A TECHNIQUE FOR SELECTING THE THRESHOLD IN APPLICATIONS OF THE POISSON-EXPONENTIAL FLOOD MODEL

A problem usually encountered when modeling partial duration flood ser ies with the Poisson-exponential model is the difficulty in finding a threshold or base level over which the model is applicable. This thres hold must define a flood series such that the number of events per tim e period is Poisson-distributed, the events are mutually independent, and their magnitudes are exponentially distributed. This article descr ibes and uses empirical techniques designed for dealing with such a pr oblem. These techniques make use of the probability-generating functio n for testing the assumption of Poisson-distributed flood count. data and a test based on the standardized P-P probability plot for the expo nential distribution. Examples using these simple and graphical approa ches are presented and discussed. Results show that these techniques a re very promising among mathematically based methods for threshold sel ection in partial duration series analysis.