Preface: Extreme Hydrological Events

Beijing Normal University, Beijing, ChinaCorrespondence to: C. Cudennec (cudennec@agrocampus-ouest.fr)The security of Mankind is multi-faceted as it is chal-lenged by many hazards and threats, with variable expo-sures across space and time. It is now recognized that theimpacts of disasters are increasing (see, for example, WMO-No. 1123, WMO, 2014) as is their complexity in many partsof the world, resulting in the Sendai Declaration “to en-hance our efforts to strengthen disaster risk reduction to re-duce losses of lives and assets from disasters worldwide”(UNGA, 2015, p. 1), with an emphasis on the role of Sci-ence (Aitsi-Selmi et al., 2015). Geophysical extreme eventsand geopolitical crisis have always been the two major causesof disasters and catastrophes. However, the risks are growingconsistently with the acceleration and increasing complex-ity of the Anthropocene and through the emergence of newnatural-technological interfaces and interactions, new cas-cading events and processes, severe changes in the driversand internal features of (sub) systems, changing geographicalvulnerability and resilience characteristics, and new hazardsaside from the traditional ones.The eight associations of IUGG – the International Unionof Geodesy and Geophysics – develop scientific knowledgeand methods concerning all geophysical processes that poserisk to society. In addition, IUGG also facilitates multidis-ciplinary approaches thanks to the richness of its commu-nities and to catalyzing mechanisms (Ismail-Zadeh et al.,2014). One of these approaches is the establishment of bridg-ing symposia during Union assemblies. At the 2015 PragueAssembly, the Union symposium U2 offered a panoramaof keynotes on “Integrated disaster risk science – Account-ing for extremes” from each of the Associations and ad hoccommissions. A major follow up event was the JH1 inter-associations symposium dedicated to “Extreme hydrologicalevents” as water is at the interface of several compartmentsand processes studied by the different disciplines. The JH1symposium was organized in cooperation with WMO, withthis PIAHS volume being a significant output.Extreme hydrological events develop at the Earth’s sur-face with low frequency and high magnitude. They are oftendriven by closely connected causal forces from other aspectsof geophysics, such as meteorological, oceanographic, andseismological processes. They are often complex in terms oftheir distribution of water over time and space, its phases,and its impacts on storages. They also often develop overthe interfaces of liquid freshwater with marine, atmospheric,icy and snowy solid, and ground waters. A large influenceof such events also exists with human and physical geogra-phies, in terms of feedback and feedforward process interac-tions, particularly in the areas of disaster risk management,disaster mitigation and prevention.Consistent with the IAHS Panta Rhei initiative (Montanariet al., 2013) and the multi-dimensional definition of WaterSecurity (Cudennec et al., 2015), this volume contains con-tributions on– the assessment, measurement, understanding, mod-elling, forecasting, prediction and management of ex-treme hydrological events;– the complexity of geophysical interfaces and science-society interactions;– new direct and indirect technologies, from specific ob-servational techniques to modelling and statistical ap-proaches;– the challenges that these issues pose in the context ofclimatic, geographical and societal change; and– conceptual advances regarding the concepts of the An-thropocene and the dimensions of Water Security.