Operationalizing Equity in Quantitative Water Resources Systems Modeling

Equity is increasingly central to water resources management as cities expand and socioeconomic inequalities persist. As climate change intensifies, urban water managers are tasked with ensuring reliable water supplies while navigating the complex distribution of benefits and burdens across diverse communities. While equitable access to clean water is a core component of Sustainable Development Goal 6: Ensure availability and sustainable management of water and sanitation for all, quantitative water resources models often lack an explicit representation of equity, instead prioritizing aggregate metrics such as cost-efficiency or system-wide reliability. Without explicit equity representations, models are limited in their ability to evaluate distributional outcomes and may unintentionally reinforce existing inequalities. In this study, we operationalize distributive equity within a quantitative water resources modeling framework to assess how equity principles influence infrastructure and operational decisions. We explore these principles in the Federal District of Brazil, a region with a high drought risk, rapid urbanization, and large income inequality. Using the WaterPaths water supply model coupled with the Borg multi-objective evolutionary algorithm, we explore how alternative distributive principles influence drought risk and equity outcomes within optimized water supply portfolios. In WaterPaths, we compare three distinct principles of distributive equity, Rawlsian, Utilitarian, and Sufficietarian, by developing three rival multi-objective problem formulations. We optimize each formulation and explore how trade-offs between conflicting objectives and equity outcomes change across the formulations. Results indicate that the choice of a specific equity principle can fundamentally shift trade-offs and alter the perceived performance of candidate management strategies. Our findings contribute to more transparent and robust system planning approaches, offering a pathway to integrate social justice directly into water resources decision-making under deep uncertainty.