Rainwater harvesting

DEFINITION: The collection and storage of rainwater to meet freshwater needs

The increasing urbanization worldwide requires an ever-increasing supply of water. Rainwater is a free resource that can be collected through various relatively inexpensive methods, and rainwater harvesting can provide water for drinking, irrigation, and replenishing aquifers.

Since many natural water sources around the world have been depleted or even completely exhausted, rainwater harvesting is becoming increasingly important to the well-being of some populations. Rainwater can be collected from roofs or from areas at ground level. Rainwater collected from roofs is often fit for drinking without any processing, but collected rainwater usually must undergo some processing before it is suitable for human or animal consumption. However, harvested rainwater is often suitable for irrigation, flushing toilets, and laundering clothes without processing. Harvested rainwater can serve to supplement available water sources; in some areas, collected rainwater is the only readily available water source. For example, small islands with high annual rainfall may be completely sustained by rainwater harvesting. In urban areas, rainwater harvesting can supplement city water supplies and reduce the likelihood of flooding.

Methods

Rainwater harvested from roofs can be channeled into storage tanks through gutters and pipes. The gutters must have an incline so that water does not stand, and must be strong enough to handle peak flow. Pollutants that may be found in rooftop water include pesticides, dust particles, animal and bird feces, minerals, and dissolved gases such as carbon dioxide, sulfur dioxide, and nitrous oxide. The level of pollutants is commonly highest in water from the first rainfall after a dry spell; such water must therefore be discarded or put through some form of decontaminating treatment, such as boiling or chemical treatment.

Ground collection of rainwater involves a number of techniques that can improve the runoff capacity of a land area: clearing or altering vegetation, increasing the land slope, and decreasing soil permeability (penetration) through compaction or the application of chemicals (or both). Ground collection methodologies provide opportunities to collect water from larger surface areas than rooftop collection allows. The placement of a subsurface dike can obstruct the natural flow of groundwater through an aquifer (underground water channel), raising the groundwater level as well as increasing the amount of water stored in the aquifer (this process is known as groundwater recharge).

Harvested water is commonly collected in tanks; these tanks must be covered to prevent evaporative loss, algae growth, and mosquito breeding. The damming of small creeks and streams, including those that contain water only during heavy rains, is an inexpensive method of storing water during dry spells. Sometimes, however, large portions of water stored in this way are lost through infiltration into the ground. Usually, this water is unfit for human consumption; thus, it is often solely used for irrigation.

Worldwide Practices and Regulations

Rooftop rainfall harvesting is widespread in China and Brazil, where the water is consumed by humans and animals; it is also used for crop irrigation and replenishment of groundwater stores. In India, where urbanization is growing at a rapid rate, rooftop harvesting is also widespread. In India, rooftop harvesting is an essential component of the water supply. In island nations, such as Bermuda and the US Virgin Islands, rainwater harvesting is commonly practiced. Likewise, in rural portions of New Zealand, the only source of water for all household activities is the rooftop collection of rainwater. In the United Kingdom, it is common practice to collect rainwater for irrigating gardens.

In some parts of the world, no statutes are in place to regulate rainwater harvesting or the allowable uses of the water harvested. In the United States, regulations vary among state and local governments, though rainwater harvesting is legal in all fifty states. California, for example, had no state laws related to rainwater harvesting until 2012, but some local governments in the state created regulations. San Francisco, for instance, allows the use of rainwater for nonpotable purposes such as toilet flushing and irrigation. In 2012, the California state legislature passed the Rainwater Capture Act. It authorized expanded rainwater harvesting in the state for reasons apart from human consumption. California’s neighbor to the north, Oregon, which receives high levels of rainfall annually, has established statutes regulating rainwater harvesting, and the city of Portland specifically permits the use of rainwater for human consumption by request. Likewise, harvested rainwater may be used for potable purposes in Texas and Ohio. For a long time, Colorado had laws prohibiting rainwater harvesting; the argument for these statutes was that harvesting could infringe on the water rights of those living downstream. The laws were changed after a 2007 study found that in an average year, almost all the precipitation that fell in the southern suburbs of Colorado never reached a stream; it either evaporated or was absorbed by plants. Two years later, a pair of state laws relaxed requirements for the use of harvested rainwater. In addition, several states, including Arizona, Rhode Island, Texas, and Virginia, began offering tax credits to those who install rainwater collection equipment.

In the 2020s, businesses, such as Toyota and Apple, built rainwater harvesting systems. Cities such as Austin, Texas, and Tucson, Arizona, passed incentives for businesses and eventually homeowners to begin using rainwater. However, as of 2024, the main barrier to widespread rainwater harvesting among homeowners was the cost of installing a rainwater system, which was about $3,000 to $4,000.

Bibliography

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Gilette, Barbara. "A Beginner's Guide to Harvesting Rainwater." The Spruce, 3 May 2024, www.thespruce.com/beginner-guide-to-rainwater-harvesting-7090759. Accessed 29 Aug. 2025. 

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National Conference of State Legislatures. "State Rainwater: Graywater Harvesting Laws and Legislation." NCSL.org, September 1, 2013.

Zorn, Justin Talbot. "America Should Harvest a Trillion Gallons of Rainwater." TIME, 6 Sept. 2023, time.com/6311363/harvest-trillion-gallons-of-rainwater/. Accessed 22 July 2024.