Drought spells and drought frequencies in West Africa = Duree et frequence des periodes seches en Afrique de l"Ouest

A comprehensive review of various definitions of droughts has been presented to develop the basis for analysis of droughts. Using the specific definition of onset of rains in each year as the sowing date, the length of dry spells were calculated from the historical rainfall data for 150 stations located in Burkina Faso, Mali, Niger, and Senegal. Probability distribution of time to the next wet day and the percentage frequencies of dry spells were computed for successive days after sowing (DAS) a crop. Dry spell analysis showed a pronounced drop in the drought risk for cereal crops from the panicle initiation phase (20 DAS) to the flowering phase (60 DAS). The relationships between mean annual rainfall and average frequency of dry spells for the selected locations in West Africa showed distinct patterns and permit the prediction of the frequency of dry spells from annual rainfall totals. Applications of the dry spell analysis for the choice of a crop/variety, supplemental irrigation, and crop water requirements have been described with examples. (Author abstract)