Why continuous rain failed to raise groundwater level in Bengaluru

City got near record rain this year, but water table hasn’t risen

November 09, 2017 01:34 am | Updated 09:11 am IST - Bengaluru

Some time in September last year, drought was tightening its grip in the State. Around the same time this year, large parts of Bengaluru were inundated while storm-water drains turning into gushing rivers.

Deep, under the surface of the city, however, a different narrative is playing out. Surprisingly, despite the record-setting rains this year, the city’s groundwater levels have significantly worsened compared to last year. Experts have attributed this to borewell exploitation, concreting landscape, and erratic intensity of the rains.

An analysis of groundwater levels between October 2016 and October 2017 — which is the latest available data for Bengaluru Urban district — shows that levels in all four taluks have significantly reduced between 2 to 9 metres in the past year, indicates data obtained from the District Groundwater Authority. Even more disconcerting is that despite over 1,200 mm received during monsoons in 2017 or, more than double of what was received in monsoons of 2016, water levels do not seem to have risen appreciably.

Between May and October this year, the 24 observation wells spread across Anekal, Bengaluru East, North and South taluks have gone up between 3% to 6%. However, in 2016, the rise was more than 27% in Bengaluru North and East, while it dipped in Anekal taluk.

Unabated exploitation

On the surface, concrete and asphalt guide water to concrete channels of storm-water drains and eventually outside the city. Under the surface, the metal proboscis of borewells tap subterranean water sources relentless. Water, it seems, is not welcome in the city.

After a season of record rains, groundwater levels have not been encouraging, and experts say this is owing to continued exploitation from borewells, run-off because of concrete roads, and erratic, intense rainfall.

Increasing exploitation by over four lakh borewells, coupled with lack of recharge in an increasingly concretised city, may perhaps have seen almost all the water flow away, instead of percolating. “In north Karnataka and other rural areas, water levels have gone up because of the rains. There, people stop using borewells during the monsoons, and there is time and surface area for water to percolate and recharge. Where is the recharging space in Bengaluru? Even during rains, borewells continue to operate and pump water here,” said K.S. Srinivas Reddy, director, Karnataka State Natural Disaster Management Authority.

Officials from the Ground Water Authority, which has tabulated data using 24 observational wells across Bengaluru Urban, agree. “Urbanisation surrounds our observational wells, which were once secluded. Now, there is pressure on the wells from neighbouring borewells which continue to draw water despite rains. Because of this, water levels will not rise as much as we expect,” an official said.

Since 2005, levels have steadily plummeted. In the rapidly urbanising Anekal, for instance, the level in some wells has fallen from 17 m in 2005 to over 50 m now. In Bengaluru North, it is 40 m, down from 20 m in 2005.

Erratic rains

Hydrologically speaking, a year of good rains will do little to ease groundwater levels that have pummelled after four years of drought, said K.R. Sooryanarayana, technical head of the Central Groundwater Board.

This is backed up by statistics of groundwater levels just before monsoon. In Bengaluru East, the levels were a staggering 77% lower in May 2017 than in May 2016. In Anekal, they were nearly a third lower. “While these rains may have recharged some places owing to terrain properties, we will need another year of good rains to see groundwater levels actually rise across the city,” he said.

Moreover, this year saw nearly 1,200 mm of rain in just two months — between August 15 and October 15. “If there is slow and steady rain, water percolates gradually, but with intense rains, it will rush out as flash run-off,” said Mr. Sooryanarayana.

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