Strategic design and finance of rainwater harvesting to cost-effectively meet large-scale urban water infrastructure needs.

Many cities are confronted with both water scarcity and urban flooding as centralized water infrastructures becoming increasingly inadequate in a changing climate. Decentralized infrastructures like rainwater harvesting (RWH) can ease both issues. Yet, most studies find RWH offers limited infrastructure capacity at high cost. Previous assessments, however, fail to consider two critical advantages: multi-functionality and high adaptability. By improving the incorporation of these advantages in our analysis of 1.06 million buildings with distinct design and water demand characteristics and 20-year hourly precipitation records in New York City (NYC), we demonstrate, contrary to existing studies, that strategically designed, financed and implemented rooftop RWH systems in all or a subset of the buildings can meet large-scale infrastructure development needs for water supply and stormwater management. RWH implementation featuring public-private partnerships (PPP) in 43-96% of the buildings can serve 17-29% of the city's non-drinking water demands while reducing the public expenditure per unit of water supply by 13-85%. The distributed citywide RWH implementations prevent 35-56% of rooftop runoff from entering the sewage system, rivers, and/or waterways per month, with observed rooftop runoff reductions as high as 90% for a single rain event.