Spatio-temporal characteristics of drought as inferred from tree-ring data in Morocco.

Abstract

Long-term tree-ring series of Cedrus atlantica have been utilized to reconstruct and characterize past drought occurrences in Morocco. Based on the close relationship between tree growth and rainfall variations, the best tree-ring indicator of drought has been found to be, x₀ = μ -.75σ, with μ and σ the mean and standard deviation of the tree-ring series. In Morocco, drought is a short event lasting 1.7 years on average with 84% of the events lasting less than 2 years. The average maximum duration exceeds 4 years. It is associated with a 40% deficit about normal moisture availability which can add up to more than 125% when the anomalies persist. The average empirical recurrence interval of drought is 8 years. Long-term persistence has been characterized by the Hurst coefficient. Over the domain of study, the mean Hurst coefficient is .765. The tree-ring series show a periodic behavior with two major peaks at 22 and 66 years. Drought characteristics have been extended from the tree-ring sites to other regions such as the agricultural plains using geostatistical methods. The drought prone areas are located in central Morocco, including the Sais, the Gharb and central Middle Atlas, which experience longer, more severe and more persistent droughts. The northern latitudes experience milder and shorter drought events. The cost for spatial extension of the results was greater kriging errors which make the estimates less reliable. However, cokriging significantly improved the reliability and the quality of the estimates. A regionalization, using cluster analysis, was found to follow the natural climatic and orographic zones of north central Morocco. The tree-ring data were used to reconstruct spatial patterns of moisture anomalies for the period 1845-1974. Three atmospheric factors appear to be important: (1) the relative strength and position of the Azores High, (2) the importance and location of the local cyclogenesis centers, (3) the importance of the northeastern perturbations coming from the Mediterranean sea. The periods 1860-1890, 1925-1950 and the 1970's are associated with dry anomalies while the periods 1900-1920 and 1950-1970 are associated with wet anomalies. They exhibit a rhythmic succession with a 20-year periodicity.