Filling the gaps: Co-designing hydroclimate projections to support US water security and policy initiatives

The US Secure Water Act of 2010 requires several US agencies to report to Congress every five years on future water-related mission vulnerabilities. Over the last 15 years, 21st century climate projection datasets from the Coupled Model Intercomparison Projects (CMIP) have been downscaled and used to drive hydrologic and streamflow scenarios across the Contiguous United States (CONUS).  The resulting datasets provide input for federal and state agency planning, guidance and policy, for water resources applications from watershed to regional scales, and for the climate-water research community. The advent of CMIP6 triggered the co-development of new, updated hydroclimate impact projections, which is now proceeding via a multi-agency effort that integrates researchers with stakeholders from US federal water, climate and energy agencies. Notably, the scientific approaches used in previous assessments have since revealed capability gaps that US agencies now seek to fill with newer, more robust methods and models. The need to address these gaps motivated a joint effort between US water agencies and researchers to strengthen the scientific underpinnings of the projections, the better to create more credible public datasets for use in agency planning and policy development. This work required creating new strategies for system vulnerability quantification, continental-scale process-based hydrological modelling, multi-decadal high-resolution surface meteorology, and water agency guided performance metrics to inform model training and evaluation. Several hundred CMIP6-based hydroclimate scenarios have been created and tailored to provide Earth system indicators directly linked to water agency planning needs. This presentation summarizes this hydroclimate dataset initiative and highlights the critical role of integrated researcher-stakeholder engagement in achieving fit-for-purpose and actionable large-domain hydroclimate outcomes.