Evaluation of the National Weather Service Operational Hydrologic Model and Forecasts for the American River Basin

This paper evaluates the National Weather Service operational hydrologic model and operational flow forecasts for several subbasins of the American River. The evaluation includes: (1) the quality of the 6-h operational flow forecasts with up to 5 days lead time; (2) the hydrologic model ability to reproduce observed mean daily flows; and (3) the reliability of the ensemble streamflow predictions of the hydrologic model to reproduce extremes of the monthly volume of full natural flow to Folsom Lake. The results indicate that the model represents the observed flow record well for sites and/or flow ranges unaffected by upstream regulation. Real time operational forecast produced by a forecaster that considers model predictions have good skill out to 18 h with precipitation forecast contributing significantly to forecast uncertainty. Certain high-flow events with a spatially distributed mix of snow/rain over the basin may not be reproduced well by the basic spatially lumped structure of the operational snow–soil–channel model. It is suggested to incorporate upstream regulation rules into the operational models for better reproduction of observed medium and low flows. Routine evaluation based on a national archive of operational flow forecasts and observations is also recommended