Rain harvesting (rainwater harvesting) is the collection, storage, and use of rainwater from rooftops, land surfaces, or other catchments. It is an ancient practice gaining renewed attention for water conservation and stormwater management.
Rainwater harvesting is the practice of collecting, storing, and using precipitation runoff from rooftops, paved areas, or prepared catchment surfaces for beneficial purposes including landscape irrigation, toilet flushing, laundry, groundwater recharge, and, with appropriate treatment, potable water supply. Systems range from simple barrel collectors under roof downspouts to large-scale cisterns integrated into building design. A typical residential rooftop rainwater harvesting system consists of a catchment surface (roof), conveyance system (gutters and downspouts), first-flush diverter (to discard initial contaminated runoff), storage tank, and distribution system with filtration and disinfection for potable use. The potential yield depends on catchment area, local precipitation, and collection efficiency, with a common rule of thumb being approximately 600 liters collected per millimeter of rainfall per square meter of catchment area, accounting for losses. Rainwater harvesting provides multiple benefits: supplementing water supply, reducing stormwater runoff volumes and peak flows, decreasing demand on centralized water infrastructure, and providing water access in areas without piped supply. Many municipalities and states offer incentives for rainwater harvesting, though some western states historically restricted the practice under prior appropriation doctrine. The technology is particularly valuable in developing countries where it can provide a low-cost, decentralized water supply for households and communities without access to improved water sources.