Evolution in the Understanding of the Characteristics of Extreme Hydrological Events

Advances in the understanding of hydrology over the last century have been driven by the evolving needs of society and technological opportunities: new ideas are generated by hydrologists as they address society's demands with the technologies of their time. This paper specifically discusses the evolution of concepts related to flood runoff generation through different mechanisms: excess infiltration, excess saturation, subsurface flow, and more recently, the emphasis on hydrological connectivity. In particular, it highlights the ideas regarding the transition from moderate flood events to extreme ones. The evolution of concepts for quantifying flood peaks through extreme value statistics is briefly summarized. Finally, the talk outlines the evolution of approaches to combine statistics with hydrological processes, starting from the theory of derived distributions and including flood frequency hydrology and regional process hydrology, in order to complete regional statistical hydrology. It also evaluates the main driving factors, depending on the characteristics of climate and catchments. It is argued that the growing societal expectations for safety and the clear human influence on the hydrological cycle now more than ever require a process-based approach to the estimation of extreme floods.