Seasonal Flow Forecasting in the Sacramento River Basin: An Application in the American River Watershed

Forecasting monthly flow in the Sacramento River Basin is vital for water resources planning and management in the state of California. One important subbasin is the American River Watershed, located to the east of Sacramento in the Sierra Nevada Mountain range and draining into Folsom Reservoir. Traditional statistical forecasting methodologies often underperform during extreme wet or dry years. As such, a new forecasting methodology has been developed to couple a physically based, deterministic component with a statistical updating component. The Watershed Environmental Hydrology-Hydroclimate Model (WEHY-HCM) provides the physically based forecast by dynamically downscaling global Climate Forecast System V2 data and inputting the forecasted atmospheric data to the snow and hydrological components of WEHY-HCM. Hindcast results for previous recent years provide an error timeseries that is then used to update the forecasted flow by statistical means. Results for the American River Watershed indicate that the methodology is successful, with forecasted 2024 January to June flows being comparable to the observed flow at Folsom Reservoir. This integrated methodology allows a physically based component to be incorporated into a purely statistical forecasting methodology. As impacts from climate change alter what a standard water year may look like, this coupled system can provide important details at a monthly scale without relying solely on the past trends of the watershed.