食物、能源和水资源是人类发展的三大生命线资源，彼此之间相互关联、相互依存、紧密交织，其中任何一个方面的变化都会对另外两方面产生深远的影响，这种互动关系在经济活动集聚的城市表现尤为显著。传统上基于部门分割视角下的管理体系，难以应对当前复杂的城市生态安全问题。尤其是全球气候变化和新冠疫情的强有力冲击，更加凸显了城市生态系统管理的困难与挑战。食物-能源-水耦合思想作为环境管理新范式，为后疫情时代城市韧性提升与资源可持续管理提供了系统思路及稳健基础。城市农业、绿色屋顶、雨水滞留池等蓝绿基础设施，是典型的城市食物-能源-水可持续管理实践，具有分散资源供应、减少洪水灾害和热岛效应的功能，目前已在许多大城市得到推广普及。而此类城市蓝绿基础设施实践多数需要大量连贯的建设空间，加剧了城市蓝绿空间规划与有限土地资源之间的竞争冲突。

城市屋顶作为重要的立体空间资源，具备开展蔬菜种植、光伏发电和雨水收集等活动的空间条件，通过直接提供食物-能源-水相关效益，能够改善城市当地食物、能源和水资源的安全性，增强城市生态系统应对不确定性扰动的抵御力。同时，由于城市地区之间存在着频繁的贸易交互活动，城市屋顶系统提供的食物-能源-水效益可以削减城市对边界外资源的调入需求和进口依赖，进而规避食物、能源及水资源产品在城市上游生产供应链上隐含的环境足迹，如能源足迹、水足迹和碳足迹。然而，城市屋顶系统在以食物-能源-水为功能导向开发时，又势必会需要大量资金投入，并引发全生命周期过程的资源环境影响，包括能源消耗、水消耗、碳排放和土地占用等，形成了城市屋顶系统与食物-能源-水-土地-碳等多种资源环境要素之间的权衡互动关系。在资源环境约束趋紧之下，揭示食物-能源-水关联视角下城市屋顶系统的资源环境效应，成为提升城市空间规划效率及资源环境可持续性的重要突破点。

本文基于“理论框架-方法学模型-综合权衡评估-规模化情景分析”的城市屋顶系统资源环境效应研究及评估体系，首先构建了食物-能源-水关联视角下城市屋顶系统提升城市可持续性的概念框架，然后建立了一套基于生命周期的城市屋顶系统资源环境效应评估方法学模型，最后以深圳市为案例区，开展了食物-能源-水关联视角下城市屋顶系统资源环境效应评估及多目标协同的规模化开发情景分析，主要研究内容及结论如下：

理论方面，基于“城市边界内屋顶系统对食物-能源-水系统的直接影响”、“由食物-能源-水关联关系引发的屋顶系统对食物-能源-水系统的部门牵引和影响传递作用”、以及“在城市与区域的频繁贸易交互驱动下，由屋顶系统直接食物-能源-水效益引发的跨尺度级联效应”，构建了食物-能源-水关联视角下城市屋顶系统提升城市可持续性的概念框架，能够揭示城市屋顶系统与食物-能源-水复杂关系网络内多要素间相互作用的关联机制，有助于多利益相关方理解城市屋顶系统与食物-能源-水关联作用内涵。

方法学方面，将传统的生态效益评估纳入到生命周期框架体系中，基于城市食物-能源-水关联关系及城市与区域的互动机制，集成物质流分析、生命周期评价、投入产出分析等代谢方法与多目标优化方法，建立了一套基于生命周期的城市屋顶系统资源环境效应评估方法学模型，涵盖四大模块：生命周期资源环境影响及经济成本评估、直接食物-能源-水效益模拟、可规避的跨城市边界环境足迹核算、多目标协同的城市屋顶系统规模化开发情景分析，能够为城市屋顶系统开发的可持续性评估及规模化推广策略研究提供技术方法。

