Rain water harvesting is a technique of collection and storage of rainwater into natural reservoirs or tanks, or the infiltration of surface water into subsurface aquifers (before it is lost as surface runoff). One method of rainwater harvesting is rooftop harvesting. With rooftop harvesting, most any surface — tiles, metal sheets, plastics, but not grass or palm leaf — can be used to intercept the flow of rainwater and provide a household with high-quality drinking water and year-round storage. Other uses include water for gardens, livestock, and irrigation, etc.
The reasons for using rainwater harvesting systems answer three questions:
What: Rainwater harvesting will improve water supply, food production, and ultimately food security.
Who: Water insecure households or individuals in rural areas will benefit the most from rainwater harvesting systems.
How: Since rainwater harvesting leads to water supply which leads to food security, this will greatly contribute to income generation.
Example rainwater harvesting in India
Tamil nadu is the first Indian state to make rainwater harvesting mandatory. On 30 May 2014, the state government announced that it will set up 50,000 rainwater harvesting structures at various parts of the capital city of Chennai. 
Around 4,000 temples in Tamil Nadu state traditionally had water tanks that were used for various rituals. The tanks also served as natural aquifers and helped recharge groundwater. Over the years, however, many of these tanks have gone out of use. Overflowing mounds of silt and garbage have replaced the water in them.
Now, following sustained campaigns by voluntary organisations and departments in charge of water distribution and use, authorities in Chennai have decided to restore around 40 major temple tanks in the city. The aim is to convert the tanks into catchments for rainwater harvesting. 
- VIDEO: Rainsong video. Example of a rainwater harvesting system that has multiple uses.
- Traditional rainwater harvesting systems to the rescue! Rainwater harvesting systems are a success in India.
- COMPONENTS OF A RAINWATER HARVESTING SYSTEM
- Smart 3R Solutions
- Rainwater harvesting sources. A list of 45 NGOs, government organizations, and some private companies that focus on rainwater harvesting programs, policies, and supplies.
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- bob® bag: bob® gives customers access to clean water.
- Rainwater pillow: The Original Rainwater Pillow is an innovative rainwater & storm water harvesting system designed to be stored in horizontal wasted space.
- Rainsoucer: Made in the U.S.A., the patent pending RainSaucer™ is a rain barrel accessory that allows you to harvest rainwater...
- Chennai - A Success Story: Rainwater Harvesting
The following projects utilize rainwater harvesting techniques, as seen on the Akvo Really Simple Reporting (RSR) pages.
- WASH program in Rural Bangladesh
- Scale up of Sustainable Water Access
- Etude technique d’avant-projet
- Rainwater harvesting for Nicolas School
- Safe water supply for Fayaco, Senegal
- Rainwater Harvesting Capacity Center
- Support on WASH - in Miyo woreda
- Upscaling CLTS for Healthy Communities
- Partnership in WASH services delivery
- Northern Region WASH Programme
- Raising awareness on rainwater harvesting
- Wetland Management & Water Harvesting
- Rain Water Harvesting in Nepal
- Rainwater Harvesting in Kenya
- Rainwater harvesting in Guinee Bissau 2
Community Exchange - Rainwater Harvesting
Here are conversations about rainwater harvesting (RWH) including best practices, troubleshooting, and advice about tanks and systems. Sourced from the Dgroups website, which is a free member-only forum, where people can ask a pool of over 3,000 members about rainwater harvesting. You can always become a member on their site for greater involvement, if you have questions you need to know for your RWH project. Go to the Community Exchange.