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

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

Uses

Rooftop rainwater harvesting helps collect water for drinking, household use, animals, small farming, and to help refill underground water sources. Kenya has already used rainwater for toilets, laundry, and farming. Since Kenya passed the 2016 Water Act, the country has focused on managing its farming industry better. In some parts of Australia, rainwater is used for cooking and drinking. Research by Stout et al. in India showed that rainwater harvesting is most helpful for small farms, which can earn income from selling crops, and for helping underground water levels grow.

In cities, using rainwater reduces the risk of water running off streets and causing floods. Combining green rooftops with rainwater collection systems has been shown to lower building temperatures by more than 1.3 degrees Celsius. Using rainwater with urban farming could help achieve goals like cleaner cities, better health, and more food and water security. The technology exists, but it needs to be adapted to use water more efficiently in cities.

Studies in five Caribbean countries found that collecting and storing rainwater helps protect crops from being lost due to dry soil or lack of water. It also reduces risks from heavy rains, such as flooding and soil erosion. Farmers on hillsides could benefit the most because they can collect rainwater and reduce soil damage.

Many countries, especially those with dry climates, use rainwater harvesting as a low-cost way to get clean water. To help crops grow in dry areas, ridges of soil are built to trap rainwater and stop it from flowing down hills. Even during times with little rain, enough water is collected to water plants. Water can be gathered from rooftops, and large storage systems like dams and ponds can hold rainwater for use on days with no rain.

Frankfurt Airport has the largest rainwater harvesting system in Germany, saving about 1 million cubic meters of water each year. The system cost 1.5 million German marks, or about $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 underground tanks with a total capacity of 100 cubic meters. It is mainly used for flushing 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. This system reduced the park’s need for drinking water by about 73%. However, the park found that recycling wastewater was a more cost-effective way to improve sustainability than using rainwater harvesting.

Technologies

Traditionally, stormwater management using detention basins had one purpose: to hold water during heavy rain. However, using smart control systems allows these basins to also collect rainwater for later use without reducing their ability to store water during storms. This method was used at the EPA headquarters to remove stored water before storms, which helps reduce the amount of water flowing into sewers during heavy rain while keeping water available for future use. This approach improves the quality of water released and reduces the amount of water that flows into sewers during overflow events.

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

Rainwater harvesting systems can be simple or complex. Simple systems require little skill to install, while more advanced systems need careful setup. A basic rainwater system works like a plumbing setup, where water from a building’s roof is directed through pipes into an underground tank. Common parts of these systems include filters, gutters, storage tanks, and, if needed, pumps and water treatment tools like UV lights or chlorine devices.

These systems should be designed to meet water needs during dry seasons. The area that collects rainwater, such as a building’s roof, must be large enough to provide 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 surface water, or using pumps to move water that has soaked into the ground or been stored in reservoirs into tanks.

Clean water sources near populated areas are becoming less available and more expensive. Rainwater is a valuable natural resource, like solar and wind energy. Large areas are covered with solar panels worldwide. These panels can also collect rainwater, which can be turned into clean drinking water through simple filtering and disinfecting processes because rainwater has very little salt. Using rainwater to make products like bottled water can help solar power plants earn extra income, even in areas with heavy rain or cloudy weather. In India, using rainwater to refill wells has been found to be an effective way to increase groundwater levels.

The Groasis Waterboxx is an example of small-scale 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 provides a separate water supply during water shortages. In developed countries, it is often used as an extra source of water. It helps during droughts, reduces 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. Surface water is easy to use for many purposes, reducing the need for underground water. It improves soil quality by reducing saltiness. It does not pollute the environment and is eco-friendly. It is affordable and cost-effective. It also provides clean drinking water because rainwater has little salt. In cities, rainwater harvesting helps both water supply and waste systems by reducing the need for clean water, less stormwater in sewers, and less pollution in freshwater.

Many studies have focused on evaluating the environmental effects and costs of rainwater harvesting systems.

