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Wednesday 19 April 2000

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Ahmedabad
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Roof-water harvesting may be answer to problems

While Gujarat is facing a situation of severe water scarcity of an unprecedented magnitude, the news that the state government plans to issue a general resolution making roof-water harvesting structure a must for all the buildings, comes as a great relief.

Though such structures are a necessity under building by-laws in Chennai, Gujarat had been dormant on this front despite representations by non-governmental organisations.

While the water resources ministry was in favour of a Bill to be passed by the Assembly, the urban development ministry felt that a GR should suffice. It remains to be seen how it is implemented.

Earlier last year, the Central Ground Water Board had launched a campaign to register all the borewell rigs functioning in Gujarat, with a view to curbing the unrestricted sinking of borewells. But such rigs are proliferating despite the prohibitive costs involved in sinking borewells at great depths.

A leading NGO, Viksat, which works extensively for water conservation, has shown eagerness to work towards making roof-water harvesting systems a reality in every household, at least, in the urban areas.

Its director Srinivas Mudrakarth believes that, "In the city areas, there is a dire need to promote groundwater recharging by collecting water from roof tops during monsoon. In Ahmedabad, for instance, most of the roofs have white tiles laid to reduce radiation. It is also observed that complexes and societies that have come up in the past three years have made this provision. Coincidentally, these areas having multi-storey complexes have a high water use rates, leading to increased groundwater extraction. Thus, the need and scope of groundwater recharging is maximum in such areas."

He also estimated that a typical complex or society has a roof area of 400 square yards. Considering an average rainfall of 765 mm (1961-95), it has been observed that 40 per cent of the season's rainfall occurs in 3-4 days' (1991-1996) period. In other words, 60 per cent of the rainfall occurs in around 35 rainy days. Therefore, under ideal conditions, there would be scope for harvesting around 400 mm of rainfall, leaving aside intense, prolonged spells where infiltration is reduced.

Mudrakartha and his team in Viksat have calculated that from a roof area of 400 sq yards (335 sq metres), the potential water collections can be 1.34 lakh litres (potential area available from each roof x 400 mm = 134 cubic metres = 1,34,000 litres). If these figures are multiplied with the thousands of such units that exist in the city, a considerably large quantity of water can be redirected into earth, which can go a long way in, at least, arresting the depletion of groundwater table.

In order to make this possible what is needed is an infiltration pit, which involves two main costs-infiltration pit and filter material. The cost of the recharge pit is put at around Rs 10,000, assuming that the building has provision for collecting and draining roof water to the recharge pit.

However, there are several factors that need to be considered while constructing infiltration pits. These include location of the borewells or wells, soak pits, proximity to and depth of building foundation and space constraints.

GREEN ALERT

Riots for water are no more a figment of imagination, cities such as Rajkot and Bhavnagar have already experienced them at the onset of this summer.



The Economic Times


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