Trends in West African floods: A comparative analysis with physiographic indices

Abstract

After the drought of the years 1970 In West Africa, the variability of rainfall and land use changes affected mostly flow, and recently, flooding is said to be an increasingly common occurrence throughout the whole of West Africa. These changes aroused many questions about the impact of climate change on the flood regimes in west african countries.This paper investigates whether floods are becoming more frequent or more severe, and to what extend climate patterns have been responsible of these changes. We analyze the trends in floods of 14 catchments within the main climate zone of West Africa. The methodology includes two types of sampling flood events, namely the AM (Annual maximum) method and the POT (Peak over threshold), and two perspectives of analysis are presented, precisely long term analysis based on two long time series of flood, and regional perspective involving 14 catchments with shorter length of series.Mann Kendall trend test and Pettitt break test are used to assess the stationarity of the time series. The trends detected in flood time series are compared to the trends of rainfall indices in one hand and vegetation indices in the second hand using contingency tables, in order to identify the main driver of change in flood magnitude and Flood frequency. The dependency between flood index and physiographic index is evaluated through a Success Criterion and the CramerV criterion calculated from the contengecy tables.The results point out the existence of trends in flood magnitude and flood frequency time series with two main patterns. sahelian flood show increasing trends, and some sudanian catchments present decreasing trends. for the overall studied catchments, the maximum five consecutive days rainfall index (Rx5d) seems to follow the trend of floods, while NDVI indices do not show significant link between with the trends of floods, meaning that this index has no impact in the behaviour of flood in the region.