Study On The Evolution Mechanism Of Water Cycle In Semi-arid Areas Under Large-scale Vegetation Restoration

Since 1970s,China's land greening has achieved remarkable results,and it has played an important role in water and soil conservation and wind prevention and sand fixation.At the same time,large-scale vegetation restoration has caused drastic changes in the underlying surface pattern,which has a profound impact on regional water cycle conditions and hydrological processes.Especially in water-scarce areas in northern China,local vegetation restoration is approaching the limit of sustainable use of water resources.A new ecological-water resource contradiction has emerged.Therefore,in-depth research on the evolution mechanism of the water cycle under the effect of large-scale vegetation restoration is significant to scientifically understand the coordinated development of ecology and water resources in water-scarce areas.,Focusing on the impact of vegetation restoration on water resources in semi-arid areas,This paper takes Beichuan River Basin,a secondary tributary of the upper reaches of the Yellow River,as the research object.First,through soil moisture monitoring in different vegetation coverage areas and rainfall infiltration replenishment tracing experiments,the regulation mechanism of artificial shrub vegetation restoration on the conversion process of precipitation-soil water-groundwater was revealed.Then combined with long-term meteorological,hydrological data and remote sensing data to analyze the evolution trend of water cycle elements at the basin scale,and clarify the effect of vegetation restoration on the evolution of key water cycle elements.Finally,through the Water Resources Multiplier Effect Model,the significance of vegetation restoration in improving the effective utilization of water resources in the basin is clarified from the perspective of generalized water resources.The research results show that:(1)Artificial shrub vegetation restoration can enhance the water permeability and water storage capacity of shallow soils,and promote the water cycle of shallow soils.The average porosity,saturated hydraulic conductivity and water content of the soil profile(D1)of the high-coverage shrubland are 1.04 times,3.83 times and 1.5 times that of the sparse grassland soil profile(D2)respectively.The cumulative increase,average growth rate,cumulative consumption,and average consumption rate of soil moisture in the D1 profile are 1.67 times,5.5 times,2.03 times and 3.2 times that of the D2 profile respectively.The cumulative increase and consumption of soil water at the depth of 0-20 cm account for more than 89%,which is the most critical layer that affects the soil water cycle.(2)Shrubs with high coverage can weaken the recharge of deep soil moisture and groundwater by rainfall to a certain extent.During the test period,the rainfall infiltration replenishment of groundwater in D1 and D2 areas were 11.08 mm and 15.46 mm,respectively,of which the dominant flow accounted for 31.4%and 42.7% respectively.The average soil moisture content in the range of 1.2?3.2m in D1 area is only 65.1%of the corresponding depth in D2 area.The impact mechanism of vegetation restoration on the attenuation of groundwater recharge mainly includes the following three aspects: First,the canopy interception reduces the rainfall and rainfall intensity reaching the surface,which reduces the total recharge and the degree of dominant flow at the same time.The second is that root water absorption causes the zero flux surface to move downward,resulting in the thinning of the unsaturated zone that can continuously replenish groundwater.The third is that the dry soil layer formed by the water consumption of deep root vegetation makes it preferentially replenish the dry soil during the process of water migration to the deep layer.The water deficit of the water,which reduces the deep replenishment.(3)Under the condition of relatively stable precipitation,the natural runoff in the Beichuan River Basin showed a decreasing trend,and the increase in ecological water consumption caused by vegetation restoration was the main reason for the decrease in runoff.From 1956 to 2016,the overall trend of precipitation above the Qiaotou Station of the Beichuan River was relatively stable,with a distribution cycle of 5 years,12 years,and more than 32 years.From 1956 to 2016,the overall trend of precipitation above the Qiaotou hydrological station of the Beichuan River was relatively stable,with distribution cycles of 5 years,12 years,and more than 32 years.The overall runoff depth showed an insignificant decrease,with an average decrease of7.5mm/10 a.The decay trend of surface runoff was about 4 times that of the base flow.Under the conditions of average precipitation for multiyear,the average annual ecological water consumption increased by 111 million m3 after vegetation restoration,which is 1.6 times the increase of man-made water resources development.(4)The annual evapotranspiration showed a decreasing trend with the restoration of vegetation,and the change of surface soil moisture was an important reason affecting the evapotranspiration and ecological water consumption changes.The average coverage rate in 2015 increased by 4.01% compared to 2000,but the annual evapotranspiration decreased by 25.96 mm.Vegetation restoration can reduce soil evaporation by reducing surface wind speed and solar radiation,which is the main reason for the decrease in evapotranspiration in the watershed.The impact of vegetation restoration on ecological water consumption is related to the amount of precipitation.The overall performance is that the water consumption intensity in the wet season is higher than that in the dry season.This reflects the good runoff regulation and water conservation functions of the vegetation restoration.This change fully reflects the good function of vegetation restoration in runoff regulation and water conservation.(5)Under the influence of vegetation restoration,the water resources structure of the Beichuan River Basin has changed,and the comprehensive water resources effect has been significantly improved.Before and after vegetation restoration,the distribution ratios of precipitation in the three links of surface runoff,ecological consumption,and base flow discharge changed from 0.275,0.569,0.156 to 0.231,0.634 0.134,respectively.The overall trend is that more precipitation participates in long-term Soil watervegetation/groundwater cycle,more water resources are transferred from linear water system to planar land system.The average annual comprehensive water resources effect after vegetation restoration increased by20.7% under the conditions of multi-year average precipitation.The changes in water cycle and distribution of water resources in this basin caused by vegetation restoration are in line with the value orientation with ecology as the core,and play a positive role in the comprehensive effects of water resources.All these studies have deepened the research on the process and mechanism of multiple water transformation under the condition of vegetation restoration in semi-arid areas,and clarified the evolution trend of the water-cycle and water resources in the Beichuan River Basin.It provides a basis for scientific understanding of the relationship between vegetation restoration and sustainable development of water resources in the water conservation area of the upper reaches of the Yellow River.