能源和水资源对人类生存和社会经济发展至关重要。随着经济的发展和人口的增加，能源和水资源短缺及其开发利用所带来的资源环境问题日趋严重。厘清能源和水资源经济政策的资源、环境、经济等效应，并对能源和水资源经济政策进行优化设计，是实现节能节水并解决相关资源环境问题的关键。
    能源和水资源之间存在密切的关联性，调控能源生产和消费的经济政策会影响水资源的开发和利用，调控水资源开发和利用的经济政策也会影响能源的生产和消费。由于能源和水资源的管理权限隶属于不同部门，现有能源和水资源经济政策效应及其优化研究，侧重从分别解决能源和水资源问题的相互独立的视角出发，而基于能-水关联视角的系统研究相对较弱；另外，现有研究重点关注能源和水资源经济政策的直接效应（主要指能源经济政策对能源使用的影响、水资源经济政策对水资源使用的影响），而略化其间接效应（主要指能源经济政策对水资源使用的影响、水资源经济政策对能源使用的影响）以及两种经济政策同时实施的叠加效应（强调两种政策同时实施的综合影响，并非单项政策效果的简单加和）。能源经济政策与水资源经济政策的制定和执行过程相互独立，缺乏交集，会造成政策实施实际效果与预期效果的偏差。
    在能-水关联视角下，对能源和水资源经济政策效应及优化进行研究，需要揭示能源和水资源这两种资源使用的关联机制，以解析两种政策效应关联性的根源；需要同时分析两种政策的直接效应、间接效应和叠加效应，以厘清政策的效应特征；需要同时考虑能源和水资源系统对外输出的功能性特征以及系统内部稳定性特征，以提升能源和水资源经济政策耦合优化的效果。为实现以上研究目标，既需要研究内容的延伸，也需要理论机制的揭示和完善，还需要相应的模型方法的构建和拓展。本论文的主要研究内容如下：
    本论文从能源和水资源的关联机制、能源和水资源经济政策效应、能源和水资源经济政策的耦合优化三个方面开展理论分析并提出理论假设，构建起全文的理论框架，为实证分析提供理论基础。对于关联机制，本论文通过能源和水资源使用的伴随性和替代性分析以及各产业部门的投入产出关系分析，从理论上揭示了两种资源使用的关联机制，厘清了两类政策交互作用的机理，提出能源和水资源要素生产和消费的关联性以及各产业部门间的关联性（投入-产出关系）是形成能-水关联性的内在因素，间接用能和间接用水可能会显著影响产业部门能源和水资源使用的关联性特征。对于效应规律，本论文通过能源和水资源供给-需求关系、庇古税理论以及一般均衡理论的分析，提出政策间接效应会随着政策强度的增加而增强，但可能呈现边际效应递减趋势；并提出两种政策同时实施时，虽然能源节约、水资源节约和碳减排的效果均大于只实施一种政策的效果，但可能产生拮抗型叠加效应。对于耦合优化，本论文通过“双重红利”假说及系统论分析，揭示了能源和水资源经济政策耦合优化的协调性机理，提出高强度的碳税和水资源费政策可能会导致能水系统网络向外部输出的功能性（环境红利、社会经济红利）与内部稳定性之间的冲突，优化的碳税率和水资源费率应存在一定的可行解范围，通过税收返还结合碳税和水资源费同时优化有利于实现三重红利目标（即环境红利、社会经济红利、能源和水资源系统稳定性红利）。
    本论文从研究框架建立和模型方法改进两个方面对研究方法进行了拓展。构建了基于能-水关联视角的能源和水资源经济政策效应分析及耦合优化的“三步三维三重”研究框架（三步指关联特征揭示、政策效应分析、政策耦合优化三个步骤；三维指直接效应、间接效应、叠加效应三个维度；三重指环境、社会经济、能源和水资源系统稳定性三重红利）。在该研究框架下，基于多学科交叉，构建或拓展了多个模型方法，例如：将叠加效应分析方法与可计算一般均衡模型相结合，构建碳税和水资源费同时实施的政策叠加效应评估模型；根据能源和水资源在社会经济系统中的流动过程，构建生态网络分析模型，分析能源和水资源经济政策对能源和水资源系统稳定性的影响等。
    在理论分析和模型方法构建的基础上，本论文在能源和水资源使用关联特征、能源和水资源经济政策效应、能源和水资源经济政策耦合优化三方面展开实证研究，对理论假设进行验证。
    首先，本论文从横向（各产业部门差异）和纵向（随时间变化）两个方面，对各产业部门能源和水资源使用的关联特征进行分析。横向分析发现，各产业部门中农业、食品和烟草业等部门的直接用水-直接用能比率、间接用水-间接用能比率、综合用水-综合用能比率最高。纵向分析发现，2002-2015年间各部门的直接用水-直接用能比率、间接用水-间接用能比率、综合用水-综合用能比率整体呈下降趋势；农业、石油加工业、化学工业等8个部门综合用水-用能比率不断减小，其中农业部门的下降率相对较小，改进潜力较大。横向和纵向分析都表明，间接用能和间接用水是影响社会经济系统能源和水资源消耗的重要因素，并影响产业部门能源和水资源使用的关联性。
    随之，本论文选取典型能源和水资源经济政策——碳税和水资源费，分别研究了其对能源、水资源使用的直接和间接效应，以及二者同时实施的叠加效应。对两种政策分别实施的政策效应研究表明，碳税在发挥节能减碳功效的同时，可减少直接和间接用水，且随着税率的提高，节水作用更加显著，但作用呈现边际效应递减的特征；水资源费在减少用水量并提高用水效率的同时，能够降低能源使用量和碳排放量，其中煤炭使用量降幅最大，且随着费率的提高，节能作用更加显著，但其作用同样呈现边际效应递减的趋势。对两种政策同时实施的叠加效应研究表明，碳税和水资源费同时实施的能源节约、水资源节约和碳减排效果均大于仅单独实施碳税或仅单独实施水资源费的政策实施效果；但两种政策同时实施将产生拮抗型叠加效应，即两种政策同时实施的政策效果小于单项政策效果的简单加和，且随着两种政策实施强度的增强，拮抗效应将更加显著。
    最后，本论文以实现三重红利为目标，研究能源和水资源经济政策耦合优化方案。基于包括节能节水率提高、CO2减排、GDP增长、能源和水资源系统稳定性提升等多目标优化分析，结果表明，优化的碳税率和水资源费率存在一定的可行解范围，但其可行解范围较窄；而过高强度的碳税和水资源费会造成能水系统网络向外部输出的功能性与内部稳定性之间的冲突，不利于三重红利目标的实现；通过合理设置碳税税率、水资源费率、税费返还比例，可以同时改善能源和水资源系统的对外输出功能性和内部稳定性，实现以三重红利为目标的能源和水资源经济政策的耦合优化。
    本论文的创新集中体现在，突破以往侧重研究能源和水资源经济政策的直接效应，以及对能源、水资源政策侧重分别研究、孤立设计的惯例，从能-水关联视角出发，探究能源和水资源关联机制，分析能源和水资源经济政策直接效应、间接效应、叠加效应，实现以三重红利为目标的政策耦合优化。本论文的研究成果可为提高能源和水资源经济政策制定的协调性，建设绿色低碳经济体系提供决策参考。

