Answering the supply–demand gap with alternative water sources: Retrofitting cities to achieve net zero urban water

Climate change, population growth, and de-teriorating infrastructure portend a global urban water crisis in the coming decades. In cities facing extremely high water stress, conservation alone will not meet the challenge. Alternative water sources are needed to fill the supply–demand gap and reach a sustainable net zero urban water balance. Potential single water source solutions have been well researched, but the full integration of alternative water supplies (e.g., stormwater, rainwater, gray water, wastewater) across municipal systems for aggregate benefit has yet to be fully explored through planning and design. This research used public-private-academic partnering, water supply and demand calcula-tions, speculative prototyping, and scenario planning and design to test the potential of transitioning a city with high water stress to a net zero urban water balance by 2050. Twenty prototypes were developed that integrated five alternative water sources across three land use scenarios in Tucson, Arizona, to replace imported water with local supplies. The article concludes that rather than managing urban water in single-purpose infrastructure, all water must be assessed as a one-resource system and part of a comprehensive urban design strategy across natural-social-technological dynamics. The article provides new forms and languages to layer, graft, integrate, and hy-bridize alternative water sources with existing city fabrics to achieve sustainable and adaptable net zero balance water systems.