CHENNAI: Rapid urbanisation has already wiped out 13.6 million cubic metres (MCM) of tank storage within Chennai city, while an additional 175 MCM of storage outside the city is now at risk, according to a new data-driven assessment of the Chennai river basin by the Tamil Nadu Green Climate Company (TNGCC) and the Council on Energy, Environment and Water (CEEW).
The report flags the steady erosion of traditional waterbodies as a central factor intensifying water stress in the basin, even as demand continues to climb. Historically, interconnected tanks played a critical role in meeting irrigation and domestic needs across the region.
The analysis notes that rapid urban expansion has not only reduced storage capacity but also weakened the basin’s natural hydrological resilience. Simultaneously, sewage treatment capacity in the region is being overwhelmed, leading to the discharge of untreated used water into rivers and the deterioration of both surface and groundwater quality.
The Chennai basin, covering Chennai, Chengalpattu, Kancheepuram, Ranipet and Tiruvallur, contributes about 33% of TN’s economy, highlighting the economic stakes of declining water security.
Using Water Evaluation and Adaptation Planning modelling, the study projects that total water demand in the basin will rise from 2,479 MCM in 2025 to 2,728 MCM by 2050 under a business-as-usual scenario. Agriculture will remain the dominant consumer, accounting for 60% of total demand by 2050, while population growth is expected to be a major driver of rising domestic demand.
The modelling also shows a widening supply gap. Unmet water demand is projected to rise from 546 MCM in 2025 to 654 MCM by 2050, a 20% increase if current trends continue. In case of high population growth, the deficit could climb to 754 MCM by 2050.
The study’s risk assessment component finds that Cooum and Kosasthalaiyar sub-basins face the highest overall water risk, followed by Kovalam and Adyar. The rankings are based on a composite index of hazard, exposure and vulnerability indicators. Among the key exposure drivers flagged are groundwater quality, proximity to the coast and the extent of rainfed agriculture. “We are dedicated to enhancing climate resilience by securing TN’s water future, in alignment with our Water Vision@2047,” CM MK Stalin said in the report, which was released during the recent TN Climate Summit.
Additional chief secretary Supriya Sahu said this study demonstrates that it is possible to undertake such assessments at hyperlocal levels. “To enable this, we apportioned data monitored by state institutions and departments at the administrative level to the sub-basin level. For future assessments, mandating data monitoring at the sub-basin level will help strengthen the system-level responses to climate-induced water risks,” she added.
Nitin Bassi, fellow, CEEW, said, “The inference from water balance scenarios demonstrate that climate change is expected to increase water availability in the Chennai river basin. Yet, improving water use efficiency through scaling up micro irrigation in potentially 13% of irrigated area and adopting circular economy approach by reusing 40% of the treated used water will be needed to reduce existing and future water deficit in its sub-basins.”