Characterizing Watershed Responses and Reservoir Operational Flexibilities: Analyses to Support Forecast‐Informed Reservoir Operations (FIRO) Planning and Assessments

Recent advances in weather forecast skill provide opportunities to manage reservoirs more flexibly using strategies like Forecast‐Informed Reservoir Operations (FIRO). This is especially critical as changes in climate, population, and land use make balancing reservoir uses to minimize flood risks and maximize water availability more challenging. FIRO viability depends on the ability to forecast reservoir inflows with sufficient accuracy and lead time that storage and releases downstream can be maintained at safe levels. In this study, we develop a transferable approach for gaining early insights into potential challenges and opportunities for FIRO by (a) identifying how precipitation amount, phase, and soil moisture mediate runoff generation, and (b) characterizing how the timing and magnitude of reservoir inflows relate to the amount of space available for storage and the ability to safely release water without causing downstream flooding. The buffering capacity of the reservoir to accommodate inflows is quantified as an operational flexibility metric, which can help to inform streamflow forecast lead time requirements. Using historical hydrologic and meteorologic data and an automated baseflow separation routine, we assessed operational flexibility at 32 reservoirs in California and western Nevada. We demonstrate that operational flexibility varies regionally, with less flexibility in snow‐dominated basins, where the amount of water that can be released from reservoirs is more limited relative to those in rain‐dominated basins. A warming climate will further challenge this flexibility, indicating that FIRO efforts in snow‐dominated basins will likely need to be coupled with other adaptation strategies.