Evaluation Of Water-carbon Relationship In Yanhe River Basin Under The Background Of Vegetation Restoration

Vegetation restoration is an important means to mitigate regional climate change and increase terrestrial carbon sequestration.However,large-scale vegetation restoration in arid and semi-arid regions can excessively consume local water resources,thus threatening regional water security.How to ensure the water demand for socio-economic development in the basin under the background of vegetation restoration,and coordinate the relationship between vegetation restoration and regional water security,is still a key problem to be solved.In this paper,the RHESSys ecohydrological model considering the dynamic growth of vegetation is built to realize the quantitative simulation of ecohydrological elements（actual evapotranspiration,surface runoff,net primary productivity,and total carbon density）in the Yanhe River Basin,a typical vegetation restoration region on the Loess Plateau.Based on this,the evolution of the water-carbon relationship in the basin is analyzed and the suitable vegetation cover in the basin to ensure regional economic and social water use is determined,which provides reference for the implementation prospect and sustainable development management strategy of the Grain for Green Project（Gf GP）.The main conclusions of this article are as follows:（1）The RHESSys model was used to model the ecohydrological process in the Yanhe River Basin,and the applicability of the model was verified.In response to the complex underlying conditions of the basin,the RHESSys ecological hydrological model,which considers vegetation growth,was introduced and built,and the model was calibrated and validated by using the measured data.The results show that in the calibration period,NSE and R² were 0.71 and 0.70,respectively,and in the validation period were 0.82 and 0.79,respectively,indicating that the RHESSys model could simulate the ecohydrological process of the Yanhe River Basin and could be used to evaluate the water-carbon relationship in the basin.the model RHESSys can realize the simulation of ecohydrological processes in the Yanhe River basin and can be used for the evaluation of the water-carbon relationship in the basin.（2）The temporal and spatial variation characteristics of ecohydrological elements in the basin were quantitatively revealed.In terms of spatial distribution,the ecohydrological elements in the Yanhe River basin show obvious spatial heterogeneity and are more consistent with the spatial distribution of precipitation gradient changes and vegetation coverage.On the time scale,actual evapotranspiration,net primary productivity,and total carbon intensity showed a significant increasing trend between 1990 and 2019,while Rs decreased at rate of0.35 mm a^-1.At the same time,the eco-hydrological elements of different ecosystems also changed after the implementation of the Gf GP.The actual evapotranspiration of forestland,cropland and grassland showed an increasing trend,while the surface runoff showed a decreasing trend.The net primary productivity and total carbon density of three vegetation types showed an increasing trend,and the increase rate of forestland was the highest.（3）The effects of climate and subsurface changes on ecohydrological elements were quantified.By calculating the contribution of climatic factors and land use changes to the changes of ecohydrological elements,and found that in the early period of Gf GP（2000～2009）,the contribution of land use change to evapotranspiration increase and surface runoff decrease were 82.66%and 51.87%,respectively,which was the main factor affecting the change.In the middle period of Gf GP（2010～2019）,the contribution of land use change to hydrological factors decreased slightly,but still dominated.However,the contribution of land use change to the increase of NPP and TC was more than 75%no matter in the early or middle period of Gf GP.（4）The evolution of water-carbon relationship in the basin before and after the implementation of the Gf GP was analyzed.The results show that ET was found to have positive correlation with NPP,indicating that the water-carbon cycle in the basin is tightly coupled.Moreover,the negative correlation of Rs-NPP and Rs-TC was found to increase after the implement of the Gf GP,which indicate the reinforcing effect of vegetation restoration on the water-carbon trade-off relationship.（5）The suitable vegetation coverage of the basin that guarantees regional economic and social water use was determined.Based on the forecast results of water supply and demand of the basin,the vegetation coverage of the basin was optimized,and the suitable vegetation coverage satisfying the balance of supply and demand of the basin was determined to be 0.425.Meanwhile,under the suitable vegetation coverage,the total water supply can reach 156.22million m³,meeting the total water demand of the basin 155.10 million m³.