Rainwater harvesting (RWH) is the process of collecting and storing rainwater instead of letting it flow away. Rainwater is gathered from surfaces like rooftops and directed into tanks, cisterns, deep pits (such as wells or boreholes), underground water storage, or reservoirs where it soaks into the ground to replenish groundwater. This method is different from stormwater harvesting because it typically collects runoff from rooftops and other surfaces for storage and later use. Harvested rainwater can be used for watering plants, feeding animals, irrigation, household needs after proper treatment, and heating homes. It can also be stored for long-term use or help refill underground water sources.

Rainwater harvesting is one of the simplest and oldest ways for homes to collect their own water. It has been used for thousands of years in South Asia and other regions. Ancient civilizations, like the Romans, built large water collection systems, including aqueducts and rooftop channels, which inspired modern gutter systems still used today. These systems can be designed for different sizes, such as individual homes, neighborhoods, or entire communities. They can also provide water for schools, hospitals, and other public buildings.

Uses

Rooftop rainwater harvesting is used to collect water for drinking, household use, watering animals, small farming, and helping groundwater levels increase.

In Kenya, rainwater has been successfully used for toilets, laundry, and farming since the 2016 Water Act focused on managing agriculture. In Australia, harvested rainwater is used for cooking and drinking. Studies in India showed that rainwater harvesting is most helpful for small farming, which helps farmers earn money from selling crops, and for helping groundwater levels increase.

In cities, using rainwater reduces runoff and flooding. Combining green rooftops with rainwater systems can lower building temperatures by more than 1.3 degrees Celsius. Using rainwater with urban farming could help meet goals for cleaner cities, better health, and food and water security. The technology exists, but it needs to be adjusted to use water more efficiently in cities.

Projects in five Caribbean countries showed that storing rainwater helps protect crops from being lost due to dry soil or water shortages. It also reduces risks of flooding and soil erosion during heavy rains. Farmers on hillsides benefit most because they can collect rainwater and reduce soil erosion.

Many countries in dry areas use rainwater harvesting to get clean water cheaply. In dry regions, soil ridges are built to trap rainwater and stop it from running down slopes. Even during dry periods, enough water is collected to grow crops. Water can be gathered from rooftops, and large storage areas like dams and ponds can hold rainwater for use during dry times.

Frankfurt Airport has the largest rainwater system in Germany, saving about 1 million cubic meters of water each year. The system cost 1.5 million DM (US$63,000) in 1993. It collects water from the airport’s new terminal, which covers 26,800 square meters. The water is stored in six tanks below the airport, each holding up to 100 cubic meters. The water is used for toilets, watering plants, and cleaning air conditioning systems.

At The Velodrome in London’s Olympic Park, rainwater harvesting was used to make the facility more sustainable. It reduced the need for drinking water by 73%. However, it was found that recycling wastewater was a better way to use money for sustainability than using rainwater harvesting.

Technologies

Stormwater management using detention basins was traditionally used for one purpose only. However, using smart control systems allows these basins to also collect rainwater without reducing their ability to manage stormwater. This method has been used at the EPA headquarters to move stored water before storms, which helps reduce the amount of water flowing during heavy rain and keeps water available for future use. This approach improves the quality of water released and reduces the amount of water that flows during sewer overflow events.

Check dams are built across streams to help surface water soak into the ground. The amount of water that soaks into the ground near check dams can be greatly increased by loosening the soil, a method similar to how explosives are used in mining. This helps refill underground water sources quickly, making surface water available for use during dry seasons.

Rainwater harvesting systems can vary in complexity. Simple systems can be set up with basic skills, while more advanced systems require specialized equipment. A basic system works like plumbing, connecting water from a building’s roof through pipes to an underground tank. Common parts of these systems include filters, gutters, storage tanks, and, if needed, pumps and devices that clean the water, such as UV lights or chlorine.

These systems should be the right size to meet water needs during dry seasons. The area that collects rainwater, like a building’s roof, must be large enough to gather enough water. The storage tank must also be large enough to hold the collected water. Simple systems use methods like collecting water from rooftops, capturing water from the ground, or using pumps to move water from reservoirs into storage tanks.

