Why Karnataka-Tamil Nadu Cauvery dispute keeps recurring — a river that has changed, a framework that hasn't

Two July, one river.Farmers led by an association form a human chain as they wade into Cauvery river to protest over water dispute, in Tiruchirappalli district of Tamil Nadu. (PTI)On July 27, 2025, the Stanley Reservoir at Mettur, in Tamil Nadu's Salem district, stood at 103 ft against a full level of 120 ft. Storage had climbed from 50 ft to 103 ft in ten days. The Cauvery was in spate. Salem's low-lying villages were being evacuated. Karnataka had opened the Krishnaraja Sagar (KRS) and Kabini dams after both filled to the brim, and 1.40 lakh cusecs were crossing the inter-state gauge at Biligundlu on the Karnataka-Tamil Nadu border.That year, Mettur touched full capacity on seven separate occasions, news reports at the time said. Tamil Nadu received 317.28 thousand million cubic feet (tmcft) of water at Biligundlu by December — 140.03 tmcft more than its 177.25 tmcft annual entitlement. The dispute went quiet.Twelve months later, on July 28, 2026, the Cauvery Water Regulation Committee (CWRC) ordered Karnataka to release 3,500 cusecs a day for fifteen days, a little under 4 tmcft in all. Tamil Nadu had asked for 9.94 tmcft, or 7,670 cusecs per day — more than double of what was allotted.Karnataka refused the order and moved the Cauvery Water Management Authority (CWMA) on appeal. The CWMA rejected it this week.Pro-Kannada groups have marched in Bengaluru, Mysuru and Mandya. Karnataka chief minister DK Shivakumar told reporters this week that KRS — opened in 1932 across the river near Mysuru and Karnataka's principal Cauvery reservoir — was at 36% of its capacity. Kabini was at 83%, Harangi at 95% and Hemavathi at 67%. Combined inflows into the four reservoirs are estimated to be around 25,000 cusecs.There is another issue. Officials estimate that Karnataka may need to still release 6,000 to 7,000 cusecs from KRS and Kabini for the 3,500 cusecs ordered to reach Biligundlu, because the riverbed in between is dry enough to swallow the rest.Shivakumar’s cabinet colleagues have said that the state had released roughly 3.5tmcft by July-end.Tamil Nadu now plans to move the Supreme Court over Karnataka not following the order. The state, in its submission to the CWRC, said it received 3.543 tmcft at Biligundlu between June 1 and July 26, against a cumulative deficit of 35.391 tmcft for the season.Shivakumar has called an all-party meeting in Bengaluru for August 3 to decide the state's next step. The Karnataka CM also asked his Tamil Nadu counterpart to also defer his visit to Bengaluru for the same day.For both years, this and the last, Karnataka owed Tamil Nadu 31.24 tmcft in July, according to a Supreme Court-approved formula set in place in 2018. In 2025, the state delivered several times that. In 2026, it is refusing to meet the obligation.Also read: Vijay's Jana Nayagan screening halted due to Cauvery protestsThe basinThe Cauvery rises in the Western Ghats of Kodagu district in southern Karnataka and flows some 800 km east through the Deccan plateau into Tamil Nadu, draining at the Bay of Bengal. The river runs entirely on the monsoons — the southwest monsoon that fills Kodagu between June and September, and the northeast monsoon that waters the Tamil Nadu delta between October and December.Four states share it. Karnataka, the upper riparian, uses the water to irrigate the Old Mysore region and to supply drinking water to Bengaluru, Mysuru and Mandya. Tamil Nadu, lower in the river's way, depends on it for the Cauvery Delta — the state's rice bowl, spanning Thanjavur, Nagapattinam and neighbouring districts. Kerala's Wayanad and Puducherry's Karaikal enclave hold smaller stakes, so the century-old dispute runs primarily between Karnataka and Tamil Nadu.As things stand, Tamil Nadu is entitled to 54.6% of the basin's water, Karnataka to 38.5%, Kerala to 4.1% and Puducherry to under 1%. The remaining is reserved for environmental flow and outlets to the sea.But this formula has seldom been adhered to. Karnataka has fought release orders in 2016, 2017, 2018 and 2023. Each of those confrontations followed a broadly similar arc: pro-Kannada protests, a Karnataka appeal to the CWMA or its predecessor body, an eventual release smaller than the order, and distress in the Delta.This recurring dispute is driven, researchers say, by two variables: a changing climate – that robs even Karnataka the share of water that was assumed — and increasing demand.WhiplashSince the Supreme Court's February 2018 verdict fixed the four states' shares, the basin has swung sharply between flood and drought.In August 2019, Karnataka's Kodagu took 935 mm of rain in nine days against a normal August of 600 mm. In September 2022, Bengaluru's Outer Ring Road went under water. In 2023, Karnataka told the CWRC that 161 talukas in the state were severely drought-hit and cumulative inflows into its four Cauvery reservoirs were down by more than half against the long-term average. In November-December 2024, Cyclone Fengal dropped 48.37 cm