| The Yangtze River Basin is the largest river basin in China and has a unique position in the world due to the complexity and diversity of its surface environment.The Yangtze River Basin is also an important economic zone in China,and the central government calls for the development of the Yangtze River Economic Belt to"take care of major protection and not to engage in major development",which places higher expectations on the management of natural resources and land in the Yangtze River Basin.In the past two decades,China's rapid socio-economic development and the implementation of ecological protection and afforestation projects have had a significant impact on the land cover of the Yangtze River basin and have caused a series of significant environmental and ecological changes.In order to investigate the extent of the impact of land cover change on carbon and water fluxes in the Yangtze River basin,this study innovatively assessed the relevant effects of land cover change in terms of land cover type and land cover greenness,and constructed a remote sensing data-driven ecosystem model to measure the spatial and temporal variability of monthly-scale carbon and water fluxes from 2001 to 2018.The individual and integrated effects of climate variability,vegetation activities and land cover changes on primary productivity,evapotranspiration and water production were quantified with a series of model experiments.In addition,two typical areas within the Yangtze River basin were selected for in-depth analysis of specific issues.The study is important for improving the understanding of the interaction mechanism between land cover change and surface ecological processes in the subtropical region and for better ecological civilization construction in the Yangtze River Economic Belt.The results of the study show that(1)all land cover types in the Yangtze River basin changed significantly from 2001 to 2018,but their proportions were generally stable.The dense tree cover showed the largest change in area among multiple land covers.The annual mean normalized difference vegetation index increased significantly at a linear rate of change of 0.004/yr.(2)From 2001 to 2018,the average gross primary productivity(GPP)in the Yangtze River basin showed a significant upward trend with a linear rate of14.21 g C/m~2/yr.Both the long-term trend and interannual fluctuations of GPP were dominated by land cover changes.(3)From 2001 to 2018,the average annual evapotranspiration in the Yangtze River basin showed a significant upward trend with a linear rate of change of 3.05 mmyr.For the long-term trend of evapotranspiration,land cover change played a dominant role,while the role of climate change was relatively small.The effects of land cover and climate on the interannual fluctuations of evapotranspiration were comparable.Land cover change leads to a significant decrease in water yield in the Yangtze River basin,however,this negative effect is masked by the large interannual fluctuations in climate(especially precipitation).(4)From 2001 to 2018,land cover in the Yangtze River basin dissipated 162 g of water for every 1 g of carbon absorbed through photosynthesis.Under effects of land cover change alone,for every 1g of carbon absorbed by land cover in the Yangtze River basin,the water dissipated through transpiration increased to 277 g.(5)Land cover change in the upper Han River basin caused a significant decreasing trend of water production by 3.2 mm/yr from 2001to 2018,and the runoff coefficient(ratio of water yield to precipitation)significantly decreased from 2003 to 2018.The greening of land cover also increased the possibility of hydrological drought,increasing the frequency of severe drought by 11.6 percentage points.(6)Both GPP and evapotranspiration showed a significant decrease during the earthquake impact period(2007-2010)in the extremely hard-hit areas of the Wenchuan earthquake in the typical district,and recovered rapidly during the post-earthquake recovery period(2010-2018).The results of this study have the following implications for policy formulation and implementation:(1)The implementation of large-scale ecological conservation projects in the Yangtze River basin since the late 1990s has increased forest cover and played an important role in improving regional ecology,which is important for China's carbon reduction policy and achieving the goal of"carbon neutrality".(2)The implementation of afforestation and ecological protection policies should be vigilant to the negative impact on China's water resources,and the Yangtze River basin needs a sound water management policy to cope with the rising evapotranspiration from the greening of the land in order to mitigate the impact of future droughts.(3)A series of land use and agricultural policies implemented in China in the past two decades have achieved positive results in increasing food production and ensuring food security.However,China's agricultural production should take effective measures to cope with the important challenge of increasing extreme weather events.(4)The implementation of reforestation and ecological protection in the upper Han River basin has had important positive effects.However,it will inevitably increase the ecological water consumption of vegetation and thus affect the water supply of the South-North Water Transfer Central Project.Therefore,a comprehensive land use and water resource management policy is needed to adopt a diversified water supply strategy.(5)After the Wenchuan earthquake,the principle of"combining natural restoration and artificial treatment,with natural restoration as the mainstay"was used to restore damaged vegetation,which has produced good practical results. |
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