The rainwater harvested depends upon the catchment area, the rainfall pattern in the area and the drianage/ collection system used. 

To understand the potential for rainwater harvesting, lets take the example of a house in DELHI with a terrace area of 100 sqm. Taking the average annual rainfall in Delhi as 600 mm, and ASSUMING 70% harvesting efficiency (as some rainwater will be lost due to evaporation, collection etc.),

we can calculate the AMOUNT of water harvested thus:

Volume of water harvested = 100 X 0.6 x 0.7=42,000litres

This volume is more than twice the annual drinking water requirement of a 5-member family, whose average daily drinking water requirement is 10 lpcd.

The rainwater harvested depends upon the catchment area, the rainfall pattern in the area and the drianage/ collection system used. 

To understand the potential for rainwater harvesting, lets take the example of a house in Delhi with a terrace area of 100 sqm. Taking the average annual rainfall in Delhi as 600 mm, and assuming 70% harvesting efficiency (as some rainwater will be lost due to evaporation, collection etc.),

we can calculate the amount of water harvested thus:

Volume of water harvested = 100 x 0.6 x 0.7=42,000litres

This volume is more than twice the annual drinking water requirement of a 5-member family, whose average daily drinking water requirement is 10 lpcd.