RELATING THE GLOBAL VEGETATION INDEX TO NET PRIMARY PRODUCTIVITY AND ACTUAL EVAPOTRANSPIRATION OVER AFRICA

Net Primary Productivity (NPP) and Actual Evapotranspiration (AET) are key processes characterizing the vegetation activity. At a continental scale, Annual Net Primary Productivity (ANPP) and annual AET are related to the integrated Normalized Difference Vegetation Index (NDVI). When applied to the African continent, the ANPP/SIGMANDVI relation gives an estimation of the productivity with a precision comparable to those given by climate models. The estimation of annual AET is possible within climatic zones defined here by their Rain-Use Efficiencies (RUE). On the other hand, on a monthly basis, the AET/SIGMANDVI relations are less clear and are strongly dependent on the physiognomy of the vegetation community considered. These results show the limits of using SIGMANDVI for estimating the elementary processes of terrestrial vegetation at a continental scale.