Many studies examine the correlation between the use of resources such as water, energy and land, and the production of food. These nexus studies focus predominantly on large scale systems, often considering the social dimensions only in terms of access to resources and participation in the decision-making process, rather than individual attitudes and behaviours with respect to resource use. Such a concept of the nexus is relevant to urban agriculture (UA), but it requires customisation to the particular characteristics of growing food in cities, which is practiced mainly at a small scale and produces not only food but also considerable social, economic, and environmental co-benefits. To this end, this paper proposes a new conceptual basis for a UA Nexus, together with an assessment methodology that explicitly includes social dimensions in addition to food, energy and water. The conceptual basis introduces People, together with Food, Energy and Water, as a fundamental factor of the UA Nexus. On this basis, a methodology is developed measuring not only resource efficiency and food production but also motivations and health benefits. It comprises a combination of methods such as diaries of everyday UA practices, a database of UA activities, life cycle assessment (LCA), and material flow analysis to connect investigations developed at a garden scale to the city scale. A case study shows an application of the methodology.

Urban areas are considered net consumers of materials and energy, attracting these from the surrounding hinterland and other parts of the planet. The way these flows are transformed and returned to the environment by the city is important for addressing questions of sustainability and the effect of human behavior on the metabolism of the city. The present work explores these questions with the use of systems analysis, specifically in the form of a Multi-sectoral Systems Analysis (MSA), a tool for research and for supporting decision-making for policy and investment. The application of MSA is illustrated in the context of Greater London, with these three objectives: (a) estimating resource fluxes (nutrients, water and energy) entering, leaving and circulating within the city-watershed system; (b) revealing the synergies and antagonisms resulting from various combinations of water-sector innovations; and (c) estimating the economic benefits associated with implementing these technologies, from the point of view of production of fertilizer and energy, and the reduction of greenhouse gases. Results show that the selection of the best technological innovation depends on which resource is the focus for improvement. Urine separation can potentially recover 47% of the nitrogen in the food consumed in London, with revenue of $33 M per annum from fertilizer production. Collecting food waste in sewers together with growing algae in wastewater treatment plants could beneficially increase the amount of carbon release from renewable energy by 66%, with potential annual revenues of $58 M from fuel production.

This study uses Taiwan's WEF nexus as a case study to demonstrate how the resource flow and associated environmental impact of the WEF nexus can be assessed as basis for evaluating strategies for promoting the sustainable use of natural resources. In this study, material flow analysis (MFA) and life-cycle assessment (LCA), were combined. The MFA was used to examine the interdependence of the three natural resources, and the LCA was used to evaluate the environmental impacts of the WEF system. The WEF nexus analysis shows that tap water supply, oil refining, the cogeneration of steam and electricity, thermoelectric power plants, irrigation, animal husbandry, and aquaculture are the main interwoven nodes and have the most prominent impact on the three natural resources. When the unit products from the WEF system were determined, LCA was implemented for these products to identify 15 types of environmental impacts. The environmental impacts for the WEF system were then calculated based on the use of unit products. The results of LCA showed that the most prominent impacts are the impact of public electricity on climate change; oil products on ozone depletion and ionizing radiation; tap water on metal depletion; and animal husbandry on terrestrial ecotoxicity. Based on the assessment of the alternative resource management strategies, if both water and energy policies are modified simultaneously, the impact of the overall WEF system on most environmental impact categories could be reduced.

Implementing effective resource management is crucial for urban sustainability. Potential resource management strategies should be assessed under the framework of a resource nexus to avoid problem shifting. The urban metabolism of food, energy, and water is driven by lifestyle, industrial structure, and infrastructure. This study employed material flow analysis to identify resource metabolism through the phases of supply, process, demand, and final sink. The resource intensity of urban activities and the risk of the nexus of resources were quantified to illuminate management strategies. This study investigated the food-energy-water nexus (FEW nexus) for a small and multi-sector island city, Kinmen, and found that the nexus risk of water for food is the highest. Water and energy consumption have excessive loads on resource metabolism in a multi-sector city, and the main demand sectors increase the nexus risk in water for food. The results indicated that higher risk results from higher resource consumption intensity, particularly in areas of economic growth. Resource management of the FEW nexus needs the best tradeoff strategy to meet the goals of urban metabolism sustainability. The risk assessment framework can support the design of optimal resource management strategies to pursue urban sustainability. Consequently, given the limitations of water treatment technology, the impact of energy risk mitigation is poor (below 4% of energy risk in 2015) and the energy risk will continue to increase (by about 10% based on the economic activity). As a result, imported water is the best tradeoff strategy to meet the FEW nexus safety for Kinmen City as a low-resource and sightseeing activity area.

Nowadays, food production systems play a relevant role as the steady increase of global population and food demand. The water-energy-food (WEF) nexus is a suitable approach to tackle resources management associated with these three pillars recognizing synergies and trade-offs. Different approaches have been used in the literature to measure the WEF nexus, being material flow analysis (MFA) and life cycle assessment (LCA), two of the most proven methodologies. The MFA approach is based on the amount of resources consumed, while using the LCA perspective considers all flows of the system (LCA footprints approach) or considering only the flows associated with water, energy, and food pillars as the inventory data (WEF-LCA approach). This manuscript compares the three mentioned approaches to identify their strengths and weaknesses. To do this, a sample of 100 Spanish dairy farms is analysed, where a single WEF nexus index (WEFni) is obtained using Data Envelopment Analysis. Results show that only four farms achieved a WEFni equal to 100 in all approaches, while the main difference between them is the number and type of resources considered for calculating the WEF nexus, which could imply a partial identification of hotspots of food systems.