Clean water near populated areas is becoming harder to find and more expensive. Rainwater is a valuable renewable resource, similar to solar and wind energy. Large areas worldwide are covered with solar panels, which can also collect rainwater. With simple filters and cleaning methods, rainwater can become clean enough to drink. Using rainwater to make products like bottled water can help solar power plants earn extra income, even in areas with frequent rain or clouds. In India, collecting rainwater into existing wells has been found to help raise groundwater levels effectively.

The Groasis Waterboxx is an example of simple technology used to help grow trees in dry areas. It collects rainwater and dew.

The Global Rainwater Management Program (GRMP), suggested by UNCCD and the Global Water Partnership, focuses on managing rainwater effectively.

Advantages

Rainwater harvesting gives a separate water supply when there are water shortages. In developed countries, it is often used to help with the main water supply. It provides water during droughts, helps reduce flooding in low areas, and lowers the need for wells, which can help keep groundwater levels steady. During dry seasons, it increases water availability by filling dry wells and borewells. Surface water is easy to use for many purposes, which reduces the need for groundwater. It improves soil quality by reducing salt levels. It does not cause pollution and is good for the environment. It is affordable and cost-effective. It also helps provide clean drinking water because rainwater has little salt or other minerals. In cities, rainwater harvesting helps both water supply and wastewater systems by using less clean water, reducing stormwater in sewers, and lowering pollution in freshwater sources.

Many studies have looked at how to measure the environmental effects and costs of using rainwater harvesting systems.

Rainwater harvesting gives a separate water supply during water shortages. In places where clean water is expensive or hard to find, it is an important source of clean water. In developed countries, rainwater is often used as a backup to the main water supply, but it can also lower household water costs or usage. Rainwater can be safe to drink if it is treated first, such as by boiling to kill germs. Adding a first flush diverter to the system is a common way to keep water clean.

During droughts, rainwater saved earlier can be used. If rain is rare or unpredictable, a rainwater harvesting system can help collect water when it rains. In dry areas, many countries use rainwater harvesting as a cheap and reliable water source. To help crops grow in dry areas, ridges are built to trap rainwater and stop it from running downhill. Even with little rain, enough water is collected for farming. Water can be collected from rooftops and stored in large tanks.

Rainwater harvesting also lowers the need for water from wells, helping keep groundwater levels from dropping.

Life-cycle assessment is a way to check the environmental effects of a system from start to finish. Devkota and others created a method for rainwater harvesting and found that building design and purpose (like schools or homes) greatly affect how well the system works environmentally.

To study how rainwater harvesting systems work, a new measure called the demand to supply ratio (D/S) was created. This helps find the best building design and purpose for using rainwater to flush toilets. Systems that use rainwater save both drinking water and stormwater that would go into sewers, reducing pollution more when buildings are connected to combined sewer systems than separate ones.

Even though rainwater harvesting can help poor communities, the cost of setting up systems can be high, depending on the technology used. Governments and aid groups can help by providing materials and teaching people how to build and care for these systems.

Some studies show that rainwater harvesting is a good solution for water shortages and other uses because it is affordable and eco-friendly. Building large water systems, like dams, can harm ecosystems, cause social problems, and are not always useful, especially in poor areas. On the other hand, rainwater harvesting systems are proven to give communities a long-term water source, along with benefits like flood control and managing water runoff. Systems that do not need major construction or regular help from outside the community are better for the environment and more likely to be used by local people for a long time.

Using in-situ technologies, which are built on-site, can lower costs for rainwater harvesting. These technologies are good for rural areas because they need fewer materials. They provide a reliable water source that can help grow more food. Above-ground tanks can store water for homes, but these can be too expensive for poor people.

Limitations

Rainwater harvesting is a common practice in countries that experience drought. Many studies have created guidelines and methods to choose good locations for harvesting rainwater. Some research has identified areas suitable for building dams and developed a model in ArcMap 10.4.1. This model uses factors like land slope, how much water flows over the land, land cover, stream size, soil quality, and water movement to decide if a location is good for harvesting rainwater.

