| Impacts of land use and land cover change(LUCC) on climate change is one of the important contents of global change research.Terrestrial vegetation variation is an important characterization of LUCC,which can affect surface energy balance and water cycle by altering biogeophysical properties such as green vegetation fraction and surface albedo,then exerting an important effect on regional climate metrics and water balance.As the important component of national strategy of“Ecological Protection and High-quality Development of the Yellow River Basin”,the Mu Us Sandy Land is located in arid and semi-arid region where the eco-environment is particularly vulnerable and is sensitive to climate change.The region used to be one of the most severely desertified areas in our country.Since 2000,with the implementation of various ecological restoration projects,the vegetation conditions and the eco-environment of the Mu Us Sandy Land have been significantly improved.The region has become the most successful example of revegetation and reversing desertification.However,the biogeophysical impacts of large-scale vegetation variation on regional climate and water balance lack quantitative assessment,and need to be studied in depth.In this study,the Mu Us Sandy Land was chosen as a typical case.Firstly,based on the data of normalized difference vegetation index(NDVI),the spatiotemporal change of vegetation variation was detected,and the relative contribution of climate change and human activities was distinguished.Secondly,based on numerical simulation,WRF-Noah land-atmosphere coupling regional climate model was used to quantitatively evaluate impacts of vegetation variation on regional climate metrics such as air temperature and precipitation,and then the related biogeophysical processes and mechanisms were clarified.On this basis,impacts of vegetation variation on regional water balance were further evaluated.Finally,the potential impact of land surface evapotranspiration on regional precipitation was modeled from the perspective of extreme change in soil moisture,and then the relative importance of local land surface(e.g.,land surface evapotranspiration)and large-scale atmospheric processes(e.g.,external moisture transport)to precipitation was discussed by comparing with the impacts of external moisture transport on precipitation.This study can not only provide guidance for vegetation restoration and ecological construction in arid and semi-arid region,but also provide a reference for understanding of the attribution of local climate change,scientific response to climate change and development of adaptive strategies.The study is expected to enrich and perfect the basic theory of the relationship between vegetation and climate.The main results and conclusions are as follows:1.From 2001 to 2018,the area of region where has experienced a significant increasing trend(p<0.05)in growing season(May to September)NDVI accounts for about 86%of the Mu Us Sandy Land,with a rate of 0.049 decade-1.The significant improvement of vegetation conditions is jointly affected by climate change and human activities.The contribution rate of human activities exceeds 80%,while that of climate change during the same period is less than 20%.2.The summer land surface biogeophysical properties are considerably modified due to revegetation in Mu Us Sandy Land.Green vegetation fraction and leaf area index largely increase while surface albedo significantly decreases.The simulations show that summer mean air temperature significantly decreases by 0.12-0.32?in response to such modifications,with the magnitude of reduction in nighttime minimum(0.15-0.47?)greater than that in daytime maximum(0.04-0.13?).The asymmetrical cooling effect leads to the significant increase in diurnal temperature range(0.1-0.37?).At the same time,the revegetation has a slight wetting effect.The combined response of air temperature and specific humidity to revegetation leads to the widespread reduction of heat content of surface air by 0.1-0.4 k J/kg,bringing the slight cooler and wetter weather conditions during the summer.In addition,revegetation causes the decrease in intensity and frequency of extreme warm events,and the increase in intensity and frequency of extreme cold events in summer to a certain extent.The cooling effect of revegetation on daily mean air temperature is mainly attributed to the increase in evapotranspiration,and the weakening of the diurnal cycle of ground heat flux plays a stronger role in the change of the minimum air temperature.3.The simulation shows that the summer evapotranspiration increases by 0.17mm day-1 averaged over the sandy land due to revegetation,with a relative change of8%,which is equal to an increase of about 3.5×108 m3 in water consumption over the entire sandy land during the summer.However,because of the increase in water vapor associated with revegetation does not generate a positive precipitation feedback,and the cooling effect tends to stabilizes the atmosphere,which will offset the positive impact of the increase in water vapor on precipitation to some extent.Therefore,the effect of revegetation on regional precipitation is negligible.As the water deficit cannot be compensated by precipitation feedback,the regional water balance is impaired,leading to the decrease in moisture of 0-200 cm soil layer,with the magnitude of reduction in soil moisture of the deep layers greater than that of the surface layer.4.Variation of land surface evapotranspiration has little potential to affect precipitation in the Mu Us Sandy Land.The variation of precipitation is dominated by external moisture transport.The precipitation under the high condition of moisture flux convergence(MFC)is more than 70%higher than that under the low condition of MFC.High MFC also brings more intermediate-to-heavy rainfall(>10 mm day-1).Precipitation change can be decomposed into direct contribution that affects supply capacity of water vapor and indirect contribution that affects precipitation efficiency.High MFC significantly increases precipitation mainly by improving precipitation efficiency.The increase in evapotranspiration caused by changes in soil moisture has a certain positive impact on precipitation only during the periods of high MFC,which mainly through indirect contribution.However,such effect is relatively weak and has an insignificant effect on precipitation.In summary,the enhanced evapotranspiration induced by revegetation in the Mu Us Sandy Land has a significant summer cooling effect,which has a positive effect on alleviating the negative outcomes of local climate warming and extreme temperature events on ecosystems to some extent.However,such change cannot promote the increase in regional precipitation.Precipitation of the region is strongly affected by external moisture transport,while changes in land surface evapotranspiration have little potential to affect regional precipitation.This further demonstrates that even if the land surface condition(such as large-scale revegetation)changes to a large extent,the subsequent change in evapotranspiration may have very limited biogeophysical impact on precipitation.It is worth noting that the water deficit caused by revegetation cannot be compensated by precipitation feedback,therefore leading to the reduction in soil moisture,which may aggravate water resource scarcity and has negative impact on sustainability of present vegetation restoration and ecosystem services.Therefore,our study suggests that future vegetation restoration and ecological construction in arid and semi-arid region should comprehensively balance the relationship between vegetation-climate-hydrology,and that vegetation construction should be adapted to local climate and ecological carrying capacity to achieve regional sustainable development. |
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