on Puducherry in 24 hours, the highest one-day fall the region had recorded in three decades, submerging Delta paddy days before landfall. In June 2025, Mettur reached full capacity for the first of seven times that year. And by July 2026, the basin was back in distress.Scientists have tried to explain these swings. A 2024 study in the journal Theoretical and Applied Climatology tracked how fast water was leaving the Cauvery basin through evaporation from the ground and uptake by plants. The current pattern, the researchers found, already matched the highest-emissions global warming pathway that climate scientists use in their worst-case projections.Separately, a 2025 study using the latest generation of global climate models projected sharper temperature spikes and more extreme rainfall for the basin through the century.The division formula was derived from the Cauvery tribunal’s 2007 assumption that the river carries roughly 21 billion cubic metres, or about 740 tmcft, of water in a typical year. That number was meant to describe a river with a stable long-term average. But two decades of data indicates that the basin now swings far more sharply between wet and dry than the framework anticipated.DemandBoth Tamil Nadu and Karnataka have demanded more from the river over the years.Karnataka CM Shivakumar told reporters this week that the state needs at least 40 tmcft of Cauvery water for drinking supply to Bengaluru, Mysuru, Mandya and adjoining districts. This quantity, he said, will be protected regardless of any distress-sharing decision. The remainder of the state's entitlement is used largely for irrigation across districts.The arithmetic works when rain is plentiful. But in the deficient years, Karnataka's own basin needs and its downstream commitment cannot both be met.In Tamil Nadu, the Cauvery Delta produces over one-third of the state's annual paddy crop. The minimum support price — the government's guaranteed floor rate at which state agencies buy grain from farmers — favours paddy over almost every alternative. So do canal access and long-established procurement infrastructure. Nothing in the policies stack rewards to switch to less water-intensive crops. So, when water from Karnataka arrives late, Delta farmers turn to their borewells.The Delta's aquifer has recorded this strain. A May 2025 study in the journal Water found that groundwater pumping in the basin during the rabi (winter) cropping season consumes 24 times more energy than during the kharif season, while irrigating 40% less land — the signature of a basin drawing from ever-deeper borewells. Pump density in the Cauvery Delta averages 23 pumps per sqkm. A 2023 paper in Agricultural Water Management also modelled aquifer depletion in the Delta and traced saltwater intrusion moving inland as the water table dropped.The News Minute reported in March this year on farmers in Cuddalore district who lost their summer paddy crop to Cyclone Fengal in November 2024 but were back at their borewells within weeks, some drawing from 600 ft below the surface in villages where every household once had a working well within 200 ft.Deficient monsoons drive these borewells deeper, and wet years recharge them only partially.Also read: Why Assam floods: The obvious answer is Brahmaputra, but there's so much more to itA long historyThe legal framework for Cauvery water distribution goes back to two colonial-era agreements. In 1892 and 1924, the Madras Presidency and the princely state of Mysore signed accords dividing the flow. Karnataka has argued for a century that both were skewed towards Tamil Nadu.In 1990, after Tamil Nadu moved the Supreme Court, the Cauvery Water Disputes Tribunal was constituted. Its interim award of 1991 set Tamil Nadu's entitlement at 205 tmcft at Biligundlu. Its final award of 2007 fixed the shares at 419 tmcft for Tamil Nadu, 270 for Karnataka, 30 for Kerala and seven for Puducherry.On February 16, 2018, a bench headed by then Chief Justice Dipak Misra modified the award. Karnataka's share rose by 14.75 tmcft to 284.75. Tamil Nadu's share was cut to 404.25 tmcft, offset by a 10 tmcft groundwater allowance in the basin. And Karnataka's obligations at Biligundlu were locked into a monthly schedule — 9.19 tmcft in June, 31.24 tmcft in July, and tapering thereafter.The CWMA and its CWRC were set up later that year to police the schedule. The court also placed drinking water on a "higher pedestal" than irrigation.When there isn't enough water to meet both the Delta's paddy crop and Bengaluru's taps, someone loses. There is no permanent formula for who. The Cauvery Monitoring Committee tried to write one in 2003 and failed. The Union water resources ministry tried again after that. Karnataka has rejected any fixed pro-rata cut, arguing that distress must be assessed on ground realities each year. Tamil Nadu has argued that the 2018 monthly schedule already implies pro-rata sharing.The 2026 standoff again surfaces that tension – which, over time, has been made worse by the climate crisis and the growing population and agriculture demands.