Geostatistical Analysis and Numerical Simulation of West African Sahel Rainfall

Abstract

The pattern of rainfall in the Sahelian zone of West Africa has been the subject of much analysis since the major droughts of the 1970s. It is widely accepted that annual rainfall has declined since that time and that it is a regional trend. The rain gauge network has changed during this recent period of desiccation. Since the rainfall of the Sahelian region is now recognised as not being homogeneous, there is some doubt about the reliability of regional rainfall aggregated statistics. This analysis uses geostatistics to investigate the impact of changes to the rain gauge network since 1931 and numerical simulations in an attempt to validate the results. This approach can include most of the rainfall records, unlike conventional methods. It seems that the configuration of the rainfall stations in the E-W direction has accounted for the spatial variation of rainfall better since the 1970s because of the eastward extension of rainfall stations. The rainfall stations in the N-S direction do not sample adequately the generally large spatial variation for the period between ca. 1945 to 1975. The only period when the distribution of rainfall stations in both directions was adequate to sample the spatial variation in rainfall was since 1970. Thus, the results suggest that the persistent downward trend in the aggregated annual rainfall since the 1970s is a return to a more accurate estimate of the rainfall in this region; prior to the 1970s the aggregated annual rainfall was overestimated. The pattern of regional annual aggregated rainfall statistics is an artifact of the annual location of stations and the spatial variation in rainfall. Regional maps of (total summer) rainfall are based on the geostatistical analysis which takes account of the spatial variation in the rainfall. These maps are believed to be more reliable than those based on conventional interpolation approaches and form the basis for a revised median annual rainfall estimate for the west African Sahel between 1931 and 1990.