A Radar Reflectivity-Runoff Model for use in Flood Warning

Summary

A flood forecast linear model with a warning objective is proposed. This model connects directly the radar reflectivity data and hydrological variable runoff. The catchment is discretized in pixels (4 Km × 4 Km) with the same resolution of the CAPPI. Careful discretization is made so that every grid catchment pixel corresponds precisely to CAPPI grid cell. The basin is assumed a linear system and also time invariant.

The forecast technique takes advantage of spatial and temporal resolutions obtained by the radar. The method uses only the measurements of the factor reflectivity distribution observed over the catchment area without using the reflectivity - rainfall rate transformation by the conventional Z - R relationships. The reflectivity values in each catchment pixel are translated to a gauging station by using a transfer function. This transfer function represents the travel time of the superficial water folowing through pixels in the drainage direction ending at the gauging station. The parameters used to compute the transfer function are concentration time and the physiographic catchment characteristics.

The reflectivity measurements in every pixel are stored in an arrival queue form taking in to account their translation in time. The queue is updated with every new CAPPI by advancing positions of the reflectivity values. Reflectivity values stored in the queue may be examined at any moment to create a reflectivity histogram with the arrival time in the abscissa. Only intense rain events were considered for which this linear model may apply.

The Z-Q model calibration is made by the correlation of branch of the hydrograph (up to the peak) because the main purpose is flood warning. The model was tested in the catchment of the Jacaré-Guaçu River (São Paulo State - Brazil). The results showed that the Z-Q model shows a potential for operational forecast system in the catchments that have short lead time.