La récente sécheresse des régions sahéliennes

Abstract

Click to increase image sizeClick to decrease image sizeThe drought which recently affected the tropical regions possessed remarkable geographical extension and severity. Particularly West Africa was concerned and the consequences of the drought were dramatic in the Sahel zone. Perceptible in certain regions from 1965, this drought became noticeable in Mauritania, Senegal, North Mali and Niger in 1968. It became more severe in 1970 and extended to Upper Volta and Chad. Numerous rainfall stations measured absolute record low precipitation values. In many parts of the Sahel, the driest year was 1972, with precipitation occurrence frequency neighbouring the centennial value for the Sahel and humid tropical regions further south. Although rainfall returned to near-normal in 1974, the effects of the drought have hardly begun to disappear. The runoff data obtained by hydrological services in collaboration with ORSTOM clearly reflect the extreme severity of the drought. The big Sahelian rivers carried annual flow volumes of occurrence frequencies between 0.1 and 0.05 ; seasonal precipitation distribution partly balanced the global annual deficit in many cases. For several rivers, 1968 was drier than 1972 or 1973. In the south, however, all the big tropical rivers - i.e. the Senegal, Niger and Chari - carried mean annual and maximum daily flows of an occurrence frequency close to 0.01, with a deficit exceeding 50 %. Low flows were extremely small : lowest discharge at Niamey was 0.6 m3/s (compared to the mean minimum annual discharge of 75 m3/s), and there was no flow at all in the Senegal at Bakel for fifteen days. Further evidence of the severity of this drought was provided by Lake Chad, which shrank to a third of the area and a quarter of the volume it had in 1964 and was divided into two separate lakes by the emergence of the Great Barrier. This drought is not unique in recent history, since hydrological records for the Senegal at Bamako and the Niger at Koulikoro since the turn of the century show other very severe drought periods in 1910-1914 and 1940-1944, in which the mean annual discharge was lower than in 1969-1973. Though records before 1930 are very incomplete, the 1913 drought can reasonably be considered to have been similar to the last one.