On the need to integrate uncertainty into U.S. water resource planning

A changing climate is expected to introduce uncertainty into water resource management decision making. We examined the latest publicly-available, state-level guidance regarding the management of water supplies and demands concerning risks associated with drought, flooding, and climate change. We found state-level guidance supplementing the federally-backed flood mitigation program to be updated most regularly (54% in the last 5 years; 84% in the last decade). Yet, the underlying floodplain mapping data these local planning efforts rely on are acknowledged by the Federal Emergency Management Agency (FEMA) to be chronically outdated. Drought planning guidance was found to be most outdated (16% last updated in the last 5 years; 18% almost two decades ago), and across the U.S., almost universally (94%) reactive (emergency response) rather than proactive (mitigation or management). Although 79-94% of states provide some level of guidance regarding water supply and demand, the projections themselves may significantly predate the guidance. Many (70%) U.S. states still lack climate change impact guidance, particularly non-coastal states and those impacted by increased water scarcity rather than flooding. Strategies are rare (4%) for addressing the impacts of increased variability and uncertainty to meet inelastic demands with finite supplies. We conclude significant gaps exist in planning to address known or projected risks of climate-related impacts. Specific recommendations, including the implementation of a nationwide water census, are provided to improve both the data and knowledge base of water management and reduce current vulnerabilities.