Coupling HEC-HMS with atmospheric models for prediction of watershed runoff

The operation of reservoirs in the Sierra Nevada mountains of California for flood control relies on forecasts of reservoir inflows. In the past, accurate forecasts of the reservoir inflows resulting from watershed runoff have been made, but only after the water has entered the main channel. During flooding events, this limits the amount of time available for the implementation of emergency management procedures. Translating precipitation forecasts into runoff forecasts can greatly improve the runoff-forecast lead time. The operational National Center for Environmental Prediction Eta model provides 48-h-ahead forecasts of precipitation in 6-h intervals in a 40 X 40 km gridded form. In this study, the mesoscale model, MM5, is used to transfer the Eta forecast data down to the appropriate space and time scales required to link the Eta model precipitation forecast results to the watershed model, HEC-HMS, for runoff prediction. An initial diagnostic study of this procedure has been performed on the Calaveras River watershed in Northern California. Initial results indicate that: (1) model parameterization choice in MM5 is necessary to refine the precipitation forecasts; (2) the method shows promise for generating 48-h-ahead forecasts of reservoir inflows; and (3) calibration of the HEC-HMS model with distributed precipitation is necessary for this methodology. This paper presents the study results along with a discussion of the methodology.