Evaluation of a WEHY-HCM Seasonal Forecasting System of Monthly Flows for the Yuba River Basin in California

We present the results and evaluation of a Seasonal Forecasting System based on the Watershed Environmental Hydrology-Hydroclimate Model (WEHY-HCM) applied to the Yuba River Basin. The forecasting system takes 15 members from the Climate Forecast System version 2 (CFSv2) each month from January to June. The Weather Research and Forecast (WRF) system is used for downscaling, while WEHY is used to calculate the flows at the station Yuba River near Smartsville (YRS). Calibration and validation of the WEHY-HCM is shown for the years 1997–2006. The hindcasting period is defined as the years from 2020 to 2023, and its results are compared to full natural flow estimations from the California Data Exchange Center (CDEC). Mixed results are obtained from the hindcast. The 2022 hindcast is shown as an example, which has good accuracy up to 6 months ahead as in January and February initializations. The deterministic forecast for 2024 has some mixed results. We developed a statistical filtering system using the hindcasting errors, based on exponential smoothing. The statistically updated results improve the 2024 forecast to higher accuracy, reducing the errors for each initialization month, except June.