Abstract Developing regions face critical water security challenges driven by rapid urban growth, economic development, and climate change. In India, these issues are particularly evident in Pune, the country's 9th most populated city. It is evolving into a sprawling urban agglomeration expected to grow from 7 to 11 million residents by mid‐century. The city's aging water‐supply system is ill‐equipped to ensure water access during droughts lasting 2–3 years, particularly for residents in informal settlements. We present a policy‐evaluation model to assess options for addressing future urban freshwater insecurity. The model uses a coupled multi‐agent systems approach that integrates human‐environment interactions and responses to future drought, population, and economic conditions. Under business‐as‐usual for a mid‐century, multi‐year drought, major reservoirs dry up and groundwater levels decrease dramatically. The water use Gini coefficient exceeds 0.5, indicating severe inequality where most low‐income individuals face: (a) unaffordable water costs (10%–18% of income), (b) vulnerability (<40 L daily), and (c) prolonged shortages (>6 continuous months). Comprehensive interventions, combining supply‐ and demand‐side measures, cut the water use Gini coefficient in half and lower water costs by two‐thirds. Implementing a strategic subset of interventions creates synergies that significantly enhance water security, yet remains insufficient for the low‐income population. This study highlights how growing inequalities in urban water access exacerbate water security challenges, even under a suite of mitigating measures. In all scenarios, additional drought emergency supply will be required to address water insecurity of the lowest 10% income population.

Unequal water access will be a major driver of increasing water insecurity in the 21st century, exacerbating the impacts of climate change. An estimated one billion people in cities in low- and middle-income countries currently face varying degrees of public water supply interruptions, subjecting them to unequal water access. This number is expected to grow, as water scarcity intensifies and supply infrastructure deteriorates. Yet, insights into the quantitative effect of water supply intermittency on urban water access inequality are so far limited. Here, we assess insights from hydro-economic multi-agent modeling case studies in South Asia and in the Middle East to analyze the effects of intermittent public water supply on the water security of heterogeneous urban household populations. We find that public water supply interruptions lead to severe disparities in household water consumption across cases. This also leads to household reliance on costly alternative water sources, jeopardizing water affordability. By 2050, climate change and population growth exacerbate the effects of water supply intermittency, causing severe deterioration in water security and increasing water access inequality. The results indicate that improved monitoring of water access inequality and reducing supply intermittency are key to mitigating urban water insecurity in the coming decades.

Pune, near Mumbai, is India’s is 9th most populated city. As an emerging megacity, Pune is projected to grow from 7.4 to 11.4 million residents by 2050. At that time, a two-year drought under moderate climate change would lead to extraordinary water supply challenges, especially for the urban poor. Without policy interventions by mid-century, the low-income urban population will be unduly affected by water shortages as indicated by a water supply Gini coefficient exceeding 0.4. This inequity occurs as low-income households experience unaffordable water costs (10%-15% of income), and most receive 6 continuous months. Using a coupled human-natural systems model, we explored various measures aimed at alleviating this catastrophe. While many actions are shown to be ineffective, a comprehensive suite of supply-side and demand-side interventions can reduce inequity, cutting the future Gini coefficient in half, and reducing water expenditures from 15% to 5% of income. The single most effective action comes from a water-market structure that enables surrounding agricultural groundwater to be pumped and provided to the city during drought periods. However, further measures are needed to secure this expensive water for the urban poor, as it can be readily captured by wealthy urban households.

&lt;p&gt;Adapting to growing urban water scarcity requires accurate assessments of present and future water security challenges. An estimated 1 billion people live in cities with intermittent public water supply, often resulting in highly unequal access to water. Under these conditions, households with below-average water access are most exposed to water insecurity. As a result, the full extent of water insecurity could substantially exceed the impacts identified by aggregate water security metrics. Here, we extend an existing coupled human and natural system model of the entire water sector in the Indian Upper Bhima basin, in order to analyze the degree to which water access inequality exacerbates urban water insecurity. The model integrates hydrologic modeling with urban water allocation institutions and water user agents, using data from a quantitative survey of almost 2,000 households in and around Pune, remote sensing data, as well as village-level census and water supply data. We use the model to assess water security impacts under historical and future droughts and various levels of supply augmentation. We find that a large share of households falls below critical water security thresholds before impacts are detected by aggregate metrics. While an unequal water distribution prevails, supply augmentation projects require several times the scale to meet given per capita water supply targets across the population than they would under a more equitable distribution. The findings demonstrate the extent to which current assessments of future urban water insecurity can underestimate the challenges ahead.&lt;/p&gt;

Systems models of the Food–Water–Energy (FWE) nexus face a conceptual difficulty: the systematic integration of local stakeholder perspectives into a coherent framework for analysis. We present a novel procedure to co-produce and systematize the real-life complexity of stakeholder knowledge and forge it into a clear-cut set of challenges. These are clustered into the Pressure–State–Response (PSIR) framework, which ultimately guides the development of a conceptual systems model closely attuned to the needs of local stakeholders. We apply this approach to the case of the emerging megacity Pune and the Bhima basin in India. Through stakeholder workshops, involving 75 resource users and experts, we identified 22 individual challenges. They include exogenous pressures, such as climate change and urbanization, and endogenous pressures, such as agricultural groundwater over-abstraction and land use change. These pressures alter the Bhima basin’s system state, characterized by inefficient water and energy supply systems and regional scarcity. The consequent impacts on society encompass the inadequate provision with food, water, and energy and livelihood challenges for farmers in the basin. An evaluation of policy responses within the conceptual systems model shows the complex cause–effect interactions between nexus subsystems. One single response action, such as the promotion of solar farming, can affect multiple challenges. The resulting concise picture of the regional FWE system serves resource users, policymakers, and researchers to evaluate long-term policies within the context of the urban FWE system. While the presented results are specific to the case study, the approach can be transferred to any other FWE nexus system.

Green walls are becoming a popular infrastructure choice in densely built urban environments, due to their multiple benefits. However, high and vastly variable water requirements of these systems are preventing their further widespread. Only a small number of studies have investigated water needs of green walls, even though this can help to design more optimal systems with increased benefits. Additionally, the knowledge on interactions between plant uptake and climate conditions (temperature and humidity) is lacking. The aim of this study was to understand daily water requirements of five plant species (C. appressa, N. obliterata, L. muscari, M. parvifolium and O. japonicus) used in greywater treating green walls, across different seasons, temperature, and humidity conditions of temperate-oceanic climate (common in parts of Australia, US and Europe). The results showed that during summer, dominant water uptake processes were plant uptake and transpiration, resulting in three to four times higher water needs than during winter, when evaporation is a major effect. Top levels of the multi-level green wall exhibited significantly higher plant activity compared to bottom levels, showing four times greater water uptake. Temperature and humidity changes during winter caused the change in water uptake of plants, pointing to different growing and activity patterns of tested plants. During summer only N. obliterata showed temperature and humidity dependence. Annual plant water uptake and other practical recommendations are given based on the results. Even though this study focused on water requirements of greywater treating green walls, its findings can also inform traditional green wall designs.