What is rain water harvesting ?

It is the activity of direct collection of rain water.
Rain water can be stored for direct use or can be recharged into the ground water aquifer.

Rain Water Harvesting - process of collecting, conveying & storing water from rainfall in an area – for beneficial use.

Storage – in tanks, reservoirs, underground storage- groundwater

Rainwater harvesting - yield copious amounts of water. For an average rainfall of 1,000mm, approximately four million litres of rainwater can be collected in a year in an acre of land (4,047 m2), post-evaporation.
As Rainwater harvesting- neither energy-intensive nor labour-intensive
It can be a cost-effective alternative to other water-accruing methods. With the water table falling rapidly, & concrete surfaces and landfill dumps taking the place of water bodies, Rainwater harvesting is the most reliable solution for augmenting groundwater level to attain self-sufficiency.