Extreme rainfall analysis in West Africa during the last 60 years

Abstract

In West Africa, the sharp decrease of rainfall since the 70s has occurred concurrently with an increase of flood damages. Although the growing vulnerability is obviously incriminated, the question of a potential evolution of extreme rainfall is raised - especially in the context of the global warming that is expected to lead to an intensification of the global water cycle. This PhD thesis aims to improve our understanding of the extreme rainfall regime in West Africa. The three objectives of this work are: (i) to provide an integrated regional insight into the spatial organization of extremes, (ii) to study the evolution of extreme precipitation regime in connection with the decadal variability of annual rainfall and (iii) to characterize the extreme rainfall by producing rainfall hazard maps and by studying scaling properties of extreme rainfall at different space-time resolutions. The work is based on: daily rainfall data from West African National networks available since 1950 and updated in the Central Sahel until 2010; high resolution rainfall data available since 1990 over the AMMA-CATCH Niger observatory. Conventional statistical models of the extreme value theory are adapted to incorporate covariates in order to represent spatial and temporal non-stationarities in extreme rainfall distributions. The models with covariates are shown to be highly robust making them reliable tools to estimate the high quantiles and to detect regional trends in extreme series. The theoretical framework of fractals is used to model the space-time scaling properties of extreme rainfall. It is shown that a "simple scaling" relation satisfactorily describes rainfall intensities from 1 to 24 hours temporal resolutions. Our climatological results confirm that the drought of the late 20th century was primarily associated with a decrease in the occurrence of rainfall, the intensity remaining relatively unchanged. However a singular behaviour is identified over the last dec