WEAP Model Based Evaluation of Future Scenarios and Strategies for Sustainable Water Management in the Chennai Basin, India

Water scarcity is a growing global challenge that affects billions of people worldwide. According to UNICEF and the World Health Organization, an estimated 2.2 billion people lack access to safe drinking water, and many more face water shortages or contamination (UNICEF & WHO 2017). Water scarcity is a result of several factors, including climate change, population growth, and poor water management practices. The impacts of water scarcity are far-reaching and can have severe consequences for human health, economic development, and ecosystem function. Water scarcity has led to conflict, migration, and social unrest in many parts of the world. The World Economic Forum has ranked water scarcity as one of the top global risks in terms of its potential impact on society and the economy (Global Risks Report 2022).

It is necessary to develop effective interventions to promote sustainable water management practices, increase water efficiency, and improve water governance to address the global water scarcity challenge. The UN's Sustainable Development Goal 6 aims to ensure access to water and sanitation for all and to improve water management practices globally (UNSD 2022). Achieving this goal will require significant investment in water infrastructure, technology, and human resources, as well as strong political will and international cooperation (High-level Panel on Water: Action Plan 2016).

The problem of inadequate water supply is severe in developing countries like India, where rapid population growth, urbanization, and climate change are exacerbating water scarcity and putting pressure on limited water resources (Gulati & Banerjee 2016). More than half of India faces high to extreme water stress, and Chennai, the capital of Tamil Nadu, is no exception. Despite being close to the Bay of Bengal, the city faces acute water scarcity, particularly during summer. The city's groundwater sources are rapidly depleting and affected by seawater intrusion, whereas the surface water sources are also under stress due to inadequate rainfall and poor water management practices. The increasing population, rapid urbanization, and climate change are exacerbating the water scarcity in the city, leading to a significant water deficit. Hence, there is a need to develop effective interventions to improve the city's water management practices and address the water scarcity challenge.

A previous study by Paul & Elango (2018) focused on predicting the future water supply-demand gap in Chennai using the water evaluation and planning (WEAP) model, and it provided valuable insights into potential interventions to bridge the water supply-demand gap. Srinivasan et al. (2010) integrated hydrological and economic models to assess the impacts of water supply alternatives, such as reservoir augmentation and groundwater extraction to identify economically optimal water supply strategies considering uncertain hydrological conditions. Murugesan et al. (2015) focused on drinking water supply and demand management in Chennai and identified the challenges and gaps in drinking water management. Srinivasan (2008) proposed an integrated framework for analysing water supply strategies in Chennai, aiming to understand the complexities of urban water supply and the factors influencing water demand and supply dynamics. Arunkumar & Mariappan (2011) conducted a water demand analysis for municipal water supply using the EPANET software, providing insights into optimizing water supply infrastructure and improving efficiency. Ahmadi et al. (2020) focused on assessing current and future water supply and demand in rapidly developing megacities, including Chennai. Their research utilized a modelling approach to analyse water availability, population growth, and water stress indicators, aiming to understand the potential risks and challenges associated with water scarcity in urban areas.

Despite the growing concerns about water scarcity and resource management in the Chennai Basin, there remains a distinct lack of comprehensive studies that holistically examine the intricate interplay between water balance, demand projections, and viable mitigation strategies. Previous research has focused mainly on isolated aspects of this complex challenge, leaving a critical gap in our understanding of how various factors interact to shape the region's water resource dynamics.

This study aims to bridge this research gap by employing a robust methodology driven by the WEAP model to comprehensively assess the present and future water availability and demand scenarios. By combining a detailed examination of groundwater recharge, reservoir capacities, desalination plants, water treatment facilities, and alternative storage options, our study offers a comprehensive view of the water balance landscape in the Chennai Basin, providing previously unexplored insights.

The Chennai Basin faces water scarcity issues due to variability in rainfall patterns, increasing demand from various sectors, and limited water storage capacities. Therefore, effective water resource planning and management are crucial to ensure a sustainable water supply and meet the growing demand.

Comprehensive data and other relevant information were collected to analyse the current water supply-demand situation and assess the potential impacts of climate change on water resources in the study area. Necessary data were obtained from multiple sources to ensure a robust and reliable foundation for this study. The Chennai Metropolitan Water Supply and Sewerage Board (CMWSSB) and Water Resource Department (WRD) provided critical information on the current water balance in Chennai, including reservoir levels, water transfers between reservoirs, and surplus water in rivers. This data served as a primary source to understand the existing water supply-demand dynamics in the study area.

