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While optimality is a foundational mathematical concept in water resources planning and management, ''optimal'' solutions may be vulnerable to failure if deeply uncertain future conditions deviate from those assumed during optimization. These vulnerabilities may produce severely asymmetric impacts across a region, making it vital to evaluate the robustness of management strategies as well as their impacts for regional stakeholders. In this study, we contribute a multistakeholder many-objective robust decision making (MORDM) framework that blends many-objective search and uncertainty analysis tools to discover key tradeoffs between water supply alternatives and their robustness to deep uncertainties (e.g., population pressures, climate change, and financial risks). The proposed framework is demonstrated for four interconnected water utilities representing major stakeholders in the ''Research Triangle'' region of North Carolina, U.S. The utilities supply well over one million customers and have the ability to collectively manage drought via transfer agreements and shared infrastructure. We show that water portfolios for this region that compose optimal tradeoffs (i.e., Pareto-approximate solutions) under expected future conditions may suffer significantly degraded performance with only modest changes in deeply uncertain hydrologic and economic factors. We then use the Patient Rule Induction Method (PRIM) to identify which uncertain factors drive the individual and collective vulnerabilities for the four cooperating utilities. Our framework identifies key stakeholder dependencies and robustness tradeoffs associated with cooperative regional planning, which are critical to understanding the tensions between individual versus regional water supply goals. Cooperative demand management was found to be the key factor controlling the robustness of regional water supply planning, dominating other hydroclimatic and economic uncertainties through the 2025 planning horizon. Results suggest that a modest reduction in the projected rate of demand growth (from approximately 3% per year to 2.4%) will substantially improve the utilities' robustness to future uncertainty and reduce the potential for regional tensions. The proposed multistakeholder MORDM framework offers critical insights into the risks and challenges posed by rising water demands and hydrological uncertainties, providing a planning template for regions now forced to confront rapidly evolving water scarcity risks.

Rising development costs and growing concerns over environmental impacts have led many communities to explore more diversified water management strategies. These ''portfolio''-style approaches integrate existing supply infrastructure with other options such as conservation measures or water transfers. Diversified water supply portfolios have been shown to reduce the capacity and costs required to meet demand, while also providing greater adaptability to changing hydrologic conditions. However, this additional flexibility can also cause unexpected reductions in revenue (from conservation) or increased costs (from transfers). The resulting financial instability can act as a substantial disincentive to utilities seeking to implement more innovative water management techniques. This study seeks to design portfolios that employ financial tools (e.g., contingency funds and index insurance) to reduce fluctuations in revenues and costs, allowing these strategies to achieve improved performance without sacrificing financial stability. This analysis is applied to the development of coordinated regional supply portfolios in the ''Research Triangle'' region of North Carolina, an area comprising four rapidly growing municipalities. The actions of each independent utility become interconnected when shared infrastructure is utilized to enable interutility transfers, requiring the evaluation of regional tradeoffs in up to five performance and financial objectives. Diversified strategies introduce significant tradeoffs between achieving reliability goals and introducing burdensome variability in annual revenues and/or costs. Financial mitigation tools can mitigate the impacts of this variability, allowing for an alternative suite of improved solutions. This analysis provides a general template for utilities seeking to navigate the tradeoffs associated with more flexible, portfolio-style management approaches.

A considerable fraction of urban water supply capacity serves primarily as a hedge against drought. Water utilities can reduce their dependence on firm capacity and forestall the development of new supplies using short‐term drought management actions, such as conservation and transfers. Nevertheless, new supplies will often be needed, especially as demands rise due to population growth and economic development. Planning decisions regarding when and how to integrate new supply projects are fundamentally shaped by the way in which short‐term adaptive drought management strategies are employed. To date, the challenges posed by long‐term infrastructure sequencing and adaptive short‐term drought management are treated independently, neglecting important feedbacks between planning and management actions. This work contributes a risk‐based framework that uses continuously updating risk‐of‐failure (ROF) triggers to capture the feedbacks between short‐term drought management actions (e.g., conservation and water transfers) and the selection and sequencing of a set of regional supply infrastructure options over the long term. Probabilistic regional water supply pathways are discovered for four water utilities in the “Research Triangle” region of North Carolina. Furthermore, this study distinguishes the status‐quo planning path of independent action (encompassing utility‐specific conservation and new supply infrastructure only) from two cooperative formulations: “weak” cooperation, which combines utility‐specific conservation and infrastructure development with regional transfers, and “strong” cooperation, which also includes jointly developed regional infrastructure to support transfers. Results suggest that strong cooperation aids utilities in meeting their individual objectives at substantially lower costs and with less overall development. These benefits demonstrate how an adaptive, rule‐based decision framework can coordinate integrated solutions that would not be identified using more traditional optimization methods.

5Emerging water scarcity concerns in many urban regions are associated with several 6 deeply uncertain factors, including rapid population growth, limited coordination across ad-7 jacent municipalities and the increasing risks for sustained regional droughts. Managing 8 these uncertainties will require that regional water utilities identify coordinated, scarcity-9 mitigating strategies that trigger the appropriate actions needed to avoid water shortages 10 and financial instabilities. This research focuses on the Research Triangle area of North 11 Carolina, seeking to engage the water utilities within Raleigh, Durham, Cary and Chapel 12Hill in cooperative and robust regional water portfolio planning. Prior analysis of this region 13 through the year 2025 has identified significant regional vulnerabilities to volumetric short-14 falls and financial losses. Moreover, efforts to maximize the individual robustness of any 15 of the mentioned utilities also have the potential to strongly degrade the robustness of the 16 others. This research advances a multi-stakeholder Many-Objective Robust Decision Mak-17 ing (MORDM) framework to better account for deeply uncertain factors when identifying 18 cooperative drought management strategies. Our results show that appropriately designing 19 adaptive risk-of-failure action triggers required stressing them with a comprehensive sample 20 of deeply uncertain factors in the computational search phase of MORDM. Search under 21 the new ensemble of states-of-the-world is shown to fundamentally change perceived perfor-22 mance tradeoffs and substantially improve the robustness of individual utilities as well as 23 the overall region to water scarcity. Search under deep uncertainty enhanced the discovery 24 1of how cooperative water transfers, financial risk mitigation tools, and coordinated regional 25 demand management must be employed jointly to improve regional robustness and decrease 26 robustness conflicts between the utilities. Insights from this work have general merit for 27 regions where adjacent municipalities can benefit from cooperative regional water portfolio 28 planning. 29 1 INTRODUCTION 30 Understanding and exploring the combined states-of-the-world (SOWs) that shape wa-31 ter supply risks is a fundamental challenge in any drought mitigation application, defining 32 perceptions and preferences related to key performance objectives, vulnerabilities, and/or 33 the robustness of a system. Fundamental to this challenge is the choice of what uncertain 34 factors should be sampled or included in generating alternative scenarios. Broadly, there 35 are two dominant methodologies that are at present used in water supply planning contexts: 36 (1) pre-specified deterministic scenarios/narratives and (2) globally sampled stochastic sce-37 nario analysis. As an example of deterministic scenario-based planning, Liu et al. (2008) 38 analyzed four semi-arid river basins in the south-western United States seeking to find water 39 management strategies based on integrat...