The Influence Of Urbanization On Carbon-water Coupling In Urban Systems In The Chengdu-Chongqing Urban Agglomeration

In recent years,the rapid urbanization around the world has led to an increase in urban water demand and CO₂emissions.Cities have become the major water consumers and carbon emitters,resulting in deterioration of the ecological environment and the occurrence of disasters such as floods and extreme drought.However,carbon emissions and water consumption are closely related through human activities such as energy production,sewage treatment,and trade in urban development.The complicated water-carbon coupling causes difficulties for urban water conservation and emission reduction.Thus,the research on the changing mechanism of carbon-water coupling and its response to urbanization process is essential for the sustainable development of cities.As an emerging dual-core urban agglomeration,the Chengdu-Chongqing urban agglomeration（CCUA）is an important strategic platform in western China.And the study on CCUA will strengthen the understanding of the carbon-water coupling and the sustainable development path in the early stages of urban agglomeration development.Therefore,this study evaluated the urbanization level and coupling coordination degree of the CCUA from 1992 to 2015 by the entropy method and coupling coordination degree model.And the development process was divided into three periods according to the piecewise fitting results.Based on the logarithmic mean Divisia index（LMDI）model,the CO₂emissions and water use changes in each period was decomposed.The changing trend,driving force and spatio-temporal evolution of carbon emissions and water use were analyzed.Besides,the decoupling index was used to analyze the changing trend and influencing factors of the carbon-economy and water-economy coupling.And the carbon-water coupling coordination degree was calculated to analyse the changing trend and characteristics of carbon-water relationship.At last,the index of carbon emissions generated by unit water consumption was promoted to analyze the sustainable development in CCUA and its core areas.Based on the research above,this study aimed to analyze the characteristics of carbon-water relationship in different periods of urban agglomeration development,trying to explore the economic and policy factors behind the research results.The main conclusions are as follows:（1）In the Primary Development Period（1992-2002）,both the urbanization level and the inter-regional coupling coordination degree in CCUA rose steadily and slowly.In the Accelerated Development Period（2002-2011）,the urbanization and inter-regional coordinated development accelerated.And in the High-speed Development Period（2011-2015）,the CCUA developed at a stable speed which was significantly improved compared with the first period.Besides,the nine Central Districts of Chongqing Municipality（NCD）and Chengdu are the core areas of CCUA,with the first and second fastest growth rates among all the research units,respectively.（2）The CO₂emissions of CCUA increased at rates of 0.066 Mt·y^-1and 0.165Mt·y^-1in the first two periods,respectively,and remained basically stable at a growth rate of-0.021 Mt·y^-1in the third period.The economic scale effect was the main driving factor for CO₂emission growth,while the CO₂emission intensity effect was the major restraining factor.The impact of economic scale effect was weakened while the impact of the CO₂emission intensity effect was stengthened,making the CO₂emission in CCUA dominated by the promotion effect of economic scale effect in the first two periods but dominated by the inhibition effect of CO₂emission intensity effect in the third.（3）The water consumption of CCUA increased by 0.160 km³·y^-1,0.643km³·y^-1and1.720 km³·y^-1in the three periods,respectively.The economic scale effect was the main driving factor,while the water intensity effect was the main inhibiting factor.And the influence of the economic scale effect was strengthened while the influence of the water intensity effect was weakened,which led to the mutual offset of the effects of the two main influencing factors in the first period and the leading role of the economic scale effect on the water use changes in the latter two periods.（4）On the regional scale,the regional contributions to CO₂emissions and water use in Sichuan were higher than those in Chongqing.The CO₂emission changes in CCUA were dominated by the two core areas,while the spatial distribution of water use changes was obviously scattered with a lack of stable and powerful leading areas.On the time scale,the CCUA mainly focused on the development of Chengdu and the CND in the second period.The contributions of the core areas to CO₂emissions and water use increase ranked first in its own provinces,respectively.In the third period,the development focus shifted from the core areas to the surrounding areas,as most of the surrounding areas contributed more to the carbon emissions and water use increase.And the core areas changed rapidly from carbon emission promoters to inhibitors,while the contributions of Chengdu and the NCD to water use growth decreased by 13.73%and11.21%,respectively.（5）In the first two periods,there was a gradually increasing decoupling effect between CO₂emissions and economic development.In the last period,the decoupling further increased from relative decoupling to absolute decoupling.And the change of CO₂emission intensity was the main driving factor,which grew stronger over time.However,the relative decoupling effect between water use and economic growth was gradually weakened.And the change of water use intensity was the main driving factor,with its promotion effect decreasing over time.（6）The carbon-water decoupling effect in CCUA was strengthened period by period while the peak of carbon emissions generated by unit water consumption in CCUA appeared in 2012.It indicated that CCUA showed temporary low-carbon development characteristics in the third period.The carbon-water decoupling effect in Chengdu was significantly accelerated by periods,while that in NCD was slightly enhanced with large fluctuation range.Meanwhile,Chengdu reached the peak of carbon emissions generated by unit water consumption earlier than the NCD.However,the index of carbon emissions generated by unit water consumption increased faster and decreased slower in Chengdu,while it increased slower and decreased faster in the NCD,indicating that the NCD responded more easily to water saving and carbon reduction strategies than Chengdu.