Energy and water resources are essential for human beings and socio-economic development. With the population growth and socio-economic development, the energy and water deficit problems and the environmental problems associated with energy and water consumption are becoming more and more obvious. Economic policies are commonly used for energy and water resources conservation. To mitigate these problems, it is necessary to explore the resource, environmental and economic effects of the two kinds of policies, and to optimize the two policies simultantously.

Energy and water are closely interconnected. The policies designed to manage energy production and consumption could influence the consumption of water resouces, and the policies designed to manage water resources withdrawal and consumption could influence the consumption of energy. Energy and water resources are managed by diffierent departments, and accordingly previous research on the effects and optimization of energy and water policies usually analyzed the two policies separately. However, few research has been performed from an energy-water nexus perspective. In addition, previous research focused on the direct effects of energy and water policies (i.e., energy policy’s effects on energy conservation, and water policy’s effects on water resources conserveration), but neglected the indirect effects (i.e., energy policy’s effects on water resources conservation, and water policy’s effects on energy conserveration) as well as the cumulative effects (i.e., the comprehensive effects under the joint implementation of two policies are different from the sum of the effects under the separate implementation of the two policies). If the two kinds of policies are designed and implemented separately, the targets of these policies may not be achieved.

Under the energy-water nexus perspective, three main research aspects need to be performed for the effect analysis and policy optimization. It is necessary to reveal the correlation mechanism of energy and water resources consumption, which is useful to understand the interconnection mechanism of the two kinds of policies. It is necessary to considerd the direct, indirect and cumumative effects simultaneously, which is useful to comprehensively clarify the effects of the two policies. In addition, it is necessary to impove the output attributes and sustainability of energy and water systems simultanouly, which is useful to jointly optimize the two kinds of policies. To perform these three research aspects, the theory and models need to be expanded and improved. The main achievements of this paper are as follows:

The theorical research of this paper consisted of three aspects, i.e., the correlation mechanism for energy and water resouces, the effects of energy and water policies, and the joint optimization of the two kinds of policies. In terms of the correlation mechanism, based on the analysis of the accompanying and substitution attributes between energy and water and the analysis of the input-output relation among industrial sectors, this paper revealed the correlation mechanism of the enegy and water, explained the interconnection mechanism of the two policies, and hypothesized that the key mechanism of energy-water nexus were the resources consumption correlation and industrial sector correlation, and the indirect energy use and indirect water use could obviously influence the correlation characteristics of energy-water nexus. In terms of the police effects, based on the analysis of supply-demand relation and the theories of Pigovian Taxes and General Equilibrium, this paper revealed the characteristics of the indirect and cumulative effects, and hypothesized that with the intensity increase of the energy (water) policy, the water (energy) conservation effects could increase, following a trend of diminishing marginal utility, and also hypothesized that when the two kinds of policies were implemented simultaneously, the effects could be greater than the effects imposed by only one kind of policy, antagonisitic effects were exerted for energy conservation, water conservation and carbon emission reduction. In terms of joint optimization, based on the analysis of double-dividend hypothesis and system theory, this paper revealed the conficts between energy and water policies, and hypothesized that high intensity of policies could lead to the conflict between the output attributes (the environmental and socio-economic dividend) and the sustainability attribute of water and energy systems. In addition, this paper hypothesized that the possible range of the carbon tax and water fee could be narrow, and the simultaneous optimization of tax revenue recycling, carbon tax and water resources fee could be positive to achieve three dividends (the environmental dividend, and socio-economic dividend, and system sustainability dividend)

This paper also expanded the research methods by improving the research framework and the models. This paper established a new framework for effect analysis and joint optimization of energy and water policies under the energy-water nexus perspective. This framework consisted of three steps (i.e., correlation mechanism revelation, policy effect analysis, and joint optimization), three effect dimensions (i.e., direct, indirect and cumulative effects), and three dividends (the environmental, socio-economic and system sustainability dividends). Under this framework, this paper established and expanded several models. For example, this paper combined the cumulative effect analysis model and the Computable General Equilibrium model, and established a new method to analyze the cumulative effects of carbon tax and water resources fee; in addition, this paper established an energy-water resources system network to simulate the cycling processes of energy and water, and adopted the ecological network analysis method to anslyze the sustainability of the energy-water resources system.

After the theorical analysis and method establishment, this paper further performed empirical research to verify the hypothesis established in the theorical analysis section.

For the correlation characteristics analysis, this paper considered the horizontal (cross sectors) and longitudinal (cross years) dimensions, and direct, indirect and total water/energy use. In cross-sector analysis, the direct, indirect and total water-energy use rate for agriculture sector, food and tobacco sector, and electrical supply sector were the greatest, indicting more intensive water conservation measures were required for these sectors. In cross-year analysis, the three rates all declined in 2002-2015, indicating significant improvement of water conservation and also further requirements for energy conservation. Both the horizontal and longitudinal dimension analysis verified that the indirect energy use and indirect water use were important factors influencing the energy and water consumption in the society, and obviously influence the correlation characteristics of energy-water nexus.

For the policy effect analysis, this paper adopted the carbon tax and water resources fee as the example policies, and analyzed the direct and indirect effects as well as the cumulative effects. Carbon tax could reduce the carbon emission and the consumption of direct and indirect water. With the increase of carbon tax intensity, the water conservation effect could be more effective. Water resouces fee could reduce the water consumption, and decrease the energy use and carbon emission (the use of coal decreased most significantly). The indirect effects followed the trend of diminishing marginal utility. When the two policies were implemented simultanoulsy, the effects of energy conservation, water conservation and carbon emission reduction were greater than the effects imposed by only one kind of policy, but antagonisitic effects would occur (the comprehensive effects when the two policies were implemented simultanously were less than the sum of effect when the two policies were implemented separately), with the antagonisitic effects increasing as the increase of policy intensity.

For the joint optimization of the two policies, the three dividends were adopted as the optimization objectives. Based on the analysis of water/energy conservation rate, carbon emission, GDP, sustainability of energy and water system, it showed that the optimized carbon tax and water resources fee existed, but too high intensity of carbon tax and water resources fee could lead to the conflict between output attributes and the sustainability attribute. In addition, reasonable setting of carbon tax rate, water resources fee rate and revenue recycling ratio could benefit both the ouput attributes and sustainability of the energy and water system, and achieve three dividends.

In summary, previous research focused on the direct effects and separate optimization of the energy and water policies. This paper adopted the perspective of energy-water nexus, revealed the correlation mechanism of energy and water, considered not only the direct but also the indirect and cumulative effects, and optimized the two policies simultaneously for three dividends. This research could be helpful for the design of water/energy conservation policies, and be useful for the establishement of low-carbon economic system.