案例应用方面，以深圳市为案例研究区，针对三种不同屋顶系统（裸屋顶系统、绿色屋顶系统和露天种植屋顶系统），开展了食物-能源-水关联视角下城市屋顶系统资源环境效应评估。在城市边界内视角下，屋顶系统的直接能源-水效益和生命周期能源-水消耗权衡结果表明，裸屋顶系统、绿色屋顶系统和露天种植屋顶系统均具有净能源效益和净水效益。单位面积裸屋顶系统、绿色屋顶系统和露天种植屋顶系统的直接能源效益分别为其生命周期能源消耗的2.3–2.4倍，直接水效益分别为屋顶系统生命周期水消耗的2.9–10.8倍。在跨城市边界视角下，单位面积露天种植屋顶系统可规避的跨边界能源足迹为其生命周期能源消耗的4.5倍，可规避的跨边界水足迹达到了屋顶系统生命周期水消耗的37.2倍。屋顶系统的直接食物效益和直接能源效益是规避跨边界环境足迹的最主要驱动因素。在露天种植屋顶系统中，直接食物效益分别驱动了93%和94%的跨边界能源足迹和水足迹避免，而直接能源效益是跨边界碳足迹避免的主要驱动因子，能够规避68%的城市跨边界碳足迹。在多目标协同的规模化开发情景下，若结合开发三种不同屋顶系统（裸屋顶系统34.2 km²、绿色屋顶系统34.3 km²、露天种植屋顶系统33 km²），将产生高达27.6亿美元的直接食物-能源-水效益，同时也伴随着巨大的生命周期能源-水-土地-碳影响和经济成本。此外，将90.2%（48.71 km²）的城市可用屋顶开发成露天种植屋顶系统，可以实现深圳市本地蔬菜自给自足，由此还能间接规避城市跨边界食物供应链上的能源足迹8.33E+10 MJ和水足迹3.07E+08 m³，分别相当于深圳市21%的能源需求和14%的居民用水需求。本研究强调了要以系统观规划城市屋顶系统、提升屋顶系统资源环境可持续性，并加快推进城市屋顶农业、屋顶光伏部署及雨水收集实践方案，研究结果能够为城市屋顶空间规划及资源环境可持续管理提供决策支持和信息辅助。

Food, energy and water are the three lifeline resources for human development, which are interrelated, interdependent and closely intertwined. Changes in any one of them will have a profound impacts on the other two, and these interactions are especially significant in cities with concentrated economic activities. Traditionally, the sectoral management approaches can hardly cope with the current complex urban ecological safety issues. In particular, the powerful impacts of global climate change and the Covid-19 have highlighted the difficulties and challenges of urban ecosystem management. As a new paradigm of environmental management, the thinking of food-energy-water nexus provides a systematic idea and a robust foundation for urban resilience enhancement and sustainable resource management in the post-epidemic era. As typical urban food-energy-water sustainable management practices, urban agriculture, green roofs, rainwater retention ponds and other green and blue infrastructures have been popularized in many large cities for their functions of decentralizing resource supply and reducing flooding as well as heat island effect. Most of these urban green and blue infrastructure practices require a large amount of coherent construction space, which intensifies the competition conflicts between urban green and blue spatial planning and limited land resources.

As an important three-dimensional spatial resource, urban rooftops could be equipped with spatial conditions for vegetable cultivation, photovoltaic power generation and rainwater harvesting, which can improve the security of local food, energy and water resources in cities and increase the resilience of urban ecosystems to uncertain perturbations by directly providing the food-energy-water related benefits. At the same time, due to the frequent trade interactions between urban areas, the food-energy-water benefits provided by urban rooftop systems can further reduce the city's demand for transferring and importing resources from outside its borders, thereby avoiding the environmental footprints, such as energy, water and carbon footprints embodied in the food, energy and water products in the upstream urban production supply chain. However, when urban rooftop systems are developed with food-energy-water as the functional orientation, they inevitably cause resource and environmental impacts in the whole life cycle process, including energy consumption, water consumption, carbon emission and land occupation, as well as capital investment, forming the trade-off interactions between urban rooftop systems and various resource and environmental factors such as food-energy-water-land-carbon. Under the tightening constraints of resource and environmental, revealing the resource and environmental effects of urban rooftop systems from the perspective of food-energy-water nexus becomes an important breakthrough point to improve the efficiency of urban spatial planning and resource and environmental sustainability.