Additionally, reports and publications from organizations such as the United Nations Development Programme (UNDP), the Indian Meteorological Department, the National Institute of Hydrology, and research publications were also used. These sources provided valuable information on climate patterns, rainfall, hydrological characteristics, and water availability in the study area.

By utilizing multiple sources and considering potential limitations, the data collection process aimed to provide a thorough understanding of the current water supply-demand situation and the potential impacts of climate change on water resources in the study area. This extensive data collection process served as a crucial foundation for the subsequent development of the water supply-demand model and the analysis conducted in this study.

We employed the WEAP model, a widely recognized tool for assessing water balance and potential future scenarios in our study area. WEAP is a decision-support software that simulates complex water systems, including surface and groundwater sources, demand sectors, and storage infrastructure (Sieber & Purkey 2007). The WEAP model, developed by the Stockholm Environment Institute (SEI), operates on a monthly time step and employs the fundamental accounting principle for the water balance. It allows users to represent the water system by defining various sources of supply, such as rivers, groundwater, and reservoirs, as well as withdrawals, water demands, and ecosystem requirements (SEI 2001).

We subdivided the study area into hydrological sub-basins, defining components like rivers, reservoirs, groundwater basins, and demand nodes. Incorporating climate, land use, and socioeconomic data, WEAP drove simulations to explore future conditions and assess interventions' effectiveness.

Simulations evaluated the impacts of measures like groundwater recharge, rainwater harvesting, and land management on water availability. We also projected outcomes by integrating multiple interventions to address water supply-demand dynamics.

In this study, the WEAP model was utilized to simulate and analyse the water balance in the study area, incorporating data and information collected during the data collection phase. The model was designed to incorporate critical components such as reservoirs, catchments, desalination plants, demand sites, and rivers, considering the different water supply sources, including surface water, groundwater, and desalination. Additionally, the model considered water demands from various sectors, including irrigation, domestic use, and industrial use.

The construction of the WEAP model involved configuring the system components and establishing their spatial relationships within the catchment under investigation. This included defining the water sources, withdrawal points, transmission infrastructure, reservoirs, wastewater treatment facilities, and the specific water demands from different sectors.

The model development process also required the formulation of scenarios based on different sets of future trends, considering factors such as policies, technological developments, and other variables that could affect water demand, supply, and hydrology. These scenarios were evaluated against criteria such as water resource adequacy, costs, benefits, and environmental impacts.

The demand sites in the study area were categorized into two main categories: irrigation demand and Chennai city demand, which includes industrial and domestic water demands. The water demand for irrigation was estimated based on crop types, irrigation methods, and cropping patterns obtained from the data collection phase. The Water Resources Information System (WRIS) website provided valuable data on irrigation requirements, indicating a total irrigation use of 1,222 million cubic metres (MCM) per year in the Chennai Basin. The land area under irrigation was determined to be 240,00 hectares (ha), resulting in an average water requirement of 5,555 cubic metres (m³) per ha per year.

For Chennai city, the water demand for domestic sectors was considered. The domestic water requirement was estimated at 135 litres per person per day. Future water demand projections were made using a linear trend method based on population data. The estimated populations in the Chennai Basin for 2001, 2011, and 2021 were 10.06 million, 12.37 million, and 15.21 million, respectively.

Catchments are essential in water resource modelling as they contribute to river flows and groundwater recharge, significantly impacting the water supply system. This study incorporated several catchments within the study area into the model to capture their influence on water availability. These catchments include:

• Araniyar Basin: The Araniyar River Basin is a major catchment in the study area, covering an area of 1,730 km². About 847 km² of the basin falls within Tamil Nadu and 883 km² in Andhra Pradesh. The sub-basin is characterized by the presence of the Araniyar River, which is the major river flowing through this catchment. The Araniyar River flows easterly and southeasterly within Tamil Nadu, covering a distance of approximately 66 km before it ultimately falls into the Bay of Bengal. The outlet of the Araniyar sub-basin into the sea is below the Pazhaverkadu lake.

