Freezing Level Forecast Error Can Consume Reservoir Flood Control Storage: Potentials for Lake Oroville and New Bullards Bar Reservoirs in California

The atmospheric freezing level ( Z FL ) determines the rain‐snow transition zone at the surface, how much rainfall is available for runoff, and the flood risk during a precipitation event. An accurate Z FL forecast is thus critical for reservoir operation, especially in mountain watersheds with narrow elevation bands like the Feather and North Fork Yuba in California, where a 500‐m elevation gain can amount to >50% of the watershed area. Using a ±350‐m Z FL forecast error, we find inflow volume uncertainties of <10% to >50% of the flood pool storages at Lake Oroville and New Bullards Bar reservoirs, depending on the Z FL , antecedent moisture condition, and the precipitation event magnitude. The uncertainties can increase by up to >3% per inch (25.4 mm) of precipitation, depending on the Z FL and antecedent moisture condition. This result substantiates the significant impact of Z FL forecast error and the critical need of Z FL forecast accuracy to support reservoir flood control operations in the two watersheds.