Rainwater as a renewable resource to achieve net zero urban water in water stressed cities.

• Tucson has the capacity to achieve net zero urban water with rainwater resources. • Passive systems help, but active systems are required for net zero urban water. • The median household storage required to support net zero urban water is 28.2 cubic-meters. • A roughly $10,000 investment per household is needed to reach net zero urban water. • Optimal rainwater harvesting areas do not correlate with socioeconomic status. Globally, cities are facing increased water stress with an imbalance between available water resources and projected urban water demands. Water experts have pointed to rainwater harvesting (RWH) as one answer; however, the capacity of such a solution to address deficits on an urban scale is largely untested. This study proposes a model to evaluate the necessary network of RWH systems to achieve net zero urban water, meaning a sustainable local supply capable of meeting long-term demand. This approach was applied to a water stressed city, Tucson, Arizona, in the semi-arid Southwestern United States. A daily water balance model optimized for the smallest required storage volumes to reach net zero urban water at a resolution of ten years of consecutive daily rainfall. The potential passive and active RWH network was modelled under four investment scenarios using remote sensing, localized daily rainfall, and municipal water meter data. In the most financially and physically plausible scenario, rainwater replaced imported water and a 30% demand conservation was assumed. The median required storage was 28.2 cubic-meters per 100 square-meters of roof area, or an estimated $10,000 investment. Comparatively, the median RWH household storage installation in Tucson is 5.3 cubic-meters (1395 gallons). In addition to widespread investment in large household storage, this result would require paradigm shifts across built, economic, and social systems to fully integrate the decentralized network across the urban fabric. This study shows that net zero urban water can be achieved with RWH under multiyear drought conditions with large storage volumes. [ABSTRACT FROM AUTHOR]