• Kosasthalaiyar Basin: The study also incorporated the Kosasthalaiyar River Basin as another significant catchment within the model. This catchment covers a total area of 3,727 km², with 2,850 km² falling within Tamil Nadu and 877 km² in Andhra Pradesh. The main river flowing through this basin is the Kosasthalaiyar River. The Kosasthalaiyar River receives water from various sources, including the Lava and Kusa Rivers of the Nagariar River (originating from Andhra Pradesh), the Nandhiar River, and the Kosasthalaiyar (which acts as a surplus channel for the Kaveripakkam and Mahendravadi tanks in Ranipet District). These rivers contribute their flows to the Poondi Reservoir, after which it is referred to as the Kosasthalaiyar River. The Kosasthalaiyar River Basin receives water from downstream watersheds through various courses: Allikuzhiodai, Rajanodai, Kattankal, Poochikal, and the Red Hills surplus course. Finally, its confluences with the sea through the Ennore creek.

• Cooum-Adyar-Kovalam (CAK) Basin: The CAK Basin is a complex system of interconnected rivers and catchments. It plays a crucial role in providing water to Chennai city. The Cooum sub-basin covers an area of 473 km², while the Adyar sub-basin spans 869 km². These two sub-basins are characterized by the presence of major rivers, namely the Cooum River and the Adyar River. The Cooum River mouth is below the Napier Bridge, while the Adyar River mouth is below the Thiru Vi. Ka. Bridge. The Kovalam sub-basin comprises multiple isolated watersheds along the east coast, each having its own drainage system. It covers an area of 782 km². The Kovalam Creek serves as the outlet for this sub-basin and is situated below the Muttukkadu backwaters.

By including these catchments in the model, the study ensured a comprehensive representation of the water dynamics in the Chennai Basin, considering the contributions of each catchment to the overall water supply and availability. The hydrological characteristics, rainfall patterns, and catchment properties specific to each area were integrated into the model to capture their individual and combined impacts on the water resources within the basin.

The water supply and resources incorporated in the model encompassed rivers, groundwater, reservoirs, desalination plants, transmission links, and runoff and recharge from catchments. Historical flow data and hydrological characteristics were utilized to simulate the natural flow regimes in the rivers. Based on historical data and hydrogeological information, groundwater availability and storage changes were considered. Reservoirs were characterized by their storage capacities, inflow and outflow rates, and operational rules. Desalination plants were included to represent alternative water supply sources. Transmission links were incorporated to account for water movement between different nodes in the system. Runoff and recharge from catchments were factored in to capture natural water replenishment processes.

The model interconnected the nodes representing these supply and resource components through simulated flows and transfers. This enabled the simulation of water movement within the system and facilitated an integrated analysis of water availability and allocation across different sources.

The runoff and recharge generated from the Araniyar River Basin, Kosasthalaiyar River Basin, and CAK River Basin were also considered along with inter-basin transfer. The simplified coefficient method was used for rainfall-runoff calculation. Based on a previous study, groundwater recharge in aquifers was assigned as 16% of total rainfall (Pazhuparambil Jayarajan et al. 2022). The remaining fraction of runoff was assigned to rivers and lakes available in the catchment. Although the rivers were not considered as direct supply sources in the model, their runoff played a significant role in overall water availability and allocation. The model simulated the river inflows and outflows to assess water availability and support the analysis of water allocation within the study area. The recharge from catchments also contributed to replenishing groundwater resources, which also played a crucial role in the overall water supply system.

Data from the WRIS website were utilized to quantify the net groundwater availability. In the model, the groundwater supply from both aquifers was allocated to the demand sites, including irrigation and Chennai city demand. This allocation of groundwater resources to fulfil the water requirements of these demand sites played a crucial role in understanding and managing water supply within the study area.

Reservoirs play a crucial role in the water supply system of the Chennai Basin, serving as storage infrastructure. The model characterized reservoirs based on their storage capacities, inflow and outflow rates, and operational rules. The reservoirs considered are Chembarambakkam Reservoir, Poondi Reservoir, Red Hills Reservoir, Cholavaram Reservoir, and Thervoy Kandigai Reservoir (Figure 2).

In addition to these components, other important water sources such as desalination plants, irrigation return flow, indirect reuse of treated wastewater through nearby lakes, and water supply from Veeranam Lake were also considered. The model simulated the interactions and flows between these different components, allowing for a thorough assessment of the water dynamics in the Chennai Basin.

