1. Controlling the runoff of rain water and make it to perculate to increase ground water table for future purpose.

2. It involves collection and storage of rainwater at surface or in sub-surface aquifier, before it is lost as surface runoff. It not only increases water availability but also checks the declining water table.

Objectives :

The following are the main objectives of rain water harvesting.

• It checks runoff water & avoid flooding. 
• It replenishes the ground water table and enables the wells. 
• It helps to overcome the adequancy of surface water to meet demand of water. 
• If the ground water is blackish, it will reduce the salinity. 
• It also reduces ground water contamination and improve water quality. 
• It helps to increase agricultural product and improves ecology of an area.

Methods of Rain Water Harvesting :

1. In-situ method The method of percolating the rain at the point of its fall itself is called In-situ rainwater harvesting. 

For example –

• Infilteration pits : The rain water can be collected and stored in pits, dug in farm lands and used for irrigation.
• Contour bunds and trenches : The Construction of bunds, trenches or check dams. 
• Roof top water harvesting : The Rain water that falls on the roofs of buildings or in courtyards is collected and stored in underground. Sumps, or wells or in drums and used for domestic purpose such as cleaning, washing etc.

2. Ex-situ method : This method of rainwater harvesting involves storing of running water. It is collected in check dams, huge percolation tanks etc. It is an expensive method.