Hybrid physical-statistical framework for seasonal streamflow forecasting in the Upper Feather River Basin, California.

Seasonal streamflow forecasts are essential given climate-driven extremes that breach stationarity in traditional methods. The complex hydrology and competing demands necessitate improved forecasting in the Upper Feather River Basin (UFRB), a key California State Water Project source upstream of Oroville Dam. We introduce a hybrid framework combining dynamical downscaling via WRF and the WEHY-HCM snow-hydrology model with a lead-time-dependent exponential-smoothing filter that adaptively corrects bias and quantifies uncertainty. Applied to December-July ensemble forecasts for water year 2024 using hindcast error training (2018-2023), this approach reduced RMSE by 8.7-318.3 million m³ across eight initialization months and eliminated systematic bias. The resulting 10-90% exceedance bands captured ~ 80% of observed flows, offering reliable confidence intervals. This hybrid method delivers accurate, low-bias streamflow forecasts for reservoir operations, flood mitigation, and irrigation planning in the UFRB and provides a transferable template for other basins facing hydroclimatic variability.