Hydro-climatic Effects Of Vegetation Restoration In The Agro-pastoral Ecotone Of Northern China

Land use/cover change is one of the research frontiers of earth system science.Over the past few decades,human activities have significantly altered about three-quarters of the global land surface.To prevent vegetation degradation,desertification,and sandy eastward and southward migration,the Chinese government has formulated and implemented several large-scale vegetation restoration programs in the agro-pastoral ecotone of northern China（APENC）,including the“Grain for Green”,the“Beijing-Tianjin Sand Control Program”,and the“Northern China’s Vegetation Belt”,which has obviously improved local ecological and environmental conditions.However,the regional climatic-hydrological responses to such large-scale vegetation restoration and the mechanism remain unclear.Against this background,based on remote sensing,observation,and reanalysis data,combined with various models,this work systematically investigates the mechanism of large-scale vegetation restoration-precipitation feedback and spatiotemporal propagation of drought in the APENC during2000-2015.The main results are as follows:1.Analysis of remote sensing and observation products shows that precipitation（P）,leaf area index（LAI）,and normalized difference vegetation index（NDVI）show a significantly increased trend during 2000-2015,indicating the vegetation in the APENC was greening.The reasons for the greening in the APENC mainly were due to the implementation of revegetation measures and the increase in P.Two vegetation experiments,namely dynamic vegetation（DV）and no dynamic vegetation（no-DV）,were designed to estimate evapotranspiration（ET）using the Priestley-Taylor Jet Propulsion Laboratory model.Results show that ET increases significantly in both experiments,due to the change in climate to a warm-wet state.DV ET is significantly higher than no-DV ET,suggesting vegetation restoration further increases soil water consumption through transpiration.Based on DV ET,no-DV ET,a further investigation of the change in precipitation recycling ratio（PRR）was undertaken using Dynamic Recycling Model.Results suggest that there is a significantly increased trend in PRR in the APENC during 2000-2015,with an average of 10.8%.The above results reveal for the first time the contribution of large-scale vegetation restoration to precipitation,demonstrating that large-scale vegetation restoration in the APENC enhances the positive local vegetation-precipitation feedback.2.The moisture sources that supply P in the APENC were identified and the contribution of local and remote atmospheric moisture fluxes to P were quantified.Moisture sources are tracked using the Water Accounting Model-2layers model（WAM2）driven by ERA-Interim.The moisture sources of P in the APENC are dominated by the westerlies,the Indian Ocean monsoon,and the East Asian monsoon.The westerlies-dominated moisture sources include Europe,northern Eurasia,Central Asia,and northwestern East Asia.The sources dominated by the Indian Ocean monsoon include the Indian Ocean,the Bay of Bengal,the Arabian Sea,Southeast Asia,and the Qinghai-Tibet Plateau.East Asian monsoon-dominated sources mainly include the Pacific Ocean and the eastern region of East Asia.The APENC itself and the south region,and the eastern region of central Asia tend to be the core area that contributes moisture most intensively.Terrestrial moisture contributes to 84.01%of P in the APENC.The study further quantifies the contribution of moisture sources to P when drought occurs during the growing season.Results indicate that the occurrence of drought in the growing season in the APENC is mainly caused by the decrease in terrestrial moisture.The reduction of moisture in the APENC itself and the area controlled by the East Asian monsoon dominated the occurrence of growing season drought.In the occurrence of drought,some moisture sources with increased contribution have a mitigating effect on the drought persistence in the APENC.3.The Weather Research and Forecasting model embedded with evapotranspiration-tagging（WRF-tagging）was used to investigate the atmospheric pathways of ET as well as where and to what extent ET returns as P in the APENC.First,the study updated the default land use and vegetation indices in the WRF-tagging with high-resolution and real-time datasets.Convection-permitting WRF-tagging modeling reproduces the spatial distribution of P and ET reasonably well after updating surface characteristics.The water vapor originating as ET in the tagged region(ET_tag)is advected in the atmosphere below 600 h Pa to hundreds of kilometers by the prevailing winds.The contribution of ET_tag to precipitation decreases gradually from the tagged region to other surrounding areas.Moisture recycling shows that 5.83%of P comes from local ET during the growing season in the core region of the APENC,with a maximum contribution of 9.65%occurring in the northeastern part of the tagged region.Furthermore,to quantify the impact of LUCC on moisture recycling,the study designed two different vegetation scenarios（Afforestation and Degradation）by changing land use and vegetation indices in the model.Results show that the precipitation recycling ratio increased to 6.31%in the Afforestation scenario,and decreased to 5.19%in the Degradation scenario,demonstrating the non-neglectable positive feedback of vegetation on P.An analysis of land surface change-precipitation feedback processes indicates that the LUCC-induced change in precipitation efficiency dominates P changes.Vegetation restoration increases local water vapor through ET and horizontal water vapor inflow through large-scale cycles,contributing more water vapor to local and surrounding regions.The increased water vapor moistens the planetary boundary layer（PBL）,favors lower the PBL height,lifting condensation level,and increases atmospheric instability,which tends to further trigger deep convection and rainfall and leads to more P.4.Based on a 3-dimensional drought identification method,this study constructs the links among the propagation of different drought types to explore the simultaneous spatiotemporal evolution of droughts.It assesses the performance of high-resolution WRF for drought variation detection in the APENC,then identifies and tracks for the first time the spatiotemporal evolution of each individual drought event in the APENC based on the WRF output.Compared with ERA5-Land,droughts computed by the WRF simulations can characterize well the drought evolutions,meaning that downscaling models like WRF could provide reliable hydrometeorological results when assessing and analyzing long-term wet-dry variations.Based on the standardized precipitation index（SPI）,a total of 185 drought clusters were identified during 2000-2015,and 24 drought events of at least 3 months duration occurred in the APENC.The characteristics of the different drought events varied considerably.Droughts in the APENC have a larger percentage of propagation to the northeast and northwest and migrated hundreds of kilometers from the source areas.The results of the propagation of meteorological droughts to agricultural droughts indicate that 58%of the meteorological drought events progressed further to agricultural droughts.Agricultural droughts exhibit spatial variations consistent with meteorological droughts.Agricultural droughts have 1.69 months onset lag time and 2.12 months termination lag time.The average duration of agricultural drought（5.14 months）was 1.33 times longer than the duration of meteorological drought（3.86 months）.This study clarifies the impact of large-scale vegetation restoration on regional moisture recycling,reveals the mechanism of land use and land cover change-precipitation feedback,quantifies the water vapor source area of P,and identifies spatiotemporal propagation of droughts in the APENC.The results of this work provide important theoretical information and technical support for the construction and enhancement of regional ecological security barriers in the APENC.