Anatomy of a Rainfall Index

Abstract

One particular index has been commonly used to monitor precipitation in drought-prone regions such as the West African Sahel and the Brazilian Northeast. The construction of this index involves standardizing the annual total rainfall for an individual nation and then averaging these standardized rainfall deviations over all the stations within the region to obtain a single value. Some theoretical properties of this ?Standardized Anomaly Index? are derived. By studying its behavior when applied to actual rainfall data in the Sahel, certain aspects of the practical utility of the index are also considered. For instance, the claim that the Sahel has recently experienced a long run of relatively dry years does not appear to be sensitive to the exact form of index that is employed. On the other hand, it is shown by means of principal components analysis that no single index can ?explain? a large portion of the variation in Sahelian rainfall, implying that much information, that is at least potentially useful, is lost when one relies only on a single index. The implications of these results for assessments of the impact of drought on society in arid and semiarid regions are discussed.