INEP: What is rain water harvesting? The source of water to wells, tanks and rivers is rain water. Rain water conservation has been happening from thousands of years and there is nothing new in what we are doing now. As civilizations grow we have more buildings, bigger villages and cities. When such changes take place, the availability of water decreases. There is a need to conserve rain water apart from having water in tanks, rivers and other places. There is a need to conserve water in our own homes. We have a system called roof top rain water harvesting for this purpose. The water collected on the terrace can be filtered, stored and used.