Water collected from rainwater harvesting systems may be limited during dry periods in arid urban areas, such as parts of the Middle East. Rainwater harvesting is helpful in developing areas because it provides water for farming and daily use. However, the collected water must be properly filtered to make it safe for drinking.

Rainwater should be tested to ensure it is safe for use. In Gansu province, for example, people use solar cookers to boil harvested rainwater before drinking it. These methods are called "appropriate technology" and offer low-cost ways to disinfect stored rainwater for drinking.

Although rainwater is naturally clean and often better than water from rivers or groundwater, the process of collecting and storing it can make the water polluted and unsafe to drink. Rainwater collected from rooftops may contain waste from animals, plants, dust, pollution, pesticides, and chemicals from the sea, such as calcium, magnesium, sodium, potassium, chloride, and sulfate. It may also contain gases like carbon dioxide and nitrogen oxides. In Europe, high levels of pesticides have been found in rainwater, especially after dry periods. These contaminants can be reduced by directing the first flow of rainwater to waste. Better water quality can also be achieved by drawing water from the top of storage tanks instead of the bottom and using multiple tanks, drawing water from the last one in the series. Prefiltration is a common step used to keep the system clean and remove large particles before water enters the tank.

A method of collecting rainwater and using solar energy to clean it for drinking in rural homes has been developed by the Nimbkar Agricultural Research Institute.

A water supply system should match the water quality to its intended use. However, in many developed countries, high-quality drinking water is used for all purposes, which wastes resources and harms the environment. Using rainwater that has been filtered for non-drinking purposes, such as flushing toilets, watering plants, or doing laundry, can help create a more sustainable water system.

Rainwater storage tanks can also become homes for mosquitoes that carry disease. To prevent this, steps must be taken to stop female mosquitoes from laying eggs in the tanks. Adding fish that eat mosquito larvae or using chemicals can help control mosquito populations.

Country examples

Rainwater harvesting is a method that many Canadians are using in their daily lives. While exact numbers about how many people use it are not known, rainwater can be used for several purposes, such as reducing stormwater, watering plants, doing laundry, and filling portable toilets. It is also helpful for watering plants because it is low cost. Many Canadians now use rainwater harvesting systems for stormwater reduction, irrigation, laundry, and bathroom plumbing. Rules made by provincial and city governments help manage how rainwater is collected and used. Since the mid-2000s, changes to Canadian laws have increased the use of rainwater harvesting in homes, farms, and businesses. However, some rules are still unclear in certain provinces. Local rules and city codes often control rainwater harvesting.

Several groups and companies in Canada help teach people about rainwater harvesting, provide tools for it, and install systems. These groups include the Canadian Association for Rainwater Management (CANARM), the Canadian Mortgage and Housing Corporation (CMHC), and CleanFlo Water Technologies. CANARM focuses on teaching people about rainwater harvesting and raising awareness about it.

In the early 2000s, India began building rainwater harvesting systems and creating rules to address water shortages. In 2001, the state of Tamil Nadu required all new buildings to have rainwater harvesting systems to protect groundwater. In Rajasthan, people in the Thar Desert have used rainwater harvesting for a long time. Recent efforts in Rajasthan have revived old water systems, such as the chauka system in Jaipur. Large cities like Pune, Mumbai, and Bangalore have rules requiring new buildings to use rainwater harvesting. In 2002, Mumbai required all new buildings larger than 1,000 square meters to have rainwater systems. This rule was expanded in 2007 to include buildings as small as 300 square meters. The goal was to help buildings store enough water for non-rainy seasons. Systems used filters and collected rainwater from rooftops. As of 2021, 3,000 new or rebuilt buildings in Mumbai had rainwater systems. However, some people report that stored water becomes salty or polluted. Experts say that local officials have not done enough to ensure these systems work properly.

Rainwater harvesting has been used in rural India for many years, even though it has become more popular in cities recently.

In the United Kingdom, rainwater harvesting is becoming more important. It is used for tasks like watering gardens, flushing toilets, and washing clothes. In large buildings like supermarkets, rainwater is stored in large tanks and used for toilet flushing. In parts of southern England, people have less water available than in some Mediterranean countries.

