| Large-scale vegetation restoration has been successful in curbing land degradation,improving ecosystem services and mitigating climate change.Around the world,a large number of countries and international organizations are planning or implementing ambitious vegetation restoration projects,such as the rainforest restoration project in Brazil and large-scale afforestation programs in central Africa.However,there are few studies on how to determine the safety threshold of vegetation restoration based on the local hydrological and climatic conditions to ensure the sustainability of the project,which brings challenges to the planning and implementation of vegetation restoration projects.As a key ecological barrier in arid and semi-arid areas,the agro-pastoral ecotone of Northwestern China(APENC)plays an important role in curbing desertification and ensuring national ecological security.Over the past decades,unreasonable development activities have brought serious ecological and environmental problems to the study region.In order to improve the degraded ecological environment,China has carried out a series of ecological restoration projects since the 1970s,which have led to an extensive restoration and expansion of vegetation in the APENC.However,restored and newly planted vegetation needs water,nutrients,light and carbon dioxide to grow.Of particular concerns are the water demands of newly planted ecosystems.In the APENC,ecosystems and human activities depend on the same water source,namely precipitation.Large-scale vegetation restoration programs in the region could exacerbate the conflicts between ecosystems and human needs for water.Balancing vegetation and human water consumption is therefore critical to promote sustainable vegetation restoration and ensure the water needs of socio-economic systems in the region.However,the land surface conditions and water-energy processes in this region show high temporal and spatial variability,and the lack of high-density and long-term observation data in the area makes it difficult to accurately estimate the change of regional ecosystem water consumption under the background of the continuous promotion of vegetation restoration projects.How to determine the reasonable safe threshold of vegetation restoration in this region has become an urgent scientific problem.In this study,1)the mechanism of how regional characteristics impact the actual evapotranspiration(ET)process in the APENC was clarified;2)and a Revised Budyko Framework(RBF)was developed to analyze the variability of ET at regional scale,and quantify the contribution of different factors,especially vegetation conditions,to regional ET changes;3)based on the RBF model,the framework for optimizing vegetation restoration threshold was developed according to the regional water resources carrying capacity,and an optimized vegetation restoration scheme was proposed for the first time in the APENC.The main research conclusions are as follows:(1)First,the performance of three models is compared in the research area with observed data.The results show that the WRF-CLM4.0 model(r=0.89,RMSE=0.66mm/d,NSE=0.90)and Complementary Relationship(CR)model(r=0.91,RMSE=0.85 mm/d,NSE=0.85)are better performed in the simulation of the actual evaporation than the WRF-Noah model(r=0.82,RMSE=0.94 mm/d,NSE=0.81)and have good application value in the simulation of ET in the study area.Then,by comparing the model mechanism and model structure on the ET simulation process,we found that,in the APENC:1.Soil moisture is the main factor affecting the ET simulation in the region,and the consideration of the weighting coefficient of soil moisture greatly affects the model simulation of ET.2.Because of the highly surface heterogeneity and lack of data in the study area,the ET simulation models considering the land surface water-energy process with fewer parameters is more competitive.(2)A simple RBF was developed to simulate and attribute the variability of ET on the APENC.The model is mainly based on the widely used Budyko hypothesis.In this study,vegetation index NDVI is introduced to expand the application of this hypothesis on the intro-annual scale and the consideration of the impact on vegetation change.At the same time,the concept of water supply including precipitation and surface soil water storage is introduced,which promotes the application of the model with high spatial and temporal resolution.Furthermore,the improved model framework aims to achieve model parameter minimization and reduce data requirements in data-deficient areas.The RBF model achieves model simplification and is applicable to meet the demand on ET simulation in data lacking areas.The simulation results of RBF were evaluated with observed data under different environmental and vegetation conditions in arid and semi-arid areas,and the results show a good performance(R~2=0.96,NSE=0.95 in the APENC;R~2=0.91,NSE=0.90 in the Qinghai-tibet plateau;R~2=0.78,NSE=0.78 in desert irrigated corn field).Moreover,compared with the FAO crop coefficient model and CR model,the performance of RBF were better than both the classical models.This framework provides a simple and high spatio-tempora resolution to quantify the contribution of climate,land surface soil moisture storage,and vegetation conditions to the ET variability,especially in arid and semi-arid regions where observed ET data are scarce.(3)Based on the newly developed RBF model and the long-term climatic conditions,the framework for optimizing regional vegetation restoration is constructed,where the maximum water consumption(ET)of regional vegetation is taken as the limiting factor and the time variation of water consumption of regional residents,industry and animal husbandry is considered.This study proposes the optimal vegetation restoration threshold value and optimal restoration strategy for the APENC.This threshold is the NDVI value allowed by the maximum vegetation water consumption ratio,which represents the upper limit of vegetation restoration potential in the APENC.The optimal vegetation restoration strategy is the annual vegetation restoration intensity,that is,the feasible range of annual growth rate of regional NDVI value.Through the calculation of this framework and the analysis of the"Master Plan of Major Projects for the Protection and Restoration of National Important Ecosystems",the national vegetation restoration project policy,it can be concluded that the maximum threshold of local vegetation restoration is 0.31,and the feasible restoration intensity is 0.0021～0.0036,that is,the annual vegetation expansion is 0.82%～1.27%.The proposed optimal vegetation restoration strategy was based on the feasible vegetation restoration intensity and past regional vegetation restoration experience.When planning the vegetation restoration speed(in terms of restoration area or planting density)in the near future,it should be 1/3 to 3/5 of the average restoration progress of the past 20 years,and 1/2 to 4/5 of the progress of the past 10 years. |
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