Extreme rainfall characterisation under climate change and rapid population growth in the city of Niamey, Niger

Since recent years, the Sahel semiarid region has experienced devastating floods-causing significant losses and damages. The present paper attempts to characterise extreme rainfalls responsible for pluvial floods in the city of Niamey, in Niger, under climate change and rapid population growth. Past damaging rainfall records spanning 1992-2015 were used to estimate the optimal temporal scale and to define a threshold for extreme rainfall. The characteristics of extreme rainfalls were then assessed under stationary and non-stationary conditions using peaks over threshold (POT) with the generalised pareto distribution (GDP). In the non-stationary POT, population data was used as threshold covariate whereas air temperature was used as scale parameter covariate. A suitable temporal scale of 3 h was found, whereas the threshold depth was 28.71 mm under stationary conditions and between 21 and 27 mm for the time dependent threshold. The analysis of the extreme rainfall series revealed no significant trend neither in the magnitude nor in the frequency. The influence of air temperature in the characterization of extreme rainfall were less compared to rapid urbanisation, represented herein by population growth. By 2040, 3-hourly rainfall depths of 20 mm could be considered as extreme rainfall.