The calibration and validation process of the WEAP model specifically focused on the storage levels in the Chembarambakkam and Poondi reservoirs, which are critical for water supply in the Chennai Basin. The model parameters related to inflow, outflow, evaporation, and other relevant processes were adjusted during calibration to minimize discrepancies between the simulated and observed storage levels during the water year 2015–2016.

The calibration phase involved iteratively adjusting the parameters to achieve the closest match between the simulated storage levels and the observed data for the specified period. This process aimed to find the best-fit parameter values that accurately represented the real-world behaviour of the system.

The calibration and validation processes ensure that the WEAP model provides accurate and reliable predictions for the Chennai Basin's water supply and demand dynamics. The results enhance the model's credibility and confidence, allowing it to simulate different scenarios and evaluate potential water management strategies effectively.

Water supply and demand projections were conducted until 2050 using the calibrated WEAP model to assess the future water availability and demand dynamics in the Chennai Basin. These projections incorporated historical data, hydrological characteristics, and operational parameters of water sources such as rivers, reservoirs, catchments, and desalination plants.

These findings emphasize the growing water demand in the Chennai Basin and highlight the need for proactive water resource planning and management. The increasing water requirements for domestic and irrigation purposes underscore the importance of implementing sustainable and efficient water management strategies to ensure an adequate and reliable water supply for the region's growing population and agricultural needs.

Then, the model was run with three rainfall scenarios: excess, normal, and deficit. These scenarios represented varying climatic conditions and their potential impact on water resources. The excess rainfall scenario simulated higher-than-normal precipitation, while the deficit rainfall scenario represented lower-than-normal rainfall. The normal rainfall scenario served as a baseline for typical climatic conditions.

The projections reveal significant challenges in meeting the water demand in the Chennai Basin as we approach the year 2050. Under both normal and deficit rainfall conditions, the unmet water demand is expected to increase over time.

Under normal rainfall conditions, the estimated unmet demand is projected to be 210 MCM in 2023, indicating a considerable shortfall in water supply. As we move forward, the situation worsens, with the unmet demand reaching 313 MCM in 2030, 462 MCM in 2040, and a substantial 602 MCM in 2050. These figures highlight the growing disparity between the available water supply and the ever-increasing demand from Chennai city and irrigation sectors.

The deficit rainfall scenario presents an even more challenging situation. In 2023, the projected unmet demand under deficit rainfall conditions is a staggering 1,140 MCM, reflecting the severity of the water shortage. This number increases to 1,228 MCM in 2030, 1,350 MCM in 2040, and a concerning 1,476 MCM in 2050. These figures illustrate the significant strain on water resources and the urgency to address the widening gap between supply and demand.

These findings highlight the need for effective mitigation strategies to bridge the projected supply and demand gaps. Mitigation measures such as increasing groundwater recharge, expanding reservoir capacities, enhancing desalination plant capabilities, utilizing additional storage options, and exploring alternative water sources should be carefully evaluated and implemented.

The results of the water supply and demand projections serve as valuable resources for policymakers, water managers, and stakeholders. They provide crucial insights for making informed decisions and formulating effective strategies to meet future water demand. By implementing appropriate mitigation measures, the Chennai Basin can work towards a more sustainable and resilient water supply system, ensuring the long-term availability of water resources while preserving the ecological balance.

Desalination plants are critical components of the water supply system in the Chennai Basin, particularly in meeting the city's water demand. Desalination involves removing salt and other impurities from seawater or brackish water to produce fresh water suitable for various uses. In the study, desalination plant expansion was considered a potential mitigation option to address the water scarcity issue. The existing desalination plants in Chennai, including the Minjur and Nemmeli desalination plants, were incorporated into the WEAP model to assess the effects of increasing their capacity.

The Minjur desalination plants, operational since 2010, has a capacity of 100 MLD. The Nemmeli desalination plant, operational since 2013, can treat 100 MLD of seawater per day. These plants have been instrumental in augmenting the water supply in Chennai. To further enhance the water supply, a third desalination plant is under construction at Nemmeli and is expected to be completed before the end of 2023. This new plant will have a capacity of 150 MLD, further increasing the desalination capacity in Chennai. Furthermore, the construction of the fourth desalination plant in Nemmeli is scheduled for completion in 2025. This plant will have a capacity of 400 MLD, significantly boosting the overall desalination capacity in the region. By 2025, the total supply from desalination plants is estimated to reach 750 MLD.

