A statistical study of NDVI sensitivity to seasonal and interannual rainfall variations in Southern Africa

The relation between rainfall and the Normalized Difference Vegetation Index (NDVI) in Africa south of 15 degrees S (1983-1988) is studied. For 115 1 degrees by 1 degrees grid-points, the spatial distribution of annual NDVI and rainfall means is highly comparable. Both parameters have overall decreasing values from Mozambique to South-Western Africa. The strongest correlations occur when NDVI monthly values are compared with the bimonthly preceding rainfall amounts, attesting a time response of one to two months. At these time and space scales, NDVI does not appear to be sensitive to the seasonal and interannual rainfall variations in the Namib desert, South Namibia and western Cape Province. Along the Indian Ocean coast, it is weakly sensitive to the seasonal cycle only. It becomes largely sensitive to the seasonal cycle in Zimbabwe, and in South-Western Zambia. A high sensitivity to the interannual rainfall variability is only observed on the Southern African Plateau, around the Kalahari basin. Multivariate analyses show that geographical conditions of seasonal and interannual rainfall-NDVI associations strongly differ. While a sensitivity to seasonal rainfall is observed in areas where mean annual rainfall varies from 300 to 900 mm and where the contrast between wet and dry seasons is strong, the sensitivity to interannual rainfall anomalies is observed only for relative dry areas, where mean annual rainfall varies from 300 to 500 mm. In both cases, the relation is observed whatever the soil type or vegetation formation.