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

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 towards earlier flow recession with some scenarios forecasting a complete loss of passable flow conditions for outmigrating smolts during drought. Significant mitigation of these adverse changes appears possible; simulating reservoir releases and reduced evapotranspiration (ET) through forest management produces increases in summer flows large enough to fully offset the predicted declines associated with even the direst climate projections. Adapting to reductions in flow during the salmonid outmigration season is more challenging, although reservoir releases or forest management could provide passable flow conditions that extend 9-35 days further into the outmigration season. In contrast, cessation of all surface water diversion did not meaningfully offset the predicted future reductions in streamflow. Streamflow enhancement effects were much 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 with storage-limited hydrogeologic conditions. Successful adaptation to climate change in terms of streamflow may require increased emphasis on strategies such as reservoir releases and/or vegetation management that would reduce 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.