The Impact of Freezing Level Forecast Uncertainty on Reservoir Operation in the Yuba-Feather Rivers Watershed

Abstract:

The atmospheric freezing level (ZFL) determines the rain-snow transition zone at the surface, how much rainfall is available for runoff, and the flood risk during a precipitation event. Thus, an accurate ZFL forecast is critical for reservoir operation, especially in mountain watersheds with narrow elevation bands like the Yuba-Feather in Northern California, where a 500-m elevation gain can amount to >50% of the watershed area. In this study, we investigated the impact of ZFL forecast uncertainty on runoff generation and the flood pools of Lake Oroville and New Bullards Bar reservoirs in the Yuba-Feather watersheds. We first identified 3-day precipitation events using the Parameter-elevation Regressions on Independent Slopes Model (PRISM) record from 1981 to 2018. We subsequently grouped the precipitation event magnitudes into five return-period categories based on the PRISM record, i.e., <2 years, 2-5 years, 5-10 years, 10-20 years, and >20 years. The watershed runoff associated with each return-period category was then estimated using the Soil Conservation Service (SCS) Curve Number method. Using a ±350-m ZFL forecast uncertainty, we found inflow volume uncertainties of <10% to >50% of the flood pool storages at Lake Oroville and New Bullards Bar reservoirs in the Yuba-Feather watersheds, depending on the ZFL, antecedent moisture condition, and the precipitation event magnitude. The uncertainties increased by up to >3% per inch of precipitation, depending on the ZFL and antecedent moisture condition. This result offered a “guide curve” for reservoir sensitivity to freezing level forecast uncertainty, and emphasized the importance of ZFL forecast accuracy to support Forecast Informed Reservoir Operations (FIRO) in mountain watersheds.