Amy L Steimke, Bangshuai Han, Jodi Brandt, Alejandro Flores

Climate change directly affects the hydrologic cycle in mountainous watersheds, which has consequences for downstream users. Improved water projections under diverse potential climate futures are critical to improving water security and management in these watersheds. The hydrologic science researchers and water resource managers, however, often focus on different metrics of flow regimes in changing climates. The research community tends to more closely focus on biophysical state and flux variables of the hydrologic system. Managers, meanwhile, tend to focus on key administrative benchmarks that govern the operation of complex water storage and distribution systems. Here, we examine potential hydrologic changes in a water supply basin in the western United States in the context of both biophysical states and fluxes, as well as from the perspective of how those changes map onto key variables that govern the administration of water resources in the region. The study site consists of the Upper Boise River Basin, ID. This snowmelt-dominated, mountainous watershed that supplies water to a semi-arid, agriculturally intensive and rapidly urbanizing region. Using the Envision integrated modeling framework, we created a hydrologic model and simulated hydrologic response to the year 2100 using six diverse climate scenarios. Annual discharge increased from historical values by an average of 13% across all climate scenarios with a range of increase of 6-24%, reflecting an increase in the precipitation in the climate projections. Runoff timing was altered, with peak discharge occurring 4-33 days earlier and center of timing of streamflow occurring 4-17 days earlier by midcentury. Examining potential changes in the date junior water rights holders begin to be curtailed regionally (the Day of Allocation), we found that the Day of Allocation occurs up to 14 days earlier by 2100 across all climate scenarios, with one scenario suggesting this date could occur over a month earlier. These results suggest that current methods and policies of water rights accounting and management may need to be revised moving into the future.