The WEAP model simulations provided valuable insights into the projected unmet demand for water in the Chennai Basin under different rainfall conditions. Under normal rainfall conditions, the simulations showed an increasing trend in the unmet demand over the years. In 2030, there was no unmet demand estimated. As time progressed, the unmet demand began to rise significantly. By 2040, the projected unmet demand reached 144 MCM, with 83 MCM for Chennai city demand, and by 2050, it further escalated to 295 MCM, with 160 MCM specifically for Chennai city demand.

The simulations also considered deficit rainfall conditions, demonstrating an even higher unmet demand for water. In 2030, the estimated unmet demand under deficit rainfall conditions was 952 MCM, with 245 MCM allocated for Chennai city demand. This indicates a significant water deficit that needs to be addressed. As the years progressed, the unmet demand under deficit conditions increased substantially. By 2040, the projected unmet demand rose to 1,082 MCM, with 351 MCM allocated for Chennai city demand, and by 2050, it reached 1,212 MCM, with 470 MCM specifically for Chennai city demand.

Additionally, the simulations examined the changes in groundwater storage after the expansion of desalination plants. The results indicated an initial increase in groundwater storage under normal rainfall conditions in 2025 and 2028, followed by a decline due to the increasing water requirements.

The findings from the WEAP model simulations highlight the importance of considering the expansion of desalination plants as a viable option to address the projected water supply-demand gap in Chennai. By increasing the desalination capacity, policymakers, water managers, and stakeholders can work towards ensuring a sustainable and resilient water supply system in the Chennai Basin. However, desalination is an energy-intensive operation and disposal of reject into the sea may lead to environmental issues.

Increasing the capacity of water treatment plants has been identified as a viable strategy to address the water demand in the Chennai Basin. The existing water treatment plants were analysed using the WEAP model, which incorporated data on their operational parameters, treatment capacities, and water quality standards.

Currently, most of the treated/partly treated water, of around 700 MLD, is discharged into the rivers. As treatment capacity is expected to increase, coping with urbanization, we assumed about 700 MLD of water may be generated by tertiary treatment. Currently, about 160 MLD of treated water is being supplied to the industries after the tertiary treatment from a plant located in the northwest part of Chennai. Another treatment plant in the southwest of Chennai treats 40 MLD of domestic sewage and discharges the same to the nearby lake. From 2025, we assumed that 700 MLD will be available for domestic supply either directly or indirectly after recharge as groundwater. Utilization of treated water can contribute to meeting the growing water demand in the basin.

The simulations conducted using the WEAP model provided valuable insights into the potential benefits and challenges associated with increasing the capacity of water treatment plants. The WEAP model results allowed for the evaluation of the impact of this strategy on water availability, water quality, and the ability to meet the projected demand.

However, the simulations revealed a higher unmet demand for water in deficit rainfall years. In 2025, the estimated unmet demand under deficit rainfall conditions was 938 MCM, with 204 MCM specifically for Chennai city demand. This significant increase in unmet demand highlights the vulnerability of water resources during drought conditions. By 2030, the projected unmet demand increased to 961 MCM, with 252 MCM for Chennai city demand. In 2040, the unmet demand rose to 1,091 MCM, with 358 MCM for Chennai city demand. Finally, by 2050, the simulations indicated an annual unmet demand of 1,221 MCM, with 478 MCM for Chennai city demand.

The simulations also revealed the trend in groundwater storage. In 2025, there was an increase in groundwater storage, suggesting a positive impact on aquifer replenishment (Figure 7(b)). However, the storage levels started to decrease due to the increasing water demand. This highlights the importance of sustainable groundwater management practices to ensure the long-term availability of groundwater resources.

To enhance the storage capacity of lakes and explore additional storage options, the model considered a scenario involving the desiltation of lakes and the utilization of quarries. Desilting involves removing accumulated sediment and debris from the lake beds, increasing reservoirs' effective storage capacity and water availability.

In the model, a 10% increase in volume was applied to the volume versus height curve of the lakes to account for the enhanced storage capacity resulting from desilting. Furthermore, quarries were considered as an additional storage option, with a total storage capacity of 28 MCM allocated to them. This added storage capacity could serve as an alternative source of water supply, with 50 MLD of water supply from the quarries.

By incorporating these measures, the model assessed the potential impact of increasing storage capacity in lakes through desilting and utilizing quarries as storage reservoirs. This comprehensive analysis allowed for an evaluation of enhanced water availability and management strategies in the Chennai Basin.

