Abstract
Abstract. A dramatic increase in the frequency and intensity of floods due to the Niger River in the city of Niamey (Niger) has been observed in the last decade. Previous studies highlighted the role of the land use changes on the flood increase since 1970s. In the last decade, observations have raised the issue of a possible increase in extreme rainfall in the Sahel, which may have caused the recent and extreme floods in Niamey in 2010, 2012 and 2013. The study focuses on the 125 000 km2 basin between Ansongo and Niamey. This is the drainage area of the monsoon rainfall that leads to the rapid flow rise occurring between June and October. To understand the possible role of rainfall in flood intensification, satellite rainfall estimate is attractive in a region where the operational gauge network is sparse. This paper analyses the evolution of the Niger hydrograph in Niamey based on discharge observations, hydrological modelling and the satellite product PERSIANN-CDR, over the 1983–2013 period. PERSIANN-CDR is first compared with four other rainfall products. The salient features of the observed changes, i.e. a marked change in the mean decadal hydrograph, is well mimicked by the simulations, implying that rainfall is the first driver to the observed changes. The increase of flooded years over the period is also well reproduced but with some uncertainties in the exact number of flood days per year.
Highlights
The Tropics is a region of the world where global changes and the associated risks are increasing
The rising anthropic pressure, massive urbanization, land use changes and development of intensive agriculture are among the factors that increase the hydrological impacts of climate variability
The latest rainfall records tend to show that the Sahel region is gradually recovering, since the late 90s and that the annual rainfall amounts are getting back to the level they were before the 70s – at least in the Eastern part of the Sahel (Lebel and Ali, 2009)
Summary
The Tropics is a region of the world where global changes and the associated risks are increasing. The upper part of the basin, between its source (situated in humid Guinea) and the city of Niamey, crosses a region which has seen major changes since the 1950s: the Sahel This region has undergone one of the strongest climatic signal ever recorded, with a period of dramatic droughts in the 70s and the 80s. Several studies based on the long series of river height data, available for the Niamey Niger station have analysed the changes in the hydrograph and the recent increase of floods (Descroix et al, 2012; Sighomnou et al, 2013). To study the inter-annual variability of the Niger floods in Niamey, understand their genesis and their recent increase, the hydrological response to rainfall variability must be analysed. A further objective is to understand the detailed role of rainfall in the recent flood increase in Niamey
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