An Empiricial Study of Probability Distributions of Annual Maximum Floods

The Wakeby distribution has been proposed for flood frequency analysis because of its attractive properties. Algorithms have been developed and tested to estimate the parameters of the Wakeby distribution from observed data.Considerable theoretical and developmental work has been conducted on the Wakeby distribution. Besides, its parameters can be easily estimated. However not many studies have been reported in the literature about fitting the Wakeby distribution to empirical data and comparing the results to those obtained from other well known distributions such as the extreme value or the log person type III distribution in the analysis of observed data.In view of these considerations, annual maximum data from twenty-five watersheds have been analyzed by using the log pearson type III,mixture of extreme values and Wakeby distributions and the results are discussed in this paper. The data are from watersheds in the United States and Europe and cover a variety of climatic conditions.The parameters of the distributions are estimated by the maximum likelihood method or method of moments and the fitted distributions are compared to the histograms of observed data by using the goodness of fit tests In several instances, the log pearson type III distribution estimates failed to converge. In other instances the fit of the mixture distribution to histograms of observed data were much poorer than those of log pearson type III or the Wakeby distribution. Thr performance of the Wakeby distribution was in general as good as that of the log pearson type III distribution and in some cases superior.In view of these considerations, it is concluded that the Wakeby distribution should be strongly considered for flood frequency analysis, especially in view of the fact it is so easy to use.