Executive summary

Eleven of India’s 15 major river basins in India are at least water-stressed (Bassi et al. 2023), and a quarter of groundwater aquifers are being used beyond safe extraction limits (CGWB 2024). Further, most river basins in India are highly vulnerable to climate extremes; as a result, 600 million people in the country face extreme water scarcity (NITI Aayog 2018).

This study focuses on the Chennai basin, the majority of which (about 78 per cent) lies in the state of Tamil Nadu (CCCDM 2022), spanning five districts – Chennai, Chengalpattu, Kanchipuram, Ranipet, and Thiruvallur – which together contribute about 33 per cent to the state economy (DES 2024a). Within Tamil Nadu, the Chennai basin comprises six sub-basins – Adyar, Araniyar, Cooum, Gummidipoondi, Kovalam, and Kosasthalaiyar (Figure ES1).

The districts within the Chennai basin recorded a rise in short-duration, high-intensity rainfall events during both the south-west monsoon (SWM) (June–September) and the north-east monsoon (NEM) (October–December) in 2011–22 compared to 1981–2011 (Prabhu and Chitale 2024). Climate change has also increased the intensity and duration of cyclones in the Bay of Bengal over recent decades (Selva et al. 2025). Additionally, the basin is experiencing a surge in droughts and floods and faces a rising risk of future droughts (Anandharuban and Elango 2021; CCCDM 2022; Joseph 2022; Kaaviya and Devadas 2021).

Rapid urban expansion has further intensified water stress in the basin, predominantly through the loss of water bodies, including traditional tank systems that historically sustained irrigation and domestic water demand (Rajagopalan et al. 2024). Urbanisation has led to the loss of 13.6 million cubic metres (MCM) of tank storage within Chennai city, while an additional 175 MCM of tank storage outside the city is now at risk (Devi et al. 2025). At the same time, sewage treatment capacity in the region has become overwhelmed, leading to the discharge of untreated used water into rivers and the subsequent deterioration of both surface water and groundwater quality (Ramachandran et al. 2021; Rosado et al. 2024). The increasing frequency and severity of climate extremes, along with challenges to water quantity and quality, call for urgent action to build climate resilience in the Chennai basin. Assessing climate risk, including physical, social, economic, institutional, and policy factors, is the first step. Further, assessing the current and projected water balance in the basin, i.e., the difference between the water supply requirement and the water actually supplied, is crucial for enhancing water security and hence reducing the risks posed by climate-induced extremes. Such assessments can inform adaptation measures such as adopting a circular economy approach to water management and improving water use efficiency. Further, interventions informed by water risk assessments in the Chennai basin can support 36 targets under 11 Sustainable Development Goals (SDGs) (Figure ES2).

Figure ES2. Potential SDG targets that the state can attain from interdisciplinary climateinduced water risk assessments

Source: Authors’ analysis based on Guppy, Lisa, Paula Uyttendaele, Karen G. Villholth, and Vladimir Smakhtin. 2018. “Groundwater and Sustainable Development Goals: Analysis of Interlinkages.” Hamilton, Canada: United Nations University Institute for Water, Environment and Health and UN-Water. 2016. “Water and Sanitation Interlinkages across the 2030 Agenda for Sustainable Development.” Nairobi, Kenya: United Nations Environment Programme. Note: SCP - Sustainable Consumption and Production

Objectives of the study

We conduct the analysis at the sub-basin level. The study has the following three objectives:

• Develop and compute an index to assess climate-induced risk to water systems in the Tamil Nadu portion of the Chennai basin.
• Update the existing water balance of the Chennai basin using the latest data and incorporate scenarios to model the water deficit.
• Develop a targeted water balance scenario to assess the potential of treated used water (TUW) reuse to reduce pressure on freshwater resources.

Methodology for risk assessment and establishing water balance

The methodology for Objective I is entailed in Figure ES3.

To address Objectives II and III, we used the Water Evaluation and Adaptation Planning (WEAP) model to assess the water balance of the Chennai basin (1994–2050) under various climate, population, and TUW reuse scenarios. The model was configured for the six sub-basins of the Chennai basin and integrated data on rainfall, evapotranspiration, groundwater, reservoirs, and demand from domestic, agricultural, livestock, and industrial sectors (Figure ES4). Inputs were sourced from national and state agencies, and climate projections were based on the Representative Concentration Pathways (RCP) 4.5 scenario. The outputs were statistically bias corrected and downscaled to a spatial resolution of 25 kilometres (km). Following are the six scenarios developed to evaluate future water demand and availability:

• The business-as-usual (BAU) scenario assumes that current growth trends and wateruse patterns will continue, without climate change impacts.
• The high population growth scenario assumes population growth rates that are 25 per cent higher than BAU rates from 2011 onwards.
• Four intervention scenarios were modelled in addition to the BAU case.
  • Two scenarios without climate change projections assess the combined impacts of micro-irrigation and treated used water (TUW) reuse, assuming that 13 per cent of the cropped area (excluding rice) adopts micro-irrigation, while TUW reuse increases to 25 per cent and 40 per cent by 2030, respectively.
  • Two corresponding climate change scenarios applied the same micro-irrigation and TUW assumptions, with added climate change projections.

