Tamil Nadu’s claim of Cauvery backlog and the Supreme Court’s response | Explained

Tamil Nadu sought directions requiring Karnataka to make good the Cauvery water backlog measured at Biligundulu, the inter-State border point. It said its realised share was about 11.7 tmc ft below what was due by October 6. The request matters because downstream Tamil Nadu depends on timely releases for its delta agriculture and other needs.

The Court focused on whether a backlog can be recovered during a drought. It noted that Karnataka’s reservoirs were also affected by water scarcity and weak inflows. Its central point was that past deficits can be covered only when there is excess water available. A shortfall cannot automatically be corrected during another shortage.

The dispute remains unresolved because both States presented competing needs. Karnataka said it must reserve water for drinking supplies and environmental flows, while Tamil Nadu said the directed release of 3,000 cusecs was inadequate. The Division Bench reserved its order on Tamil Nadu’s application.

The Cauvery water backlog is the accumulated gap between the quantity Tamil Nadu was supposed to receive and the water actually measured at Biligundulu. Biligundulu is the inter-State border measuring point used to track releases from Karnataka into Tamil Nadu. The term therefore describes a delivery shortfall, not a separate reservoir or stored quantity.

Tamil Nadu told the Supreme Court that, as of October 6, it had received around 11.7 thousand million cubic feet less than its expected share. The Cauvery Water Regulation Committee calculated that 57.568 tmc ft should have been realised during the relevant period under the prevailing circumstances, but actual realisation was 45.849 tmc ft.

The size of the backlog became the centre of the dispute. Tamil Nadu later demanded that Karnataka make good 15.02 tmc ft at the Authority’s October 8 meeting. The differing figures reflect different dates and calculations, while the Court considered whether recovery was possible during continuing scarcity.

The shortfall occurred across several linked measurements. From June 1 to October 7, the Cauvery catchments received 30% less rainfall than normal. The deficit was worse downstream of the Krishnaraja Sagara and Kabini reservoirs, up to Biligundulu, where rainfall was 43% below normal. These figures show that the shortage was both basin-wide and especially severe near the measuring point.

The CWRC calculated that 57.568 tmc ft should have been realised during the relevant period. Actual realisation was 45.849 tmc ft. The difference between those figures was about 11.7 tmc ft, matching Tamil Nadu’s claim before the Supreme Court. At the Authority’s October 8 meeting, Tamil Nadu cited a larger backlog of 15.02 tmc ft.

These numbers explain why the dispute intensified. Less rain reduced inflows and available storage, while Tamil Nadu still expected scheduled releases. Karnataka argued that its own reservoirs had suffered a 54% deficit in cumulative inflows.

Upper- and lower-riparian locations create different pressures during drought. Karnataka lies upstream and holds four key reservoirs in the Cauvery basin. When inflows collapse, it must decide how much stored water to release and how much to retain. Tamil Nadu lies downstream and depends heavily on water arriving at Biligundulu for its Cauvery delta and seasonal cultivation.

Karnataka reported a 54% deficit in cumulative inflows to its four reservoirs. It said limited storage had to support drinking-water requirements and environmental flows, together needing 45 tmc ft. Tamil Nadu argued that about 3,500 cusecs should flow at Biligundulu in October, rather than the 3,000 cusecs directed by the CWMA.

The States therefore experience the same drought through different risks. Karnataka fears exhausting supplies before replenishment, while Tamil Nadu fears inadequate downstream delivery. Karnataka pointed to the northeast monsoon as a possible source of replenishment, but Tamil Nadu stressed that this monsoon is highly variable and unpredictable.

If releases remain below Tamil Nadu’s expected share, the immediate concern is the Samba paddy crop during the current cultivation season. Tamil Nadu said the Cauvery delta’s farmers and their livelihoods could suffer because paddy depends on timely water. The State also argued that the authorised flow should be about 3,500 cusecs in October, not 3,000 cusecs.

Tamil Nadu called 3,000 cusecs grossly inadequate because it did not address the accumulated backlog. Lower flows would leave less water reaching the delta and would make it harder to sustain the crop. The dispute is therefore about both measured quantities and the timing of deliveries during a fixed agricultural season.

Drinking-water supplies are also part of the wider risk. Karnataka said it needed to reserve scarce storage for drinking water and environmental flows, together requiring 45 tmc ft. The States thus face competing consequences: reduced releases threaten Tamil Nadu’s agriculture, while larger releases could strain Karnataka’s essential supplies.

The Cauvery Water Disputes Tribunal’s final award sets the basic allocation framework for the river’s waters. The Supreme Court modified that award in February 2018. The current institutional system exists mainly to implement that legally settled arrangement, rather than to create a wholly new allocation each time a drought occurs.

The Cauvery Water Management Authority is the implementation mechanism for the modified award. The Cauvery Water Regulation Committee is its main assisting arm. The Committee works out relevant release and realisation figures, while the Authority issues directions such as providing 3,000 cusecs for 15 days from October 9. Biligundulu serves as the measurement point for delivery.

The Court remains involved when a State challenges compliance or seeks additional directions. In this dispute, Tamil Nadu approached the Court over its claimed backlog, while Karnataka explained its scarcity. The article says the present institutions are limited to implementing the award, so the Union government should work toward a broader Cauvery River Basin Organisation.

Drought makes river-sharing difficult because supply falls while demand does not disappear. Monsoon rainfall determines inflows into reservoirs, but rainfall can vary across the basin. A 30% basin-wide deficit and a 43% deficit near Biligundulu meant that water was not equally available across locations. Reservoir storage then becomes the buffer for competing users.

The present dispute shows the mechanism clearly. Karnataka reported a 54% deficit in cumulative inflows and said 45 tmc ft had to support drinking water and environmental flows. Tamil Nadu needed dependable releases for Samba paddy and argued for about 3,500 cusecs in October. Karnataka pointed to the northeast monsoon, but Tamil Nadu noted its spatial and temporal variability.

Fair division therefore requires decisions that account for timing, location and user needs, not only annual totals. The article says deficit spells will continue and El Niño may raise extreme-weather risks in 2027. It urges coordinated farm planning and a Cauvery River Basin Organisation beyond the current arrangement.