Rainwater harvesting gives a separate water supply during shortages. In areas where clean water is hard to find or expensive, it is a key source of clean water. In developed countries, rainwater is often used as extra water, but it can also lower household water costs or usage. Rainwater can be safe to drink if treated, like boiling to kill germs. Adding a first flush diverter helps remove contaminants.

During droughts, stored rainwater from earlier months can be used. In places with unpredictable rain, harvesting systems help collect water when it rains. Many dry regions use rainwater harvesting as a cheap and reliable water source. In arid areas, ridges are built to trap rainwater for crops. Water can be collected from rooftops and stored in tanks.

Rainwater harvesting also reduces the need for well water, helping keep groundwater levels from dropping.

Life-cycle assessment is a method to study environmental impacts from a system’s creation to its end. Devkota et al. developed this method for rainwater harvesting, showing that building design and purpose (like homes or schools) affect the system’s environmental impact.

To improve rainwater harvesting systems, 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 for tasks like toilet flushing. Connecting buildings to combined sewer systems saves more water and reduces pollution compared to separate systems.

Although rainwater harvesting can help poor communities, the cost depends on the technology used. Governments and organizations can help by providing materials and teaching people how to build and maintain systems.

Studies show rainwater harvesting is a good solution for water shortages because it is affordable and eco-friendly. Building large water systems like dams can harm ecosystems and cost more, especially in poor areas. Rainwater harvesting systems are better for communities because they are sustainable, help prevent floods, and reduce runoff. Systems that local people can build and maintain are more likely to be used long-term.

Using in-situ technologies lowers costs for rainwater harvesting. These technologies are good for rural areas because they need fewer materials. They provide reliable water for farming. Above-ground tanks can store water for homes, but they may be too expensive for poor people.

Limitations

Rainwater harvesting is a common way to collect and store rainwater in areas that experience dry conditions. Scientists have studied and created different methods to choose good places for harvesting rainwater. Some research has identified areas where dams could be built and used a computer program called ArcMap 10.4.1 to create a model. This model used several factors, such as the slope of the land, how much water flows over the ground, the type of land cover, the size of nearby streams, the quality of the soil, and water movement patterns, to decide if a location was suitable for harvesting rainwater.

Water collected from rainwater harvesting systems may be limited during dry periods in places like the Middle East, which have little rainfall. Rainwater harvesting is helpful in developing areas because it provides water for farming and daily use. However, the collected water must be filtered properly 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 energy to disinfect rainwater by boiling it in special solar cookers before drinking it. These methods, called "appropriate technology," offer low-cost ways to clean stored rainwater for drinking.

Although rainwater is naturally clean and often better than water from rivers or groundwater, the way it is collected and stored can make it polluted and unsafe to drink. Rainwater collected from rooftops may contain waste from animals, birds, or humans, plants like moss and lichens, dust from the air, chemicals from pollution, pesticides, and minerals from the ocean. In Europe, high levels of pesticides have been found in rainwater, especially after a dry period. These harmful substances can be reduced by directing the first rainwater away from storage. Better water quality can also be achieved by drawing water from the top of storage tanks instead of the bottom and using a series of tanks, taking water from the last one. Pre-filtering, which removes large particles before water enters the tank, is a common practice to keep the system clean.

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

Ideally, 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 uses, 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 disease-carrying mosquitoes. 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 chemical treatments can also help control mosquito populations.

Country examples

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

Several groups in Canada help teach people about rainwater harvesting, provide tools, and install systems. These 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 helping them learn about the industry.

In the early 2000s, India began investing in rainwater harvesting to solve water shortages. In 2001, Tamil Nadu became the first Indian state to require all buildings to use rainwater harvesting to prevent lowering groundwater levels. In Rajasthan, people in the Thar Desert have used rainwater harvesting for a long time. Efforts to use rainwater harvesting more widely in India have revived old systems, such as the chauka system in Jaipur. 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 metres to have rainwater systems. This rule was expanded in 2007 to include buildings larger than 300 square metres. The goal was to ensure buildings had enough water during dry seasons. Systems used a collection method, a way to clean water before use, and filtering. By 2021, 3,000 new or rebuilt buildings in Mumbai had rainwater systems. However, some people report that stored water becomes salty or briny. Experts and residents say local officials have not focused enough on making the system work, and it is unclear how effective the rules are.

