Accounting for built and natural storage is necessary to estimate the true vulnerability of downstream water supplies

Socio-environmental systems downstream of mountainous, higher elevation (upland) systems in western United States (US) are dependent on both natural (e.g., snowpacks, groundwater) and built (e.g., reservoirs) storage. Increasingly, assessments of these systems susceptibility or inability to cope with adverse effects of climate change (vulnerability) indicate the potential for severe demand shortfalls as natural storage from snow declines and increased winter runoff stresses reservoir management. Adaptation strategies such as conjunctive use have been put forward as one promising way to lessen the damage caused by these changes to natural and built storage by banking winter runoff as groundwater via reservoir management. Identification of where this type of adaptation may be most impactful for enhancing downstream resilience under climate change requires robust characterization of both natural and built storage, but also infrastructure capacity, policy constraints, and demand dynamics. However, because they are often conducted at basin-scale, existing vulnerability assessments make simplifying assumptions about these variables; a disconnect that motivates the following research questions: 1) How does storage composition influence the ability of upland reservoir inflows to meet variable demands under policy and infrastructure constraints? 2) How do uncertainty contributions from climate, policy, demand, and infrastructure vary across systems? 3) How can information about potential storage changes and uncertainty help better characterize systems most suited for adaptation strategies such as conjunctive use? Building on recent work, we first characterize historical and projected changes in reservoir inflows and snow storage using data from the Coupled Model Intercomparison Project phase 5 (CMIP5) for ~50 upland reservoirs with minimal upstream disturbance throughout the western US. Because future projections of infrastructure, policy, and demand are difficult to obtain, we rely on the development of scenarios to assess variability in future supply-demand dynamics. We then quantify uncertainty contributions from climate, policy, demand, and infrastructure to develop a policy typology for targeted adaptation efforts based on a more holistic view of water supply and demand.