Accounting For Uncertainty of Ensemble Streamflow Predictions In The Operations of Prado Reservoir In Riverside County, California

Forecast-Informed Reservoir Operations (FIRO) reservoir-operations strategy that better informs decisions to retain or release water by integrating additional flexibility in operation policies and rules with enhanced monitoring and improved weather and water forecasts. This study evaluated the forecast uncertainty quantified in ensemble streamflow predictions (ESPs) made by NOAA’s California-Nevada River Forecast Center (CNRFC) to inform reservoir releases. A water management model was developed to simulate forecast-informed alternatives for Prado Reservoir, located in Riverside County, California, that is owned by the U.S. Army Corps of Engineers. Prado Reservoir is maintained dry for most of the year, however, during storm events, the Corps and Orange County Water District (OCWD) collaborate to temporarily detain and release stormwater at a rate that supports OCWD’s managed aquifer recharge system downstream of Prado Dam. Reservoir operations were simulated for Prado Reservoir using a 27-year (1985-2011) ESP hindcast generated by the CNRFC. For each forecast cycle within the hindcast period, each member of an ESP is individually modeled to forecast to generate a storage ensemble forecast. A management policy was developed that evaluates the distribution of the storage forecast ensemble to formulate reservoir release volumes. The goals of this policy were to minimize risk of uncontrolled spillway releases that can result in flooding of properties downstream, minimize high storage levels that cause flooding of properties within the reservoir, and maximize stormwater capture for water supply. Simulations of this forecast-informed policy yield higher storage levels during the flood season while maintaining flood storage by pre-releasing water in advance of large forecasted storms. As a result of the higher storage levels maintained in the reservoir, additional water is available for release to OCWD’s recharge facilities. Model results also show a decrease in the frequency of inundation of properties within the lower elevations of the reservoir and no increase in the frequency of spillway releases that could cause downstream flooding. These investigations demonstrate that the developed policy may be a viable flood risk management approach for Prado Reservoir and warrants further investigation through additional modeling and analysis.