Delhi, June 14, 2012: 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.
With a population of over 138 million at last count Delhi is said to have an average daily demand of about 1100 million gallons or about 5 billion litres. The supply gap this summer is about 465 million gallons or over 2 billion litres per day.
Had all the pronouncements and declarations by Delhi government on making rainwater harvesting mandatory in commercial and residential constructions been implemented, at least some edge would have been taken off this present water scarcity.
Those few who have installed a rainwater harvesting system in their homes vouch for its effectiveness. One such person is Ruchi Singhal, an interior decorator, who got a system installed in 2004 with the help of the Centre for Science and Environment (CSE), a Delhi-based environmental advocacy institution. "I have a 3800 litres capacity tank under the driveway connected to the roof through pipes. There is arrangement to take the overflow from this tank to a discharge well which takes the excess water down to the groundwater table," says Singhal. She uses the collected water for nondrinking purposes like washing cars and watering the garden .
Apart from rooftop harvesting, rainwater can very effectively be used for recharging precious groundwater, says Sushmita Sengupta of CSE. The total built-up area of Delhi is around 50% according to Sengupta. This could yield a huge amount of water through drainage area recharge or a micro-watershed treatment. In both of these, run-off water (that is, water not absorbed by open ground but runs off and finally ends up in drains and the river or evaporates) is trapped either on the surface through check dams or channeled to well-like structures nearby so that it goes into the groundwater after crude filtering.
All this would ease some of the crisis that occurs every summer in the capital. For a long-term solution, there is a need to quickly finish various projects like the Renuka, Kishau and Lakhwar Vyasi dams and the canals linking them to Delhi. This will require sorting out watersharing issues with surrounding states, as with the existing Yamuna, Beas and Ganga water sharing systems, says Dilip Fouzdar, an expert on water resources management. "The capital's water needs have to be treated with priority and this requires cooperation between various states, under the stewardship of the Union government," he says. (Source: Subodh Varma, TNN, Published in The Times of India)