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

August 17, 2020

Sonoma Water Engineer Chris Delaney led the development of a forecast informed reservoir operations (FIRO) decision support system, Ensemble Forecast Operations (EFO), for Lake Mendocino with contributions from researchers at CW3E, U.S. Army Corps of Engineers (USACE), NOAA, and independent researchers. This project directly supports the FIRO priority area of CW3E’s 2019-2024 Strategic Plan by describing a crucial tool used in reservoir operations for Lake Mendocino, the first FIRO reservoir. A proof-of-concept EFO model was originally developed by Chris in 2015 as a personal research project, and has been refined over the past 5-years with research and real-time testing and evaluation using major deviation requests made to the USACE. Additionally, the significant evaluation required by the FIRO Preliminary Viability and the Full Viability Assessments, demonstrated that the EFO is a viable alternative for flood control operations at Lake Mendocino. An article documenting the computational processes of EFO has been published in the Water Resources Research scientific journal (Delaney et al., 2020).

EFO is a risk-based approach to reservoir flood control operations that incorporates ensemble streamflow predictions (ESPs) made by the California-Nevada River Forecast Center (CNRFC). Each member of an ESP is individually modeled to forecast system conditions and calculated risk of reaching critical operational thresholds (Fig. 1). Reservoir release decisions are simulated to manage forecasted risk with respect to established risk tolerance levels. EFO was developed for Lake Mendocino, a 111,000 acre-foot reservoir near Ukiah, California, to evaluate its viability to improve reservoir storage reliability without increasing downstream flood risk. Lake Mendocino is a dual use reservoir, owned and operated for flood control by the USACE and operated for water supply by Sonoma Water. EFO was simulated using a 26-year (1985-2010) ESP hindcast generated by the CNRFC, which provides 61-member ensembles of 15-day flow forecasts. EFO simulations yield generally higher storage levels during the flood management season while maintaining needed flood storage capacity by strategically pre-releasing water in advance of forecasted storms. Model results demonstrate a 33% increase in median storage at the end of the flood management season (May 10) over existing operations without marked changes in flood frequency for locations downstream from Lake Mendocino. EFO may be a viable alternative for managing flood control operations at Lake Mendocino that provides multiple benefits (water supply, flood mitigation and ecosystems) and provides a management framework that could be adapted and applied to other flood control reservoirs.

Figure 1. EFO forecasted storage (top panel) and risk (bottom panel) for February 8, 1986.

Delaney, C. J., Hartman, R. K., Mendoza, J., Dettinger, M., Monache, L. D., Jasperse, J., Ralph, F. M., Talbot, C., Brown, J., & Reynolds, D. (2020). Forecast Informed Reservoir Operations Using Ensemble Streamflow Predictions for a Multi‐Purpose Reservoir in Northern California. Water Resources Research, 56, e2019WR026604, https://doi.org/10.1029/2019WR026604.