Article Courtesy : Times of India

Author : Subodh Varma

How much is 69 billion litres of water? By a rough calculation it is equivalent to nearly 9 million water tankers — the same ones that can be seen trundling about supplying precious water to Delhiites in a swathe of residential colonies.

But, 69 billion litres of water is the amount of water that rains on Delhi's rooftops every year. The calculation is simple — take Delhi's average rainfall at about 490 millimetres, and multiply it with the total rooftop area of about 140 square kilometres as estimated by some experts.

 In a city where the search for water, especially this blistering summer, has become an almost life-anddeath struggle for thousands of families, this bounty from the skies almost seems unbelievable. But it is as true and real as the dry taps in Delhi today.

Of course it is not as simple as that. All water cannot be collected from rooftops. Some will be absorbed by the surface, some will evaporate in the heat, some will get contaminated or spill away. But the numbers do give an idea of the immense potential of rainwater harvesting in Delhi.

And this is just harvesting from the rooftops. It is only 10% of the total rain that falls on Delhi in an average monsoon. The total volume of water that comes down as rain is a mind-boggling figure, about 690 billion litres. Even a 25% usage of this would go a long way in recharging the fast depleting groundwater.

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