Abstract
Forecasts of river flow are traditionally performed using observed rainfall as input to a model of the surface hydrology. However, this sometimes allows for only a short forecast lead time, especially for small watersheds. One procedure by which this lead time can be increased involves the use of a meteorological model to produce forecasts of the rainfall. As a test of this approach, the Pennsylvania State University–National Center for Atmospheric Research mesoscale meteorological model was used to produce ten 72-h precipitation forecasts for the Susquehanna River basin (SRB) in New York, Pennsylvania, and Maryland. These forecasts were evaluated in two ways: 1) the observed and predicted hourly rainfall, averaged over the SRB, was compared, and 2) the forecast rainfall was used as input to a river-flow model, and the forecast river discharge was compared with the observed discharge. The ten study periods, from both winter and summer, each included a significant precipitation event over the SRB. In every case, the meteorological model predicted the precipitation-producing event. The average magnitudes of the error in the start time and the end time of the precipitation events were both less than 6 h, and the magnitude of the error in the duration of the precipitation events was ∼ 5 h. This represents approximately 15% of the average event duration of 33 h. The model overpredicts the precipitation volume in the SRB by an average of 61%, and the time of maximum precipitation is in error by ∼ 7 h. Because the area of the SRB represents less than 1% of the area of the forecast domain of the meteorological model, these error statistics for the SRB represent essentially a “point” verification of the model.
Two of the ten study periods were selected for use in the testing of the coupled meteorological-river-flow modeling system. The river-flow model, developed by the U.S. Army Corps of Engineers, is a commonly used system known as HEC-1. In this application, the upper SRB was divided into 25 subareas. For each of the two rainfall events, the observed and model-predicted rainfall in each of the subareas were used separately as input to HEC-1, and discharge forecasts were generated for the Susquehanna River at Sunbury, Pennsylvania. When the parameters used in HEC-1 reflected an average partitioning of rainfall between runoff and infiltration, the discharge forecasts based on observed and predicted rainfall for the two cases both showed a reasonably accurate time of peak discharge at Sunbury. However, the discharge was overpredicted by the forecast rainfall and underpredicted by the observed rainfall. When, in one case, the HEC-1 parameters were modified to reflect the existence of snow on the ground, reasonable peak discharges were predicted using the observed rainfall and the model-predicted rainfall that had been normalized for the bias error in the prediction.
