Developing Effective Flow Enhancement Strategies for Salmonid Recovery and Climate Change Adaptation in Central California's Coastal Watersheds

ABSTRACT An integrated hydrologic model of a rainfall‐dominated catchment was used to quantify existing and predict future streamflows relative to salmonid needs and evaluate the efficacy of flow enhancement strategies. Future climate scenarios indicate declines in summer rearing habitat of up to 22% and a shift toward earlier flow recession with some scenarios forecasting a complete loss of passable flow conditions for out‐migrating smolts during drought. Significant mitigation of these adverse changes appears possible; simulating reservoir releases and reduced evapotranspiration (ET) through simplified hypothetical forest management scenarios predicts increases in summer flows large enough to fully offset even the driest climate projections. Adapting to reductions in flow during the spring salmonid out‐migration season is more challenging. Nevertheless, reservoir releases or reduced ET through potential forest management actions could provide passable flow conditions that extend 9–35 days further into the out‐migration season. In contrast, cessation of all surface water diversion did not meaningfully offset predicted reductions in streamflow. Streamflow enhancement effects were greater in the portions of the watershed underlain by Coastal Belt units of the Franciscan Complex compared to areas underlain by Central Belt units, suggesting that flow enhancement work may be less impactful in basins that have geologic constraints that limit storage. Successful adaptation to climate change in terms of streamflow may require increased emphasis on strategies such as reservoir releases and/or vegetation management actions capable of reducing forest ET. Significant additional research linking specific forest treatments to long‐term ET reductions and streamflow consequences is needed to optimize streamflow benefits of forest treatments.