Compared regimes of NDVI and rainfall in semi-arid regions of Africa

Bi-monthly normalized difference vegetation index (NDVI) at an 8 km spatial resolution from the advanced very high resolution radiometers (AVHRR) was used from 1981 to 1995 to analyse the vegetation response to rainfall supply in semi-arid regions of Africa. Within the 200-600mm annual rainfall belt, for which the apparent NDVI response to rainfall was the strongest, three regions were selected which exhibited different patterns in their NDVI regimes and/or relationships with rainfall. The regions, located in western, southern and eastern Africa, were split into coherent sub-regions in terms of mean regime of photosynthetic activity through a cluster analysis. Overall, intra-regional differences were found to be of relatively minor importance compared with inter-regional differences. The mean annual rain-use efficiency (RUEa) was the highest in southern Africa, followed by that in eastern Africa, and it was the lowest in western Africa. In castern Africa, a distinctive feature was found which consisted of the dissimilar vegetation response to the two rainy seasons. For a given rainfall amount, the NDVI peak was comparatively higher for the short versus the long rains. Then, in all three regions, the correlation between rainfall and NDVI regimes was found to be significantly high. Moreover, the lag between the rainfall and NDVI peaks, which was found to be the smallest in western Africa (1 month), and the highest in southern Africa (over 1.5 month), was determined to be related to the increased rainfall rate before the peak. In view of these results, it is shown that the dominant cause for the inter-regional discrepancies, at the spatial and temporal scales considered, is the structure of the rainy season (distribution, concentration) more than the potential evapotranspiration, vegetation cover or soil type.