Key findings

Sections below present the key findings from the risk assessment and WEAP modelling exercise.

Risk assessment

The risk to water resources in each sub-basin from the changing climate, calculated as a product of that sub-basin’s hazard, exposure, and vulnerability, is presented in Figure ES5. Among the sub-basins in the region, Cooum and Kosasthalaiyar exhibit the highest risk levels, followed by Kovalam, Adyar, Araniyar, and Gummidipoondi. These ranks are comparative; a lower rank should not be interpreted as a dissolution of the need for climate action.

Figures ES6–ES11 present the top five contributing indicators for each risk subcomponent. Since sensitivity had only three indicators, all three were deemed important. Overall, the indicators have been presented in descending order of their contribution to hazard, exposure, sensitivity, and adaptive capacity.

Water balance

• Under a business-as-usual (BAU) scenario
  • The total water demand in the Chennai basin will rise from about 2,479 MCM in 2025 to 2,728 MCM by 2050, driven by a 34 per cent increase in population over the same period.
  • Agriculture will continue to account for the largest share of water demand, accounting for around 60 per cent of total demand by 2050. Unmet water demand will increase from 546 MCM in 2025 to 654 MCM by 2050, representing a 20 per cent increase.
• Under a high population growth scenario
  • The total water demand will reach 2,939 MCM in 2050, representing a 12 per cent increase from the 2025 level of 2,633 MCM.
  • Unmet water demand will rise to 754 MCM by 2050, further stressing water resources and jeopardising access for domestic and agricultural users.
• Scenarios under which unmet water demand reduces
  • The combined implementation of two primary strategies – the adoption of micro-irrigation and TUW reuse – can reduce unmet water demand in the Chennai basin.
  • Scaling up micro-irrigation to 13 per cent of the cropped area and achieving 25 per cent TUW reuse will reduce unmet water demand by about 52 per cent by 2050. Increasing TUW reuse to 40 per cent will further reduce unmet demand, reducing unmet water demand by about 63 per cent by 2050.
  • Under the same intervention scenarios, incorporating climate change effects, unmet water demand will reduce by 90 per cent and 93 per cent by 2050, respectively.

Recommendations

The following recommendations aim to reduce water risk and deficits in the Chennai basin and are considered feasible for implementation over the next five years:

• Mandate interdisciplinary water risk assessments and water balance preparations at the hyperlocal level: This study demonstrates that it is possible to undertake such assessments at hyperlocal levels, such as sub-basins.To enable this, we apportioned data monitored by state institutions and departments at the administrative level to the subbasin level. For future assessments, mandating data monitoring at the sub-basin level will help strengthen the system-level responses to climate-induced water risk and will help align assessments with natural hydrological boundaries rather than administrative ones. Such assessments should be jointly undertaken and periodically updated by the Municipal Administration and Water Supply Department and the Environment, Forest and Climate Change Department, with the former taking the lead role.
• Assess and build capacity of state institutions to enable sustained water risk and balance assessments: Although such assessments can be jointly undertaken and updated by the mentioned departments, the process requires coordination and support from other departments and institutions. It is therefore essential to conduct a training needs assessment across relevant departments and institutions followed by the design and delivery of tailor-made training programmes to strengthen institutional capacity for conducting such assessments. The Anna Institute of Management, whose mandate includes capacity building for prospective and practising administrators, can play a key role in conducting such training needs assessments and delivering targeted training programmes.
• Strengthen existing datasets that enable water risk and balance assessments: Water management complexity arises from wide hydrological span and competing sectoral demands. Strengthening existing datasets at the sub-basin level through initiatives, such as the Tamil Nadu Satellite-Based Water Bodies Information Monitoring and Safety System (TN-SWIP), as envisioned in the State Draft Water Policy 2024, can enable realtime monitoring of water bodies. Further, making such datasets publicly available through a dynamic dashboard, such as the Tamil Nadu Water Resource Information Management System (TN-WRIMS), can support more informed assessments.
• Prioritise budgetary allocations and interventions for high-risk sub-basins: Funding should prioritise sub-basins identified as high risk and design interventions to address the key indicators driving risk, particularly those common across multiple sub-basins. This should be guided by the results of the WEAP scenario analysis. For instance, the Kosasthalaiyar sub-basin faces the highest water risk but also shows strong potential to mitigate it. The results indicate that unmet water demand could be reduced by nearly 65 per cent by 2050 by scaling up TUW reuse to 40 per cent and micro-irrigation to 13 per cent. Similarly, investments and interventions in the Adyar sub-basin should prioritise decentralised, modular used water treatment plants near major demand centres to enable local reuse for non-potable domestic and industrial purposes. These efforts should be complemented by measures to promote and subsidise micro-irrigation coverage in water-intensive farms, alongside TUW reuse for groundwater recharge.

Together, these recommendations provide a clear direction for action in the near and medium term, supporting more resilient water systems under a changing climate in the Chennai basin.