Rainwater harvesting has been used in rural India for a long time, even before it became common in cities.

In the United Kingdom, rainwater harvesting is becoming more important. It is a traditional and growing way to collect water for use at home. Water is used for tasks like watering gardens, flushing toilets, and washing clothes. In places like supermarkets, large systems can collect between 1,000 and 7,500 litres of water for toilet use. In the South East of England, people have less water available compared to some countries in the Mediterranean.

Rainwater is usually collected from rooftops and filtered using methods like filters attached to downspouts, baskets, or self-cleaning filters in underground tanks. Homes using rainwater systems can save up to 50% of their water, though saving 20% to 30% is more common. In the UK, the cost of delivering and processing water is about £2 per cubic metre. Using less water from the main supply also reduces costs for sewage and waste disposal.

In the United States, until 2009, laws in Colorado limited rainwater harvesting because it was considered taking water that others had rights to. Now, homeowners who meet certain conditions can get permits to install systems. Large projects may also be allowed. A study in 2007 showed that most rainwater in Douglas County, near Denver, was not reaching streams but was used by plants or evaporated. In Santa Fe, New Mexico, new homes must have rainwater systems. Texas offers tax breaks for buying rainwater equipment. Both Texas and Ohio allow using rainwater for drinking. Oklahoma passed a law in 2012 to support rainwater and graywater projects.

In Uganda, rainwater harvesting has been used for years to help families and communities get water. Keeping systems working is a challenge, and some installations have failed because of poor care. People’s knowledge about rainwater harvesting and how to get help varies.

In Thailand, about 40% of rural people use rainwater harvesting. The government promoted it in the 1980s. When funding stopped in the 1990s, private companies provided millions of storage tanks, many still in use. This is one of the largest examples of people using their own water systems globally.

In Bermuda, all new buildings must have rainwater systems to meet residents’ needs.

In New Zealand, areas with heavy rain use rainwater harvesting in rural homes. Water is collected from rooftops into 1,000-litre tanks, and local governments often support this.

In Sri Lanka, rainwater harvesting has been used for farming and drinking water in rural homes. A law passed in 2007 encourages rainwater use. An organization called the Lanka Rainwater Harvesting Forum leads efforts. Old systems like the tank cascade are used for irrigation.

In Bolivia, rainwater systems have been built in schools to help with water shortages and support farming. In Cochabamba, groups have used rooftop systems to grow food for students.

History

Cisterns, which are structures used to store rainwater, were first built during the Neolithic Age. These early cisterns were made of waterproof lime plaster and 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 an important part of water management systems used for farming in dry areas.

Many ancient cisterns have been found in parts of modern-day Jerusalem and Israel/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 also used large cisterns during the Minoan period, from 2600 BC to 1100 BC. Four large cisterns have been found in locations such as Myrtos-Pyrgos, Archanes, and Zakroeach. The cistern at Myrtos-Pyrgos was built around 1700 BC and had a capacity of more than 80 cubic meters (2,800 cubic feet).

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 the Cuddalore district of Tamil Nadu between 1011 and 1037 AD. This tank is 16 kilometers long and can store 41,500,000 cubic meters (1,465,000,000 cubic feet) of water for drinking and farming.

Rainwater harvesting was also widely used in the Roman Empire. While Roman aqueducts are famous, cisterns were also common. In Pompeii, 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 salty and not safe to drink. To solve this, Venice’s early residents built man-made wells with insulated floors. Water passed through layers of sand and was collected at the bottom of the wells. Later, Venice began importing 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 building designs to help reduce water runoff and provide more water. Cities like Melbourne, Singapore, and Hyderabad have created rules that encourage rainwater collection in homes and businesses.