Trends in extreme rainfall events in Benin (West Africa), 1960-2000

Global dataset of derived indicators has been compiled to clarify whether the frequency and / or the severity of rainfall extremes changed during the 1960 – 2000 period in the Republic of Benin in West Africa. This period provides the best spatial coverage of homogenous daily series, which can be used for calculating the proportion of global land area exhibiting a significant change in extreme or severe rainfall. We selected 12 indicators of extreme climatic events that are based on daily totals of precipitation. The six first indicators are the annual total precipitation (RTOT); the annual total of wet days (with daily rainfall >= 1mm, Rd); the simple day intensity index (SDII) calculated as the average rainfall from wet days; the annual maximum rainfall recorded during 1, 5, and 30 days (Rx1d, Rx5d, and Rx30d). The other six indices are based on the 95th and 99th percentiles calculated from the daily rainfall data over the 1961-1990 period, that is the 30-year period required by the World Meteorological Organization (WMO) for the calculation of normals. The 95th percentile defines a very wet day. The 99th percentile corresponds to an extreme rainfall event. Based on these percentiles, three extreme precipitation indices were chosen. Very wet day and extreme rainfall frequency are based on the annual count of days with rainfall >= 95th and 99th percentiles of 1961-1990 (R95p and R99p). Very wet day and extreme rainfall intensity corresponds to the annual total precipitation recorded from days with rainfall >= 95th and 99th percentiles of 1961-1990 (R95pSUM and R99pSUM). Very wet day and extreme rainfall proportion is the percentage of the annual total precipitation recorded from days with rainfall >= 95th and 99th percentiles of 1961-1990 (R95pTOT and R99pTOT) and measures how much of the total rain comes from very wet or extreme events. Only time series which had complete records from 1960 to 2000 were used. A total of 21 stations time series were extracted from national climate archives and collated into the unique dataset described here. National maps showing trends during the 1960-2000 period were produced. Results show that only the annual total precipitation, the annual total of wet days and the annual maximum rainfall recorded during 30 days present a significant decreasing trend while the other nine rainfall indicators appear to remain stable. These results are important findings for Benin since scarce works realized in neighbouring regions of West Africa describe a situation more prone to increases in extreme rainfall (in Nigeria [1]) or the opposite (in Guinea Conakry [2] and in Niger [3]) in a similar context of a clear reduction in the yearly precipitation amount. We call for further research on this topic in West Africa since water management is a major tool for sustainable development and poverty alleviation.

[1] M. New, B. Hewitson, D.B. Stephenson, et al., “Evidence of trends in daily climate extremes over southern and West Africa,” J. Geophys. Res., vol. 111, 2006, D14102, doi:10.1029/2005JD006289.
[2] E. Aguilar, A. Aziz Barry, M. Brunet, et al., “Changes in temperature and precipitation extremes in western central Africa, Guinea Conakry, and Zimbabwe, 1955–2006,” J. Geophys. Res., vol. 114, 2009, D02115, doi: 10.1029/2008JD011010.
[3] P. Ozer, Y.C. Hountondji, O. Laminou Manzo, “Evolution des caractéristiques pluviométriques dans l’est du Niger de 1940 à 2007,” GEO-ECO-TROP, Vol. 33, pp. 11-30, 2009.