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11-05: Sahel the drought is over

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From [Source-weekly] Special Features Edition 42, November 2005 :  SAHEL: the drought is over

Pierre Hubert, Senior researcher at the National School of Mining Engineering in Paris and Secretary-general of the International Association of Hydrological Sciences (AISH) discusses the up and downs of the Sahelian drought in West Africa. This expert view was translated from Sources Nouvelles . 

  • Question: The Sahelian drought has been over since the mid 90s. How can you demonstrate this?

Analyses we made of the annual flow of the Senegal River, revealed that the drought was "broken", between 1993 and 1994. Set against the records of the past 50 years, this is a very clear signal that leaves no doubt that the Senegal River basin has entered a relatively wet phase. As the Senegal River basin has shown itself in the past to be a reliable indicator for the whole region, we can reasonably conclude that the same applies to the Sahel as a whole.

West Africa has, during these years, recorded a number of significant rises in rainfall as the floods in Dakar can testify. In addition, it is particularly significant that the locusts also returned. If locusts return, it is because they have something to feed on and that means that there is at least minimum wetness. Nevertheless, can we absolutely declare that the drought is over? The question is raised because 1999 was a second wet year following 1994. Two studies, published in 2002, provided contradictory answers and each one called for a review over a longer time frame [1].

At this point, we decided to resume the study of the Senegal River chronological records, which are now 100 year long (1904-2003), having already studied a shorter series in 1989 and 1998. This study enabled us to highlight the same patterns as before: a period of drought in the 1910s, followed by a relatively wet period in the 1920s, then a new drought starting in 1939-1940, and again a wet period between 1950 and 1960 before falling back again into drought in 1969-1970.

  • Question: ... With a new "break" during the 1990s?

Exactly. We refined our methodology of "segmenting the series" which consists of dividing the time frame into as many steady periods as possible ("stable" in the statistical jargon). We applied this procedure of segmentation to all the sub series panning the years from 1905 to 2003. The results highlight "natural" breaks, including the main changes like that of years 1921-1922, corresponding to an increased flow in the Senegal River, that of 1938-1939 (a decrease in flow), that of 1949-1950 (an increase) and again that of 1967-1968 (a decrease). In addition to these main changes came other changes which are not accidental or chance events but which, on the contrary, recur in relatively wide ranges on both sides of the series. We can see for example such a break between 1910 and 1911, in ranges running from 1904 to almost 1940, which testify to a local aggravation of drought in the beginning of the 20th century. Against the prevailing trend, there are two breaks in between 2003 and 1945: the first in 1976-1977 marking an aggravation of the last drought, and a second one in 1993-1994, marked by an increase in water flow.

  • Question: Does this allow one to assert beyond doubt the return of a wet phase?

The result shown by the records of the last 50 years seem to us sufficient to assume that we actually entered ten years ago a new climatic phase at the level of the Senegal River Basin. However, this does not solve all the problems, because increased wetness brings new risks; such as floods and locust invasions we already mentioned. Will the agricultural and social structures, which were deeply affected by the drought that began in about 1970, be able to use this gift from Heaven?… Let’s make way here for development specialists.

Remarks collected by Martine Le Bec –

1. L’Hôte Y., Mahé G., Somé B., Triboulet J.P., "Analysis of a sahelian annual rainfall index from 1896 to 2000; the drought continue", 2002, Hydrological Sciences Journal 47(4), 563-572.

Ozer P., Erpicum M. Demarée G. and Vandiepenbeeck M., "The sahelian drought may have ended the 1990’s, discussion of ‘analysis of a sahelian annual rainfall index from 1896 to 2000; the drought continue’ by L’Hôte et al.", 2002, Hydrological Sciences Journal 48(3), 489-496. offers approximately 150 theses, traditional texts and a photographic library about hydrology. This site is also gradually putting on line the Red Books of the AISH/IAHS. Three hundred books have been published since 1924 including 82 that are already accessible, representing nearly 30 000 pages of scientific literature.

Pierre Hubert, Jean-Claude Bader and Hocine Bendjoudi will report on the findings of their study "One century of Senegal River Annual Discharge" at the Montpellier HSM Seminar, "Climatic and Human Impacts on the variability of water resources". Maison des Sciences de l’Eau in Montpellier on 22-24 November 2005.