Evaluation Of The Ecohydrological Effects And Optimization Of Water-based Vegetation Pattern In The Jing River Basin Of Loess Plateau

Since the implementation of the Grain for Green Project,remarkable achievements have been made in vegetation construction on the Loess Plateau.However,the quality and sustainability of ecological construction has been significantly affected by the fact that vegetation restoration has intensified the intensity of regional water consumption,and soil dryness has occurred in some areas.How to establish a method for regulating the water-based pattern of vegetation and comprehensively improve the benefits of afforestation is a realistic problem that needs to be solved for high-quality ecological construction on the Loess Plateau.At present,vegetation construction at the regional scale is mostly based on the static soil and water resource carrying capacity,ignoring the dynamic impact of land use conversion driven by vegetation construction on the ecohydrological process.In this paper,an ecohydrological model considering the vegetation-water interaction mechanism is constructed in the Jing River basin of Loess Plateau as the study area,and a multi-factor quantitative simulation of the vegetation ecohydrological effect is realized.Based on the quantitative model,a vegetation pattern optimization and control method that integrates the constraints of sustainable soil water use in the watershed and the goal of ecological benefit enhancement has been established to provide scientific and technological support for the sustainable ecological construction of the Loess Plateau.The preliminary progress of this study is as follows:（1）Based on MODIS high-resolution vegetation data and other subsurface parameter datasets in the Jing River basin,the RHESSys ecohydrological model was introduced to establish a kilometer-scale ecohydrological process simulation scheme for the study area,and Monte Carlo simulation was used to achieve the optimization and rate determination of key model characterization parameters.At the same time,the reliability of the model was comprehensively validated using a multi-level validation approach focusing on key hydrological and ecological process elements（soil water,runoff,actual evapotranspiration,vegetation carbon density,soil carbon density）.The results show that the RHESSys ecohydrological model of Jing River Basin can effectively characterize the long series of ecohydrological processes and their time-varying characteristics before and after vegetation restoration,and can be used for the evaluation of ecohydrological effects in this basin.（2）The new index of vegetation water supply and demand balance（i.e.water suitability index）was established by taking the increase of water available in the root zone of vegetation as the upper limit of water supply and the vegetation demand under the soil stagnant water content as the lower limit of water demand during the period.The analysis found that about23.5%of the area within the Jing River basin showed an overconsumption of soil water,and the average vegetation-soil water suitability of the basin showed a significant decreasing trend during the study period（1990-2019）,and the recovery of vegetation led to a continuous increase of soil water reservoir consumption in the Jing River basin.In terms of spatial distribution,soil water was overconsumed in the southern part of the Jing River Basin,while a larger amount of available soil water remained in the north-central region.Overall,the suitability of soil water and vegetation pattern in the whole basin is poor,and it is significant to carry out optimization of water-suitable vegetation pattern in the basin.（3）The improvement of carbon sequestration capacity and the effect of soil conservation before and after the implementation of the Grain for Green Project in Jing River Basin were determined.After years of vegetation restoration,the average carbon density in Jing River Basin increased from 5.65 kg·m^-2（1990）to 6.76 kg·m^-2（2019）,which was mainly affected by precipitation and vegetation restoration.Among them,vegetation restoration contributed 66.5%to the increase of carbon sequestration in Jing River Basin.Meanwhile,the mean annual erosion intensity of Jing River Basin decreased from 6152 t·km²·a^-1 to 2699 t·km²·a^-1.More than 95%of soil erosion was distributed in the areas with NDVI<0.6,and the increase of vegetation coverage contributed 85%to the reduction of soil erosion in Jing River Basin.Therefore,vegetation restoration had a significant impact on the improvement of the carbon sequestration and soil conservation service function.Through reasonable regulation of vegetation layout,it is expected to further improve the regional carbon sequestration and soil conservation service function.（4）A vegetation pattern optimization and control method with carbon sequestration,soil conservation,and carbon sequestration-soil conservation synergy as the objectives and soil water supply-consumption balance as the constraints,respectively,was constructed.Through the rational planning and utilization of soil water resources,the average vegetation cover in the Jing River Basin under the three regulation schemes increases slightly,and the land use conversion under the carbon sequestration priority objective is more inclined to woodland and shrubs,while the soil conservation priority objective is inclined to higher vegetation cover.The average carbon density in the Jing River Basin under the carbon sequestration priority scenario was calculated to be 7.25 kg·m^-2,which was 10.7%higher compared to the original vegetation pattern.The average annual soil erosion was reduced to 989 t·km²·a^-1 under the soil conservation priority scheme,and the soil conservation effect was improved by 63.4%.After considering the benefits of carbon sequestration and soil conservation for synergistic regulation,it is found that the average annual carbon and sand reduction benefits of Jing River Basin can reach 29.605 billion yuan,which is 322 million yuan and 31 million yuan more than the carbon sequestration and soil conservation priority scenarios,respectively,and its carbon density and soil conservation capacity are between the above two scenarios.（5）Based on the IPCC6 climate prediction data（SSP2-4.5 scenario）,we evaluated the response of the Jinghe basin vegetation pattern to climate change from 2021 to 2040.The analysis found that under the dual stress of reduced future precipitation and increased temperature,42.8%of the vegetation in the region suffers from moisture supply-demand mismatch,mainly in the eastern part of the basin（five counties of Pengyang,Guyuan,Jinyuan,Pingliang,and Huating）and the southern part of the basin（the area around Changwu County）.In this study,the water suitability thresholds for vegetation in areas with unsustainable future moisture are adapted for the reference of decision makers,and vegetation species with lower water consumption should be given priority in vegetation restoration in these counties to prevent the occurrence of future droughts.Based on the concept of sustainable vegetation construction and water suitability control,this study proposes a vegetation water suitability pattern optimization control method with vegetation water supply-demand balance as the constraint and ecological benefit maximization as the goal,realizes the quantitative spatial allocation of forest,irrigation and grass under different ecological objectives,enriches the method of vegetation pattern optimization control in arid and semi-arid areas,and provides important technical support for ecological protection and high-quality ecological construction on the Loess Plateau.