Coupling HEC-HMS with Atmospheric Models for the 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 had 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 NCEP Eta model provides 48 hour-ahead forecasting of precipitation in 6-hour intervals in a 40-km by 40-km gridded form. In this study, in order to link the Eta model precipitation forecast results to the watershed model, HEC-HMS, for runoff prediction, the mesoscale model, MM5, is used to transfer the Eta forecast data down to the appropriate space and time scales. An initial diagnostic study of this procedure has been implemented on the Calaveras River watershed in Central California. This paper presents the study results along with a discussion of the methodology and future directions.