The Application of Mixture Distribution for the Estimation of Extreme Floods in Controlled Catchment Basins

In the estimation of distribution of annual maximum flows it is a generally accepted assumption that the sequence of observations originates from a homogeneous population. This assumption, however, is rarely met. The observed annual maximum flow are only in part generated by flood events. The remaining ones are the result of the effect of other hydrological processes that do not have that character. For this reason, a new solution to this problem is proposed in the paper. It is assumed that the sought distribution is a mixture of two probability distributions: a three-parameter GEV distribution, describing flows generated by events with flood character, and a two-parameter gamma distribution, accounting for maximum annual flows that do not have such a character. The paper presents both the method of estimation of the mixture distribution and its application for gauging stations selected so as to take into account possible the most diverse conditions of meteorological, hydrological and geomorphological character. The area with such a high diversification, selected for the study, is the catchment basin of upper and central river Odra (South-West Poland). In the studied water gauge profiles the proposed mixture distribution indicates correct fit. Its advantages and limitations are presented through a comparative analysis with results obtained during estimation of distributions of maximum annual flows by means of standard methods.