On the other hand, under deficit rainfall conditions, the annual unmet demand represents a shortfall in water supply due to lower-than-average rainfall. In 2030, the estimated unmet demand was 1,218 MCM, with 482 MCM for Chennai city. By 2040, the unmet demand was projected to be 1,340 MCM, with 605 MCM allocated to Chennai city. By 2050, the unmet demand was expected to reach 1,466 MCM, with 731 MCM allocated to Chennai city. These figures illustrate the significant water deficit during periods of below-average rainfall.

In terms of groundwater storage, under normal conditions, the projections indicate that the groundwater storage in the Chennai Basin will reach 207 MCM by the end of 2050. This suggests a relatively stable and sufficient groundwater supply to meet the water demands under normal rainfall conditions. However, under deficit rainfall conditions, the model estimated that the groundwater storage would be depleted, resulting in an estimated storage of 0 MCM. This indicates the dependence on alternative water sources and the potential challenges in meeting water demand solely through groundwater during periods of reduced rainfall.

Increasing groundwater recharge was explored as a mitigation strategy to address the water supply-demand gap in the Chennai Basin. A previous study by Pazhuparambil Jayarajan et al. (2022) indicated an initial groundwater recharge of 16% for the area. The scenario considered in this study involved increasing the recharge to the aquifer by an additional 10%, resulting in a total recharge of 26%. This intervention aimed to enhance groundwater resources and improve overall water availability in the study area.

Under normal rainfall conditions, the WEAP model simulations provided valuable insights into the annual unmet demand for water in the Chennai Basin. In 2025, the projected unmet demand was estimated to be 176 MCM, with 94 MCM specifically for Chennai city demand. However, from 2026 to 2050, no unmet demand was observed, indicating a balance between water supply and demand during those years.

The simulations projected a significantly higher unmet demand in deficit rainfall years than normal rainfall conditions. In 2025, the estimated unmet demand under deficit rainfall conditions was 981 MCM, with 347 MCM specifically for Chennai city's domestic and industrial demand. As we move forward, the simulations showed an upward trend in the unmet demand. In 2030, the projected unmet demand was 1,047 MCM (398 MCM for Chennai city demand), in 2040, it was 1,179 MCM (502 MCM for Chennai city demand), and in 2050, it reached 1,312 MCM (613 MCM for Chennai city demand). These figures demonstrate the significant challenges faced in meeting water demand during the deficit rainfall years, emphasizing the need for comprehensive strategies to bridge the supply-demand gap.

The simulations also show the trends in groundwater storage under normal rainfall conditions. From 2025 to 2041, there was an increasing trend in groundwater storage. However, due to the increasing water demand, the storage levels slowly decreased. In 2046, the storage level reached below the available groundwater fluctuation zone.

With a combination of all the interventions in the WEAP model, the simulations yielded promising outcomes for water supply and demand in the Chennai Basin. Under normal rainfall conditions, no unmet demand was projected until 2050, indicating a balanced water supply and demand scenario. Groundwater storage exhibited significant recovery, reaching its maximum capacity. These results indicate the effectiveness of implemented measures such as expanding desalination plant capacities, increasing water treatment capacities, and utilizing additional storage options in meeting water demand and improving groundwater availability.

Groundwater storage under normal conditions exhibited stable fluctuation, reaching maximum storage in the year 2037 (Figure 10(b)). Groundwater storage was estimated to be 102 MCM for the deficit rainfall condition. These results demonstrate that the integrated approach, considering strategies such as increasing groundwater recharge, expanding reservoir capacities, and utilizing quarries for storage, has significantly reduced water deficits in the Chennai Basin.

These findings underscore the importance of adopting a comprehensive and multi-pronged approach to water resource management. Implementing the suggested measures and strategies can enhance water supply resilience and reduce dependence on external sources in the Chennai Basin. Continuous monitoring, updating of model parameters and assumptions, and adapting strategies to changing climatic conditions and evolving water demands are crucial for sustainable water management.

Our study comprehensively analysed the Chennai Basin's water availability and demand dynamics and addressed how the water demand can be managed. By employing the WEAP model and examining various scenarios and interventions, we have gained valuable insights into the challenges and opportunities associated with water resource management. This section discusses the implications of our findings and their significance in the context of addressing water scarcity and ensuring long-term water security.

