The Supreme Court has relied on groundwater estimates from reports dating back to the 1970s and 1980s to reduce the amount of water Karnataka must release from the basin of the Cauvery river to Tamil Nadu. But studies in the last decade show the groundwater situation in Tamil Nadu has worsened.

On February 16, the court pronounced its verdict on appeals filed by Karnataka and Tamil Nadu against the final allocation of water decided by the Cauvery Water Dispute Tribunal in 2007. The court decided to reduce Tamil Nadu’s share by 14.7 thousand million cubic feet – from 192 thousand million cubic feet to 177.2 thousand million cubic feet – citing that the tribunal had not taken into account that the state has a minimum 10 thousand million cubic feet of available groundwater that can be used for irrigation.

But according to a 2011 report by the Central Water Commission, groundwater levels in the Cauvery delta region in Tamil Nadu dropped between 0.5 metres and 2.7 metres in the 1980-2009 period because of a steady rise in the exploitation of groundwater resources. The fertile delta in the lower reaches of the river includes the three major districts of Thanjavur, Thiruvarur and Nagapattinam. Nagapattinam showed the greatest lowering of the water table in this period. Moreover, the salinity of groundwater in these districts is excessive, said S Janakarajan, a retired professor at the Madras Institute of Development Studies. With the river dry for most of the year, seawater has been invading the canals in the basin and seeping into groundwater resources, he added.

The Tamil Nadu State Action Plan for Climate Change report of 2009 said 21% of the total blocks surveyed in 1980 were classified as overexploited, critical or with poor water quality. However, this rose to 25% in 1997 and shot up to 48% in 2009.

A 2016 policy note by Tamil Nadu’s Public Works Department further stated that 77% of available groundwater resources in the state were in use and that only 39% of revenue blocks (sub-divisions in districts) were classified as safe in terms of groundwater levels and quality.

The Supreme Court verdict is just the latest chapter in a 22-year-old legal battle between Tamil Nadu and Karnataka. The dispute has been the subject of massive protests and is an intrinsic part of the politics of the two states. And it does not seem to be nearing an end any time soon.

According to a report in The Hindu, if Tamil Nadu files a review petition against the Supreme Court ruling, the inclusion of the groundwater factor may form an important part of its objections.

However, experts say groundwater should not be seen in isolation. Veena Srinivasan, a hydrologist at the Ashoka Trust for Research on Ecology and the Environment, pointed out that groundwater and river water that flows on the surface – also called surface water or surface runoff – are interconnected. Therefore, the Supreme Court should see them as a single component and not separately, she said.

Nagapattinam district, in Tamil Nadu's Cauvery delta, saw the sharpest drop in the water table in the 1980-2009 period, according to a 2011 study. (Credit: Vinita Govindarajan)

The groundwater factor

In 2007, the Cauvery Water Dispute Tribunal had observed that the delta districts in Tamil Nadu had at least 20 thousand million cubic feet of available groundwater, based on studies by the United Nations Development Programme, the Central Ground Water Board and the World Bank among others. But it did not take this factor into consideration while making the final allocation, citing it as conjecture.

In its appeal, Karnataka argued that extraction of groundwater was “optimal utilisation of available resources” and that the tribunal had made a “patent error” by not reducing the allocation to Tamil Nadu despite recognising the availability of 20 thousand million cubic feet of groundwater in that state.

Tamil Nadu countered by saying Karnataka had relied on an outdated report to make its argument. It also argued that it was scientifically difficult to accurately estimate groundwater levels and the extent to which they get recharged by rainfall and surface flow. spoke to a groundwater expert from a Bengaluru-based research institute who said the Cauvery basin in Karnataka is at a higher altitude and, hence, extracting groundwater is more difficult here than in Tamil Nadu, where the aquifers – underground layers of water-bearing permeable rock – are more shallow.

However, the Supreme Court decided there was little reason to exclude the groundwater factor while determining the share of Cauvery water for each state, given the “overwhelming empirical data” that points to its availability in the region.

Interconnected hydrology

Hydrologist Veena Srinivasan said the Supreme Court’s verdict is based on a misconception that river water that flows on the surface and groundwater are separate entities.

She explained that rainwater can end up in three places. It can form surface water, or river water that flows above the ground and into the sea. It can also seep slowly into the ground and recharge groundwater. But more than 70% of rainwater in the basin is absorbed by vegetation through the roots and transpired back into the atmosphere through the leaves, she said.

“A large majority of water does not end up as any river or any aquifer,” she said. “This is important to remember because when you are talking about sharing the river water, this is actually a very small part of the rainwater received by the region.”

Srinivasan also explained the interconnectedness of surface water and groundwater by using the example of a sponge soaked in water. “If you cut a path in the sponge, the water from the rest of the sponge is going to ooze into that channel,” she said. Similarly, when groundwater levels are high, such that they intersect the river water on the surface, groundwater will seep into the river. That is called baseflow, she said. This is why rivers flow throughout the year, even when there is no rainfall or snowmelt to feed them.

“But when you make the water table go deeper and deeper, there is not going to be that baseflow to feed the river,” she added. Instead, the surface water will seep into the ground. Groundwater exploitation is a major reason why perennial rivers are drying up.

Srinivasan suggested that the Supreme Court should not only avoid accounting for groundwater and surface water separately, it should also estimate the availability of water resources in total for the whole of Tamil Nadu and Karnataka and not just in the delta region. “Because if you suck out groundwater from areas outside the delta region, it will deplete the water in the river upstream and reduce flows into the delta,” she said.