Climate study shows drying of Cauvery to persist until 2050, even as other rivers swell

While a warming climate is expected to significantly increase the flow of most major Indian rivers, the Cauvery basin stands out as an exception. The river faces a potential “near-term decline” of approximately 3.5% of its waters between 2026 and 2050, even as its northern counterparts brace for floods, according to a study by researchers at the Indian Institute of Technology (IIT), Gandhinagar, published in the peer-reviewed journal Earth’s Future.

Given the fraught history of Cauvery water sharing between Karnataka and Tamil Nadu, particularly in years of deficit rainfall, the study suggests that despite climate models projecting increased rainfall from global warming in India in the decades ahead, the Cauvery might not benefit. In such a situation, river interlinking projects, such as the proposed Godavari-Cauvery link project, might be necessary, the researchers say.

The study finds that the Cauvery experienced a 28% decline in streamflow between 1951 and 2012, based on data from Kollegal which, one of the authors told The Hindu, “well represented” actual flows in the Cauvery.

Constrained modelling approach

While the study is based on a modelling study and is reliant on an extrapolation, it attempts to reduce errors that can creep in from blindly applying climate models to predict the impact of global warming on Indian rainfall. To do that, the authors use a novel statistical framework and base their analysis on actual river flows between 1951 and 2012, measured at nine stations representing nine major river basins of India, and then extrapolates the data using a ‘constrained modelling’ approach.

The nine rivers are the Cauvery (measured at Kollegal), the Ganga (Farakka), Brahmaputra (Bahadurabad), Indus (Bhakra), Godavari (Polavaram), Krishna (Kurundwad), Mahanadi (Basantpur), Narmada (Mandleshwar), and Tapi (Burhanpur).

Fraught history

Decades of failed negotiations on water sharing between Karnataka and Tamil Nadu led to the establishment of the Cauvery Water Disputes Tribunal (CWDT) in 1990, which took 17 years to reach a final order in 2007. The tribunal calculated total available water at 740 thousand million cubic feet (TMC) in a normal year, and allocated shares accordingly. The 2018 Supreme Court verdict allocated 404.25 TMC to Tamil Nadu and 284.75 TMC to Karnataka, while also declaring the Cauvery a national asset.

In 2023, Tamil Nadu requested 24,000 cusecs per day citing severe drought, but Karnataka refused, citing its own water shortage, leading to widespread protests in both States.

Near, mid term water shortages

Water challenges apart, the study highlights a persistent problem in climate science: while models agree that India will get warmer, they vary widely on exactly how much rain will fall. By applying observational constraints, the researchers identified that only 8 out of 22 models accurately captured the seasonality of the Indian monsoon.

The source models used are the CMIP6 (Coupled Model Intercomparison Project Phase 6), which are the latest generation of global climate models used by scientists to project future climate change

Unconstrained models show a 5% increase in the Cauvery in the near term, the researchers said, as well as a 25% increase in the Indus, 8% in the Ganga, and 16% in the Krishna. When projections were restricted to these ‘constrained’ or more reliable models, the certainty of a wetter future for most rivers increased, but the outlook for the Cauvery remained grim, with the basin facing “near- and mid-term water shortages”.

Data-driven warning

The researchers, led by Dipesh Singh Chuphal and Professor Vimal Mishra of IIT Gandhinagar, were able to reduce the projection uncertainties by nearly one-third to derive their estimates. “Raw outputs have biases because of model resolution, simplified physics that global climate models use. To correct these biases and show future trends based on what is actually observed, we use constrained models,” Mr. Chuphal, the lead author of the study, told The Hindu. “As far as the Cauvery is concerned, it implies that water sharing could get tougher.”

Crucially, the researchers simulated “naturalised” flows, meaning the study focused purely on climate-driven changes without accounting for human interventions like dam operations or irrigation withdrawals. In the real world, these human pressures could potentially exacerbate the projected shortages in the Cauvery.

“Reliable information on how India’s major rivers will respond to a changing climate is critical for the food, water, and energy security of approximately two billion people,” the study concludes, offering a data-driven warning for one of India’s most water-stressed regions.