A review of recent developments in regional frequency analysis of hydrological extremes
Adequate estimation of extreme hydrological variables is essential for the rational design and operation of a variety of hydraulic structures, due to the significant risk that is associated with these activities. Local frequency analysis is commonly used for the estimation of extreme hydrological events at sites where an adequate amount of data is available. However, data are usually only collected at a relatively limited number of sites. In practice, it frequently happens that little or no streamflow data is available at a site of interest (where a dam is to be constructed for example). In such cases, hydrologists can utilize a regional flood frequency procedure, relying on data available from other basins with a similar hydrologic regime. Various methods have been developed over the last few years for the regional analysis of extreme hydrological events. These regionalization approaches aim to estimate different characteristics of the extreme hydrological phenomena of interest, make different assumptions and hypotheses concerning these hydrological phenomena, rely on various types of data, and often fall under completely different theories. The present paper aims to review and classify recent developments in regional frequency analysis of extreme hydrological variables. The specific objectives of the paper are to: i) review the main recent developments in regional hydrologic modeling that have been proposed during the last few years; ii) classify these developments into different groups according to the theoretical background of the method, its specific objectives, and the characteristics of hydrological extreme phenomena it is intended to deal with; iii) propose a comprehensive discussion of these methods, and point out the hypotheses, limitations, data requirements, and potential of each one; iv) identify the new challenges facing engineers in terms of regional frequency analysis of hydrological extremes; and v) propose potential promising directions for future research work which aim to meet these new challenges. Recent developments reviewed in the present paper include improvements in classical approaches for regional delineation and for information transfer, methods combining the delineation and estimation steps, seasonality-based methods, multivariate models for regional frequency analysis, the QdF approach, non stationary models, and approaches for the combination of local and regional data. The paper provides also a discussion of the various hydrological variables treated with regional estimation methodologies, comparative studies of these methodologies, and practical tools that were developed for regional frequency analysis. It is hoped that this document will contribute towards closing the gap between theory and practice, by narrowing the wide body of literature that is available, and by providing comprehensive propositions for regional frequency analysis approaches that meet the new challenges facing hydrologic engineers.