Based on the “theoretical framework-methodological model-comprehensive trade-off assessment-scenario analysis at the urban scale” system, this paper firstly constructs a conceptual framework for urban rooftop systems to enhance urban sustainability from the perspective of food-energy-water nexus, and then establishes a suite of life cycle-based methods for assessing the resource and environmental effects of urban rooftop systems. Finally, taking Shenzhen, China, this study carried out the assessment of the resource and environmental effects of urban rooftop systems from the perspective of food-energy-water nexus and the analysis of multi-objective synergistic development scenarios:

Theoretically, based on the “direct impacts of rooftop systems on food-energy-water systems within urban boundaries”, “sectoral traction and impact transmission of rooftop systems on food-energy-water systems induced by food-energy-water nexus”, and “transboundary cascading effects triggered by direct food-energy-water benefits of rooftop systems driven by frequent trade interactions between cities and regions”, constructing a conceptual framework for urban rooftop systems to enhance urban sustainability under the food-energy-water nexus perspective, which can reveal the complex network of urban rooftop systems and food-energy-water nexus. It helps understand the interaction mechanisms between multiple elements in the complex network of urban rooftop systems and food-energy-water nexus, and makes it clear for multi-stakeholders on the implications of urban rooftop systems and food-energy-water nexus.

Based on the urban food-energy-water nexus and the interaction mechanism between the city and the region, the methodology integrates metabolic methods such as material flow analysis, life cycle assessment, input-output analysis and multi-objective optimization methods, and establishes a life cycle-based methodological model for assessing the resource and environmental effects of urban rooftop systems, covering four major modules: life cycle resource and environmental impact and economic cost assessment, direct food-energy-water benefit simulation, avoidable transboundary environmental footprint accounting, and multi-objective synergistic scenario analysis of urban rooftop system development, which can provide technical methods for sustainability assessment of urban rooftop system development and scale-up strategy research.

In terms of case study application, the assessment of the resource and environmental effects of urban rooftop systems from the perspective of food-energy-water nexus was carried out for three different rooftop systems (bare roof system, green roof system, and open-air farming roof system). Taking Shenzhen as the case study area, the direct energy-water benefits and life cycle energy-water consumption trade-offs of the rooftop systems from an intra-city boundary perspective showed that the bare roof system, green roof system, and open-air farming roof system had net energy and net water benefits. The direct energy benefits per unit area of bare roof system, green roof system and open-air farming roof system are 2.3–2.4 times of their life cycle energy consumption, respectively, and the direct water benefits are 2.9–10.8 times of the life cycle water consumption of the roof system, respectively. From the transboundary perspective, the avoided transboundary energy footprints per unit area of open-air farming roof system is 4.5 times its life cycle energy consumption, and the avoided transboundary water footprint reaches 37.2 times life cycle water consumption of the roof system. The direct food benefits and direct energy benefits of the roof system are the most important drivers of the transboundary environmental footprint avoidance. Direct food benefits drive 93% and 94% of the transboundary energy and water footprint avoidance, respectively, in open-air farming roof systems. Whereas the direct energy benefits are the main driver of transboundary carbon footprint avoidance, which could avoid 68% of the transboundary carbon footprints. In a multi-objective synergistic development scenario at scale, when combing three different roof systems (34.2 km² for bare roof system, 34.3 km² for green roof system, and 33 km² for open-air farming roof system), it would generate up to $2.76 billion in direct food-energy-water benefits, but accompanied by significant life cycle energy-water-land-carbon impacts and economic costs. In addition, developing 90.2% (48.71 km²) of the available urban rooftops into open-air farming roof systems would enable Shenzhen to be self-sufficient in local vegetables, thus indirectly avoiding energy footprints of 8.33E+10 MJ and water footprints of 3.07E+08 m³ in the transboundary food supply chain, equivalent to 21% of the city’s energy consumption and 14% of the city’s residential water demands. This study emphasizes the need to plan urban rooftop systems with a systemic lens to enhance the resource and environmental sustainability of rooftop systems, and accelerate urban rooftop agriculture, rooftop photovoltaic deployment, and rainwater harvesting practice programs. The results of this study can provide decision support and information assistance for urban rooftop spatial planning and sustainable resource and environmental management.