India File

When Nature hits back

T V Jayan | Updated on October 08, 2018 Published on October 08, 2018

A significant drop in groundwater table soon after the severe floods that ravaged several parts of Kerala in August has surprised both lay persons and experts alike.

Most open wells, particularly in flood-affected river basins, saw out-of-the-ordinary depletion in groundwater levels in the immediate aftermath of the floods, the worst in close to a century.

The sudden plummeting in groundwater level was rather perplexing, considering that the State had received 41 per cent excess rainfall between June 1 and August 22. The heavy downpour and the uncontrolled opening of the spillway gates of almost all reservoirs in the State inundated vast stretches of river banks and floodplains and this, together with 200-odd major landslides, affected over 1.5 million people.

The fact that such a freak phenomenon was not observed elsewhere even though flash floods are a regular feature in many parts of the country also contributed equally to the astonishment.

Interestingly, a recent controversial Malayalam novel, Meesha, mentions such a drawdown in water levels following the 1924 floods which, similarly, devastated many parts of the State.

“Its author S Hareesh seems to have done thorough research for this work. I was quite surprised to see him talking of similar dip in water levels, which very few people alive today remember,” said N C Narayanan, professor at the Centre for Technology Alternatives for Rural Areas at the Indian Institute of Technology, Bombay.

But this one was different. With the media playing it up over and over, it generated a surprise, if not a scare, among people. “Open wells in almost 60 per cent of the geographical areas witnessed a sudden drop in water levels, which was rather unusual,” said AB Anitha, Executive Director of the Centre for Water Resources Development and Management (CWRDM), a Kerala government research institute located at Kozhikode. “Such drop in groundwater levels was observed in several river basins in the State,” she said. At places, the fall in groundwater levels was as high as 1.5 metres as compared to the levels a year ago.

The unique occurrence prompted the LDF government in the State to order a scientific investigation. Last week, the CWRDM submitted a report to the government, according to which the decline in groundwater table and depletion in river flows, subsequent to the floods, could be attributed to flood-induced topographical and hydrological alterations, unique topographical and hydrological characteristics of the State as well as impact of recent land use changes on hydrology.

It was exacerbated by other atmospheric and geological conditions. They include an unusually long dry spell (most parts of Kerala didn’t receive any rain for a month after the floods), the deepening of riverbed due to heavy erosion caused by high flood velocity, and high groundwater discharge to the river systems due to high hydraulic gradient, according to the report.

It is a fact that Kerala has been witnessing major land-use changes in the last few decades. Apart from deforestation in its high ranges, wetlands and paddy fields in the plains have been encroached upon due to population pressure. “There have been around 8 lakh hectares of paddy land in Kerala in the late ‘70s. This has now shrunk to close to 2 lakh hectares,” says P V Dineshan, a senior scientist at the CWRDM, the main author of the report.

“The role played by paddy fields in natural recharge of groundwater table is well established. But when these fields, which remained flooded for months during monsoon season, are diverted for non-farm activities, we are depriving the soil a means to replenish groundwater,” says Athira P, a hydrologist with the newly-established Indian Institute of Technology at Palakkad.

“The loss of topsoil would have also prevented rainwater from percolating down and thus hampered the process of groundwater recharge,” says S P Ravi, director of the non-governmental River Research Centre, who has been actively involved in protecting the Chalakudy river and other basins in the State for three decades.

Another interesting theory on why the water levels dropped down so suddenly comes from C G Madhusoodhanan, who did his doctorate in water resources engineering from IIT Bombay.

According to Madhusoodhanan, in the Western Ghats, when the groundwater levels are above a certain threshold, water is released through numerous naturally-formed micro/macro ‘soil pipes’ present in the near surface. Saturated soil water gets drained safely through the hill slopes towards the stream banks through these soil pipes.

Madhusoodhanan says in an unpublished report, “This year, by the end of July, soil in the hills and slopes of the Western Ghats and plains was super saturated. The heavy spell of rains in August third week put more pressure on these soil pipes and initiated soil erosion leading to increased size and water carrying capacity of the pipes... which drained out more water at much faster rates than normal.”

In the normal course, the increased size provides safer passage to the excess waters. But when massive land subsidence, such as that reported from Idukki, Wayanad, Thrissur and Kannur, happened, an excessive soil pipe erosion may have occurred.

The sites of landslides are also seen to be mostly confined to stream channels/ lineaments, where there is maximum water pressure from soil pipes. These landslides resulted in sudden release of stored soil waters, leading to immediate drawdown of groundwater levels in these regions, Madhusoodhanan says.

Scientists may take a while to understand why such sudden drop in groundwater levels has happened. But what it clearly tells authorities is that the Western Ghats can spring nasty surprises unless they learn to respect Nature.

Published on October 08, 2018
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