Rainwater is usually collected from rooftops and filtered using systems like filters attached to downspouts, baskets, or underground tanks. Homes with rainwater systems can save up to 50% of their water use, though 20% to 30% is more common. In the UK, water and sewage costs are about £2 per cubic meter. Reducing water use also lowers sewage costs because water companies assume all water used is sent to sewers.

In the United States, until 2009, Colorado had rules that limited rainwater harvesting, treating it as stealing water. Now, homeowners can get permits to install systems if they meet certain requirements. In Santa Fe, New Mexico, rainwater harvesting is required for new homes. Texas offers tax breaks for buying rainwater equipment. Both Texas and Ohio allow rainwater use for drinking water. Oklahoma passed a law in 2012 to support rainwater and graywater projects.

• In Uganda, rainwater harvesting has been used for many years to help communities get water. However, poor maintenance often causes systems to fail. People’s knowledge about rainwater harvesting and how to get help to use it varies.
• In Thailand, about 40% of rural people use rainwater harvesting. The government promoted it in the 1980s, and private companies later provided millions of storage tanks.
• In Bermuda, all new buildings must have rainwater systems to meet residents’ needs.
• In New Zealand, rainwater is commonly collected in rural areas using rooftop systems and stored in large tanks.
• In Sri Lanka, rainwater harvesting is used for farming and drinking water. A law passed in 2007 supports this practice, and an ancient system called the tank cascade is used.
• In Bolivia, schools use rainwater harvesting to help with water shortages and support farming for students’ meals.

History

The use of cisterns to store rainwater began during the Neolithic Age, when waterproof lime plaster cisterns were built into the floors of homes in villages in the Levant, a large area in Southwest Asia. This region is located south of the Taurus Mountains and is bordered by the Mediterranean Sea to the west, the Arabian Desert to the south, and Mesopotamia to the east. By around 4000 BC, cisterns became important parts of water management systems used for farming in dry areas.

Many ancient cisterns have been found in parts of Jerusalem and across what is now Israel and Palestine. At a site believed to be the biblical city of Ai (Khirbet et-Tell), a large cistern was discovered that dates back to about 2500 BC. This cistern could hold nearly 1,700 cubic meters (60,000 cubic feet) of water. It was carved into solid rock, lined with large stones, and sealed with clay to prevent leaks.

The Greek island of Crete used large cisterns for rainwater collection during the Minoan period, which lasted from 2600 BC to 1100 BC. Four large cisterns have been found at sites such as Myrtos-Pyrgos, Archanes, and Zakroeach. The cistern at Myrtos-Pyrgos had a capacity of more than 80 cubic meters (2,800 cubic feet) and dates back to 1700 BC.

Around 300 BC, farming communities in Balochistan (now in Pakistan, Afghanistan, and Iran) and Kutch, India, used rainwater harvesting for agriculture and other purposes. The Chola kings in India also collected rainwater. For example, water from the Brihadeeswarar temple in Thanjavur, India, was stored in the Shivaganga tank. Later, during the Chola period, the Vīrānam tank was built in Tamil Nadu between 1011 and 1037 AD. This tank was 16 kilometers long and could hold 41,500,000 cubic meters (1,465,000,000 cubic feet) of water for drinking and farming.

Rainwater harvesting was also used in the Roman Empire. While Roman aqueducts are well known, cisterns were also widely used, and their construction grew as the Empire expanded. In Pompeii, for example, people stored water on rooftops before aqueducts were built in the 1st century BC. This practice continued during the Byzantine Empire, such as in the Basilica Cistern in Istanbul.

For many years, the city of Venice relied on rainwater harvesting. The lagoon around Venice contains brackish water, which is not safe to drink. Venice’s early residents built a system of man-made insulated wells to collect rainwater. Water passed through specially designed stone floors and was filtered by sand before being collected at the bottom of the wells. Later, Venice began bringing water by boat from rivers on the mainland, but the wells remained important, especially during wars when enemies could block access to mainland water sources.

Urban implementation

In cities, rainwater harvesting systems are built into buildings to help reduce water that flows away and provide more water for use. Cities such as Melbourne, Singapore, and Hyderabad have made rules that support collecting rainwater in homes and businesses.