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Rainfall intensity proves Kerala's bane

Vinson Kurian

SILVER LINING: Clouds hang over the Thiruvananthapuram skyline dotted by trees bereft of much of their foliage right at the start of this year's summer season. The capital city has been receiving summer showers during the past few days, bringing great relief to residents. - S. Gopakumar

Thiruvananthapuram , April 6

INTENSIVE rainfall associated with normal monsoon seasons, which used to be Kerala's preserve over these years, may well have proved to be its undoing, according to Mr K.R. Gopinath, founder and Chairman of the Chennai-based KRG Rainwater Harvesting Foundation.

At more than 20 mm/hr, this pounding makes for a torrent when compared with the trickle of 1 to 2 mm/hr recorded in most other places.

This could only have promoted more runoff and less percolation in the State. People of the older generations dealt with the situation by making the water to impound on the surface by constructing tanks, contour trenches and bunds.

But the introduction of piped water gave a short shrift to the received wisdom on the desirability of retaining precious groundwater for its sustained availability. The results are there for all to see with the State, which boasts an average annual recorded rainfall of 3,000 mm, often being made to burn the midnight oil to get the `water of life' shipped in tankers.

Rainwater harvesting is a tradition-renewed scientific technology that augments ground water quantitatively and qualitatively. The main source of groundwater is rain. It is estimated that 12 million litres of rainwater can be collected in a year in one acre of land in Kerala. The quantity of seepage depends on the soil condition, though. The infiltrated water gradually builds up the groundwater table to a considerable extent.

Given Kerala's current drinking water situation, it has now become necessary to inject rainwater through open wells and bore wells to compensate the overexploitation of groundwater resources.

Rooftops of houses are excellent collection centres for rainwater, which is likely to be contaminated by foreign elements such as dust, leaves and bird droppings. These foreign elements can be prevented from entering the well by flushing the initial water flow through the pipes at the beginning of rain.

The main benefit of injection of rainwater into the well is that it creates ground water ridge or mound around the well. It enhances the quality and quantity of water. After a few rainy seasons, the well shall start giving sustainable yield of quality water.

Air bubbles should not be allowed to go into the aquifer, as consistent air bubble formation will lead to air locking of the aquifers and fractures.

The most effective remedy is to extend the down pipe below the static water level of the well, instead of allowing it to pour water from a level above.

Houses with larger open area can also make use of enormous quantity of surface water generated in the open area for recharging purposes. For this, a percolation pond or a recharge well of suitable size can be constructed at a distance of 1 to 2m from the open well or bore well.

The run off water generated on the open area can be diverted to the percolation pond or recharge well. Most of the pathogens will be eliminated during the course of transfer of water through the process of percolation through overburdened and weathered rocks.

In the case of open wells lined with masonry or bricks, it is desirable that a few "weep holes" are provided in each layer of the lining 3m below ground level.

This arrangement will facilitate entry of water into the open well or bore well, percolating downwards from the percolation pond or recharge well.

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