A COMPARISON OF THE VEGETATION RESPONSE TO RAINFALL IN THE SAHEL AND EAST-AFRICA, USING NORMALIZED DIFFERENCE VEGETATION INDEX FROM NOAA AVHRR

This article presents the results of a study of the relationship between rainfall and Normalized Difference Vegetation Index (NDVI) in East Africa and the Sahel. Monthly data for the years 1982 to 1985 have been analyzed. We have evaluated NDVI-rainfall relationships by vegetation type, using the major formations described by White (1983). In the article, a comparison of the differential response of vegetation growth to rainfall in the two study regions is emphasized. The most important conclusions of our research are as follows: (1) The spatial patterns of annually-integrated NDVI closely reflect mean annual rainfall. (2) There is a good relationship between rainfall variations and NDVI on seasonal and interannual time scales for areas where mean annual rainfall ranges from approximately 200 to 1200 mm. (3) In most cases, NDVI is best correlated with the rainfall total for the concurrent plus two antecedent months; the correlation is better in the Sahel than in East Africa. (4) The ratios of NDVI to rainfall are considerably higher in East Africa than in the Sahel. (5) Mean annually-integrated NDVI is linearly related to mean annual rainfall in the Sahel. In East Africa the relationship is approximately log-linear; above some threshold value of rainfall, NDVI values level off and vary minimally with rainfall. Two possible explanations of this last conclusion are suggested: above this threshold, rainfall is no longer the limiting factor in vegetation growth and/or NDVI is not a good indicator of vegetation growth. The latter is a likely possibility since NDVI directly reflects photosynthetic activity and becomes a poor indicator of biomass (i.e., growth) as high canopy densities are reached. The NDVI-rainfall relationship for East Africa is markedly similar to the relationship between NDVI and Leaf Area Index demonstrated by Sellers (1985) and Asrar et al. (1984). © 1990 Kluwer Academic Publishers.