The projections of water supply and demand dynamics until 2050 highlight the escalating water scarcity in the Chennai Basin. The upward trend in water requirements for domestic and irrigation purposes emphasizes the pressing need for proactive water resource planning and management. The projected increase in water demand underscores the importance of implementing sustainable and efficient water management strategies to meet the population's growing needs and agriculture.

Desalination plants emerge as crucial components in bridging the water supply-demand gap in Chennai. Expanding desalination plant capacities offers a viable solution to augmenting water supply. The simulations illustrate that increasing desalination capacity can substantially reduce the unmet demand under both normal and deficit rainfall scenarios. These findings highlight the importance of continued investment in desalination infrastructure to enhance water availability.

Balancing the need for increased water supply through desalination with the preservation of local ecosystems is a critical consideration for sustainable water management in the Chennai region. Proper site selection, rigorous environmental impact assessments, and mitigation measures are essential to minimize the ecological impact of desalination projects while meeting the growing water demands of the area (Roberts et al. 2010).

Increasing the capacity of water treatment plants proves to be another effective strategy for managing the water demand. A significant contribution can be made towards meeting the water requirements by optimizing the utilization of treated water and ensuring its availability for domestic supply. This strategy can aid in mitigating water deficits and achieving a more balanced water supply-demand scenario. However, integrating this approach into practical implications presents potential challenges in optimizing the utilization of treated water and ensuring its availability for domestic supply, which can affect its effectiveness in mitigating water deficits and achieving a more balanced water supply-demand scenario.

The analysis of increasing reservoir capacities through desilting and utilizing quarries as storage options shows potential benefits in addressing the water supply-demand gap. However, incorporating these solutions into practical implications introduces potential challenges, such as the logistical and environmental considerations of desilting and quarry utilization, which may impact their overall effectiveness in enhancing water availability and reducing unmet demand, especially under normal rainfall conditions. Furthermore, the findings related to increasing groundwater recharge underscore the importance of this strategy in improving groundwater resources and overall water availability.

The combined assessment of various interventions demonstrates the power of an integrated approach to water resource management. A substantial reduction in water deficits can be achieved by considering multiple strategies simultaneously, including desalination plant expansion, water treatment capacity enhancement, storage options, and groundwater recharge. This integrated approach optimizes water availability from various sources and enhances the resilience of the water supply system.

The insights provided by our study offer valuable guidance to policymakers, water managers, and stakeholders in the Chennai Basin. The results emphasize the urgent need to prioritize and implement a combination of strategies to ensure long-term water security. These strategies encompass a range of approaches and initiatives, each of which plays a critical role in addressing the water supply-demand gap in the region. We highlighted several key areas where policy and decisionmakers can focus their efforts.

Promoting and incentivizing rainwater harvesting practices at the individual and community levels can significantly augment the available water supply. Policy measures can include tax incentives, rebates, and educational campaigns to encourage the widespread adoption of RWH systems. The public needs to also maintain them properly to keep up their efficiency.

Investing in decentralized water treatment facilities allows for more efficient and localized water purification, reducing the strain on centralized infrastructure. Policymakers can support the development of decentralized systems by providing grants, technical assistance, and regulatory frameworks to facilitate their implementation.

Promoting the construction of zero discharge buildings, where the used water is recycled and reused, can effectively reduce the overall demand on the municipal water supply as well as the load on the sewer lines. Incentives such as zoning exemptions, expedited permitting, and recognition for sustainable building practices can drive the adoption of these eco-friendly structures.

Implementing comprehensive incentive programmes that reward water conservation and sustainable practices can have a profound impact on reducing water consumption. Such programmes can include tiered pricing structures, water-efficient appliance rebates, and financial incentives for industries adopting water-saving technologies.

The effectiveness of these measures can be enhanced through ongoing monitoring of water resources, adaptive management, and collaboration among stakeholders. Regular data collection and analysis enable informed decision-making and the identification of areas requiring immediate attention. Collaboration among government agencies, NGOs, businesses, and communities is essential to ensure a coordinated and sustainable approach to water management.

Acknowledging the limitations and potential sources of uncertainty associated with the data analysis and modelling process is essential. Some of the limitations of this study are as follows.

Data Limitations: The accuracy and availability of data could have posed challenges during the analysis. Sometimes, data may have been limited or unavailable for specific variables or periods. Assumptions or approximations may have been made to fill data gaps or overcome limitations, which could introduce some level of uncertainty.

