Les sécheresses saisonnières dans le haut bassin de l'Oum-er-Rbia (Maroc central) : aspects et fréquences

International audienceIn Morocco, the constraint of the drought in mountainous areas is often ignored. The droughts Morocco has known since 1980 remind us that these regions are faced with important water deficits, especially during the summer season. The purpose of this study is to explain the spatio-temporal distribution and the frequency of the seasonal droughts in a mid-mountain area. The study was based on the thermo-pluviometric data of two typical stations of the Upper basin of the Oum-er-Rbia, namely Khénifra and Ouiouane, representative of the Meseta and the Middle-Atlas Causse (central Morocco), respectively. Two methods were used to study the characteristics of the intra-annual droughts: the P/T ratio and the water balance. The yearly approach of the P < 2T ratio does not reveal any significant correlation between the annual rainfall and the number of dry months, a fact which underlines how bad the monthly distribution of the Mediterranean precipitation is. At the seasonal scale, the ratio shows that summer drought is regularly present every year, its duration varying from one year to the other. The drought may also accidentally affect in other seasons the year. Both types of droughts (summer and accidental) affect the Meseta as much as the Causse, but their intensity and their duration decrease with the altitude. The results derived from the water balance differ from those derived from the ratio P = 2T, in the sense that the former identify the drought as the lack of water restricting the growth of vegetables, namely the water necessary to satisfy potential evapotranspiration. Besides, the water balance expresses the hydro-spatial differences between Khénifra and Ouiouane. The latter, unlike Khénifra, provides the majority of the waters which, by partially infiltrating into the Causse in winter, widely contribute to the support of the summer flow of the Oum-er-Rbia. Finally, both methods agree and complement each other to show the influence of altitude on the spatio-temporal distribution of intra-annual droughts.Au Maroc, la contrainte de la sécheresse dans les régions montagneuses est souvent ignorée. Les sécheresses qu'a connues le Maroc depuis 1980 nous rappellent que ces régions ne sont pas épargnées par des déficits en eau importants, surtout pendant la saison estivale. Le but de cette étude est d'exposer la répartition et la fréquence spatio-temporelles des sécheresses saisonnières dans une région de moyenne montagne. L'étude a été menée à partir des données thermo-pluviométriques de deux stations caractéristiques du haut bassin de l'Oum-er-Rbia, à savoir Khénifra et Ouiouane, représentatives respectivement de la Meseta et du Causse moyen-atlasique (Maroc central). Deux méthodes ont été utilisées pour étudier les caractéristiques des sécheresses intra-annuelles : le rapport P/T et le bilan hydrique. L'approche annuelle du rapport P