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765Standardized Rainfall Anomaly43210-1-2-3-4-5-6-71930 1940 1950 1960 1970 1980 1990Mean Max MinFigure 4. Average standardized precipitation anomalies for the continental Sahel using the 1961–90 base averaging periodand the maximum and minimum anomalies observed at individual stations.through time. Hence purely statistical definitions ofmeteorological drought should be treated with caution.Perhaps of equal significance is the omissionwithin the SPI of any assessment of persistence. It israre that drought in any one year causes major hardship.It is the sequence of low rainfalls that createsdifficulties. For example, in England the drought of1976 was really caused by the low rainfalls in thepreceding year, while the drought of 1992 was theresult of the low rainfalls from 1988. The SPI thereforeneeds to be developed from merely classifyingintensities to include drought sequences, and theselection of appropriate averaging periods needs moreattention.AcknowledgmentsThe annual rainfall totals were generously providedby the Climate Research Unit at the University of EastAnglia. The river flow data was provided by theEnvironment Agency, United Kingdom.C. T. AgnewDepartment of GeographyUniversity College London26 Bedford WayLondonWC1H OAPUnited KingdomReferencesAcremen, M.; and B. Adams. 1998. Low Flows, Groundwaterand Wetland Interactions. Vol. 1, Issues Report to theDrought Network News11