Forecast Informed Reservoir Operations Using Ensemble Streamflow Predictions for a Multi-Purpose Reservoir in Northern California

Forecast informed reservoir operations (FIRO) is a methodology that incorporates short to mid-range precipitation and flow forecasts to inform the flood operations of reservoirs. The Ensemble Forecast Operations (EFO) alternative is a probabilistic approach of FIRO that incorporates ensemble streamflow predictions (ESPs) made by NOAA’s California-Nevada River Forecast Center (CNRFC). With the EFO approach, release decisions are made to manage forecasted risk of reaching critical operational thresholds. A water management model was developed for Lake Mendocino, a 111,000 acre-foot reservoir located near Ukiah, California, to evaluate the viability of the EFO alternative to improve water supply reliability without increasing downstream flood risk. To provide a comprehensive period of analysis, retrospective ensemble forecasts, i.e. hindcasts, of Lake Mendocino inflow and downstream watersheds were generated by the CNRFC using the Hydrologic Ensemble Forecast System model over a 26-year period (1985-2010). Lake Mendocino is a dual use reservoir, which is owned and operated for flood control by the United States Army Corps of Engineers and is operated for water supply by the Sonoma County Water Agency. Lake Mendocino releases water into the Russian River which provides habitat to endangered and threatened salmonid species, and water supply for numerous municipalities and a thriving vinicultural region. Due to recent changes in the operations of an upstream hydroelectric facility, this reservoir has suffered from water supply reliability issues since 2007. Results of this investigation for Lake Mendocino demonstrate that the EFO alternative may be a viable approach for reservoir management to improve water supply reliability without increasing flood risk to downstream communities. The EFO alternative provides a management framework that could be adapted and applied to other flood control reservoirs throughout the United States.