Investigating Forecasts of Streamflow Utilizing Radar Data

This paper will describe the influence that the spatial and temporal distribution of rainfall within a river basin has on resulting streamflow forecasts. Instead of using coarse rain gage data, the paper will evaluate the use of the fine spatial (4 km x 4 km) and temporal scale (1 hour) hourly digital precipitation (HDP) data that are provided by the WSR-88D Doppler radar. The hypothesis is that the greater spatial resolution of the radar-derived rainfall will produce a more accurate estimate of rainfall within the basin, thereby producing a more accurate streamflow forecast.

The headwater of the Flint River near Atlanta is studied. The entire region is divided into six sub-basins. Then a composite of radar-derived rainfall estimates and rain gage data (XMRG files) is used to generate a time series of composite 4x4 km mean area precipitation values at 1-hour intervals summed to 6 hours. These 6 hour MAPX (mean areal precipitation derived from radar) values within each sub-basin and basin are then compared MAP (mean areal precipitation derived from rain gages) for a two year period (June 1996 - July 1998) to investigate any differences in radar and rain gage rainfall. These data are then to used to produce the streamflow forecasts.

The Sacramento Soil Moisture Accounting (SACSMA) streamflow model is configured for the basin and its sub-basins. The model is run for 41 years (1955-1995) using 6-hour MAP data to initialize all model parameters. From that time, beginning in January 1996, MAPX and MAP are used separately to simulate streamflow. Specifically, MAP and MAPX are computed for each sub-basin and used to calculate separate estimates of simulated streamflow at the final river gage as well as the sub-basin gage sites. Simulated stream flows from both the entire basin, and from the separately calculated sub basins, are compared to observed data at the actual river gage. The greater spatial and temporal resolution provided by the radar-derived rainfall (MAPX) is expected to produce a more accurate streamflow simulation for the sub-divided basin approach than provided by MAP for the entire basin.