The drought is a major natural risk linked to long-term climatic abnormalities.
It appears by a drying out of the soil in other words a decline of its hydric potential resulting from an important pluviometric deficit (Lefèvre and Schneider, 2003).
If desert regions are subjected to quasi-permanent drought because of the presence of the subtropical high pressures. The zone of Sahel (Mauritania, Senegal, Niger, Chad and Côte d’Ivoire), undergo a rarefaction of rains more and more marked since 1968 to this day.
Certainly, the successive years of drought are interrupted by a return to normal (year 1 to 2) from time to time but the hydric deficit persists with peaks (73/77/82…).
Indeed, the measures made in the experimental fields of Senegal and Mauritania indicate that since 1968, isohyets 300/400 mm are uncalled-for from 100 to 200 km southward in the space of 15 years (from75 to 90) and no year was superfluous (Sircoulon, on 1992).
In the sector of the pluvial farming in Senegal, we record a clear production decrease of groundnut due to the useful shortening of the duration of rainy season and to the more and more frequent appearance of arid periods during the culture.
Although, in Sahel, the drought is at the origin of famine, of conflicts, of exodus and mortality (200 000 died only for the year 1973 when the drought reached its peak.)
The Mediterranean countries also know droughts of several months on the south bank (4 months in Tunis and Algiers, 7 months in Alexandria) and in a lesser degree the North shore (2 months in Barcelona and 3 months in Istanbul) with the exception of the South of Spain (Drain, 2006) already confronted with the water shortage (Margat, 1990).
Multiannual aridity also occurs in Mediterranean region. Besides, the fall of the agricultural productions (especially cereal), the droughts of the period 1990-1999 are also at the origin of the decay of the cedar of the ATLAS in Algeria and in Morocco. (Halitim, 2006).
In Europe, also the decay of the forest domains is also relatively important following the successive episodes of droughts between 1947 and 1976. In France, the drought from 1989 till 1992 engendered an drying out of streams (11000km), a reduction of the agricultural productions (especially those of May) and an increase of the frequency of the fires of bit in Southern zone especially (Lefèvre and Schneider, 2003).
According to the same authors, the aridity can lead disorders at the level of the constructions further to dehydrations of underground clays.
Nevertheless, the difference of rainfall between the North and the South of Sahara is very strong, because in Mediterranean region except the summer aridity, we find the rest of the year of more or less important rains (350 in 750m). Despite very different pluviometric regime, we can hold some criteria common to these arid climates:
- The global incapacity of the precipitation in view of the potential evaporation.
- A marked interannual irregularity
- The momentary excesses of water even in Sahelian region (SIRCOULON, 1992)
What constitutes a major constraint for the biological rise and a threat for the agriculture whose the socioeconomic impact is roughly more striking in the regions of the world where the followed of the populations is strictly connected to a food-producing agriculture and\or to a practice of the breeding as in Sahel.
The drought is also a factor of degradation of natural resources such as the vegetation, pastoral courses and soils, thus, accentuating the process of desertification. in fact, both of the drought and the desertification are very dependant phenomena. Therefore, the Convention to Combating Desertification encouraged countries of the world to begin to fight at the same time the desertification and the effects of the drought (OSS).