The framework for sustainability of urban areas is tied to the patterns of urban metabolism in which resources (water, food, energy, materials and chemicals) are delivered to an urban area, metabolized and changed to outputs. Under the current linear concept water, energy and other inputs generate waste and pollution. Furthermore, lack of conservation and waste within the city leads to shortages and, in the near future, to exhaustion of resources. There is a need to change the current linear urban metabolism to one that would reuse and recycle and in which used water and solids would become a resource. This would be a paradigm change of building and retrofitting the cities.
The footprints are quantitative measures of sustainability and metabolism. Footprints covered in this article are water, energy/greenhouse emissions, and ecology. When the footprints are defined, development of sustainability criteria should follow. The footprints may be global, regional or local and can be hierarchically interconnected.