 15%and reduce sediment yield by 100%.The effects of vegetation aboveground on increasing runoff-start time and decreasing runoff are in the order of grassland>shrubland>forestland.The rainfall infiltration recharge coefficient of original and root slope were significantly higher than that of the farmland.The root part of vegetation prolongs runoff-statrt time by 120%,reduce runoff by 50%and reduce sediment yield by 94%.The results showed that the order of the effects of increase of runoff-start time and decreases of runoff are shrubland>grassland>forestland,while the effects of decreasing the sediment yield were ordered as forestland>grassland>shrubland.There was a power function relationship between the increasing runoff-start time benefit and the vegetation coverage,and a logarithmic function relationship between the decreasing runoff benefit and the vegetation coverage.Underground biomass is the key index that affect the increasing runoff-start time benefit,while soil bulk density and soil>0.25 mm aggregates stability are the main hydrological physical parameters that impact the benefit of runoff and sediment reduction（4）The variation of soil bulk density has a significant effect on runoff and sediment yield and the hydrodynamic characteristics on the slope.The runoff-start time decreases with the increase of soil bulk density,the runoff increases with the increase of soil bulk density,and the change of sediment yield with the change of bulk density is affected by the rainfall intensity.At the rainfall intensity of 60 and 90 mm h^-1,the sediment yield increased with the increase of bulk density.At the rainfall intensity of 120 mm h^-1,the sediment yield decreased firstly and then increased with the increase of bulk density.The average flow velocity,runoff depth,Reynolds number,Froude number and the stream power all increase with the increase of soil bulk density,Darcy resistance coefficient and Manning coefficient decrease with the increase of soil bulk density,while the changes of flow shear force with the change of bulk density is affected by the rainfall intensity.At the rainfall intensity of 60and 90 mm h^-1,the flow shear stress firstly increases and then decreases with the soil bulk density increases,while it firstly decreases and then increases with the increases of soil bulk density at the rainfall intensity of 120 mm h^-1.The sediment yield is exponently correlated with the average flow velocity,while is quadraticly correlated with the runoff depth.The relationships of sediment yield with Reynolds number and Froude number are power functions.There was a power function between sediment yield and Darcy resistance coefficient of soil slopes with different soil bulk density.There is an exponential function between sediment yield and Manning coefficient.Additionally,the relationship between sediment yield and flow shear force is linear relations,and there is a power function relationship between stream power and sediment yield.Reynolds number and stream power are the best indices that to characterize the sediment yield on the slope.（5）Long-term vegetation restoration could affects the characteristics of runoff yield and confluence in the basin,and causes the runoff yield mechanism to change from infiltration-excess runoff to compound runoff and the saturation excess runoff.The mechanism of runoff and confluence in xichuan river basin,a tributary of Yanhe River basin,was explored.Results showed that the runoff yield mechanism of the Xichuan River basin is still infiltration-excess runoff,while the proportion decreased.However,the compound runoff and the saturation excess runoff increased durng the later two periods（1980-1988 and 2008-2016）.The precipitation,average rainfall intensity and the maximum rainfall intensity of the 91 flood events showed an upward trend,while the runoff depth,the proportion of surface runoff and the runoff coefficient all showed a downward trend.The runoff depth showed a power function relationship with the sum of the precipitation and the pre-precipitation.Runoff depth showed no significant relationship with the average rainfall intensity,while has a power function relationship with the maximum rainfall intensity.Moreover,runoff depth showed more dependent on precipitation than rainfall intensity after 2008.Compared with the period of 1974-1979,the flood runoff components of overland runoff and interflow decreased.However,the flood of overland runoff,interflow,groundwater runoff and saturated surface runoff increased during 1980-1988 and2008-2016.The increases of vegetation coverage,decreases of cultivated land and the increases of forest and grass land in the basin lead to the enhancement of underground outflow and infiltration capacity of the basin,the increase of water storage capacity,and enhance of river basin regulation and storage capacity are the fundamental reasons for the change of the mechanism of runoff yield and confluence in the basin.