TOI: Harvested rainwater can be used for artificial recharge of groundwater: June 10, 2008 | Rainwater Harvesting

IT’S TIME WE DID A RAINCHECK .............Atul Thakur | TIMES INSIGHT GROUP

There is a standard water story in Delhi every summer—the government shouting itsaelf hoarse about how Haryana is denying the capital its share of drinking water followed by a statement that talks are on at the highest level to secure Delhi's share from the Yamuna water.

While Delhi's incessant water crisis seems to have no immediate solution, the Maharashtra government is planning to construct the state's tallest dam. TOI had reported that civic officials are consulting Chinese firms for the Vaitarna dam project, which is supposed to relieve the city from its relentless water crisis.

Probably the civic officials don't acknowledge the fact that China's Gansu province also has one of the world's largest rainwater harvesting projects.

A little effort might have solved water problems for Mumbaikars and the recent series of rains in Delhi would have been more pleasant if apart from giving relief from the scorching heat they had also contributed to relieving the city from its unremitting water crisis.

Delhi gets on average 79.73 cm of rainfall annually. With a total area of 1,483 square kilometre, that means it receives roughly 1.2 million mgl (mega litres) of rainwater in a year. Obviously, not all of this rainwater can be harvested. But the rain that falls on rooftops certainly can be.

How much of a difference would it make? Let's assume Delhi harvests even 20% of its annual rainwater, not an impossible task by any means. That in itself would be enough to quench the thirst of the dehydrated capital for 57 days. Similarly, Mumbai's average annual rainfall of 240.12 cm and area of 437.71 sq km works out to 1.02 million mgl of water. Again, 20% of that could suffice to meet the city's water demand for more than 53 days a year.

Compared to the average daily demand of about 4,000 mgl in Mumbai and 4,142.9 mgl in Delhi, the supply is only 2,816.1 and 3,400 mgl water respectively.

If 20% of the annual rainwater were harvested in both cities, that would amount to 647.9 mgl for Delhi and 575.9 mgl for Mumbai, enough to close the demand-supply gap for Mumbai and to reduce it significantly for Delhi.

Of course, rooftop-harvested water cannot be used directly for drinking purposes, but it can be used for all other purposes. It can also be used to replenish the groundwater. Because of the inadequate supply of municipal water over decades, groundwater is being relentlessly tapped and on an average it’s declining by more than 20 cm per year for both Delhi and Mumbai.

As concrete or roads cover most parts of the city, most of the rainwater goes quite literally down the drain. There is little absorption and almost negligible replenishment of groundwater. The harvested rainwater can be used for artificial recharge of groundwater.

Times View

Most of metropolitan India has for a long time now been facing a major water shortage. It is, therefore, quite shocking that a law making rainwater harvesting mandatory in areas where the water table is lower than 8 metres below the surface has been so seldom enforced. This despite the law having been in existence for seven years. A law that remains on paper is of no use to anybody. Civic authorities clearly need to get their act together immediately and make sure the law is implemented. But as citizens, we must not wait till the stick is wielded before we do something that is so obviously in our own interest. Civil society too must play a role in ensuring that rainwater harvesting acquires the dimensions of a movement. Like helmets or seatbelts, this is something we need to do for our own good, not because it is mandatory.

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