We present a rigorous quantitative, systems-based model to measure a municipality’s water portfolio security using four objectives: Sustainability, Resilience, Vulnerability, and Cost (SRVC). Water engineers and planners can operationalize this simple model using readily available data to capture dimensions of water security that go far beyond typical reliability and cost analysis. A metro area’s resilience to drought is higher than that of any of its constituent municipalities, underscoring the benefits of coordinated water planning at the metropolitan area-level. Municipalities can enhance water security along cost-neutral, adaptive policy pathways by considering broader measures of water portfolio security that consider the "what if" of worst-case scenarios where system reliability fails. Because water supply is a critical lifeline infrastructure, consideration of worst-case failure in design is essential.