ON THE RELATION BETWEEN RAINFALL AND THE NORMALIZED DIFFERENCE VEGETATION INDEX FOR DIVERSE VEGETATION TYPES IN EAST-AFRICA

This study is an investigation of the relation between the Normalized Difference Vegetation Index (NDVI) and rainfall in East Africa. The temporal and spatial patterns of both variables and their inter-relationships are evaluated. The NDVI and rain-fall associations are analysed for ten vegetation formations of East Africa: four forest zones, two woodland zones, and four drier bush, thicket and grassland types. There is a strong similarity between temporal and spatial patterns of NDVI when annual rainfall is below about 1000 mm and monthly rainfall does not exceed approximately 200 mm. In this range, NDVI is also a sensitive indicator of the inter-annual variability of rainfall. The overall relation between NDVI and rainfall is log-linear and the correlation between annually-integrated NDVI and the log of annual rainfall at 65 stations is 0.89. The best correlation on a monthly time scale is between the monthly composite NDVI and a three-month average of rainfall in the concurrent and two previous months. Considerable differences in water-use efficiency are apparent among the ten vegetation formations, the highest being in the bush and thicket zones, the lowest in the coastal forest with exceedingly high rainfall.