Assumptions and Simplifications: The modelling process involved making certain assumptions and simplifications to represent the complex water supply-demand system. These assumptions may not perfectly reflect real-world conditions and could introduce uncertainties into the results. Sensitivity analyses may have been performed to assess the robustness of the model to different assumptions.

External Factors: The study may have been influenced by external factors beyond the control of the analysis, such as changes in climate patterns, population growth, or policy decisions.

Seawater Intrusion and Submarine Groundwater Discharge: In coastal areas like the Chennai Basin, the influence of seawater intrusion and submarine groundwater discharge on the water balance has not been discussed in this study. If these aspects were not considered, it could add an additional layer of uncertainty to the overall analysis.

Data Projection and Future Scenarios: The analysis has relied on projected data and assumptions about future conditions, such as population growth rates, water demand patterns, or climate change scenarios.

Despite these limitations, the data analysis provided valuable insights into the current water supply-demand dynamics and the potential impacts of different scenarios. The results and discussion based on the analysis formed the basis for informed decision-making and formulating strategies to address the water supply-demand gap in the study area.

In conclusion, our study has successfully bridged a longstanding research gap in understanding water availability and demand dynamics in the Chennai Basin. Through the meticulous application of the WEAP model and a thorough analysis of diverse scenarios, we have achieved a holistic evaluation of the challenges and prospects linked with water resource management. By unravelling insights into groundwater recharge, reservoir capacities, desalination plants, water treatment facilities, and alternative storage options, our study emerges as a significant and pioneering contribution to the field. Our findings illuminate the intricacies of water balance dynamics and provide pragmatic guidance for policymakers and water managers in pursuing sustainable solutions. Armed with these novel insights, we aspire to chart a course for more efficient and resilient water resource management strategies within the region.

Groundwater recharge is a pivotal factor in addressing the pressing issue of water scarcity in Chennai. The study unequivocally underscores the pivotal role of augmenting groundwater recharge as a formidable mitigation strategy to enhance water availability. By endorsing practices such as artificial recharge, rainwater harvesting, and enhanced land management techniques, the study accentuates the far-reaching advantages of replenishing subterranean aquifers, thereby ensuring a robust and sustainable groundwater reservoir.

Furthermore, the synergistic impact of the amalgamated interventions assessed in this study is a striking revelation, shaping the dynamics of both water supply and demand. The strategic amalgamation of approaches, including intensified groundwater recharge, expanded reservoir capacities, fortified desalination plant capabilities, the utilization of supplementary storage alternatives, and the elevation of water treatment capacities, is projected to reduce the anticipated water deficit substantially. The simulations predict that implementing these integrated interventions could significantly curtail the annual unmet demand, showcasing the promise of this multifaceted approach.

These findings unequivocally emphasize the significance of adopting an integrative stance towards water management, where various strategies synergistically reinforce each other to optimize water availability. By concurrently addressing the multiple dimensions of the water system, the study underscores the potential for achieving a harmonized and sustainable water supply in Chennai.

Policymakers, governmental bodies, and stakeholders must accord primacy to groundwater recharge initiatives and the pragmatic execution of the combined interventions outlined in this study. This necessitates collaborative endeavours, robust governance, and sustained infrastructure and awareness campaign investments. The continued success of these measures hinges on vigilant monitoring, meticulous evaluation, and adaptive strategies that respond to evolving circumstances and emerging challenges. This adaptive approach is crucial for ensuring the long-term water security of Chennai.

This study is an invaluable repository of insights into the intricate water dynamics prevalent in Chennai and serves as a blueprint for enhancing water resource management. By spotlighting groundwater recharge and wholeheartedly embracing the combined interventions, Chennai can fortify its resilience, mitigate the spectre of water scarcity, and secure an enduring and dependable water supply for its burgeoning populace.

The authors thank the Department of Science and Technology, Government of India (Grant No: DST/TM/WTI/WIC/2K17/82(G)) for financial support. Additionally, we would like to thank the Stockholm Environment Institute for their kind waiver of the license fee for WEAP.

L.E.: conceptualized the idea of this study. P.V.R.: collected data, developed model, analysed and interpreted. M.R.: collected data and interpreted results. All authors were involved in the writing and approval of the final manuscript.

All data collected, generated, and analyzed in this study are available upon request per ethical guidelines and permission of the funding agency.

The authors declare there is no conflict.