Global Changes in 20-Year, 50-Year, and 100-Year River Floods

Slater, L. and Villarini, G. and Archfield, S. and Faulkner, D. and Lamb, R. and Khouakhi, A. and Yin, J. (2021) Global Changes in 20-Year, 50-Year, and 100-Year River Floods. Geophysical Research Letters, 48 (6): e2020GL091. ISSN 0094-8276

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Abstract

Abstract Concepts like the 100-year flood event can be misleading if they are not updated to reflect significant changes over time. Here, we model observed annual maximum daily streamflow using a nonstationary approach to provide the first global picture of changes in: (a) the magnitudes of the 20-, 50-, and 100-year floods (i.e., flows of a given exceedance probability in each year); (b) the return periods of the 20-, 50-, and 100-year floods, as assessed in 1970 (i.e., flows of a fixed magnitude); and (c) corresponding flood probabilities. Empirically, we find the 20-/50-year floods have mostly increased in temperate climate zones, but decreased in arid, tropical, polar, and cold zones. In contrast, 100-year floods have mostly decreased in arid/temperate zones and exhibit mixed trends in cold zones, but results are influenced by the small number of stations with long records, and highlight the need for continued updating of hazard assessments.