Analysis of NDVI trends and their climatic origin in the Sahel 1986-2000.

The first objective of the current work was to confirm the existence of consistent NDVI trends in the Sahelian region by applying per-pixel linear regression analyses to an 8-km dataset ranging from 1986 to 2000. These analyses ascertained that evident NDVI increases took place at the peak of the rainy season (August) in most of the Sahelian belt. Three sample areas were then selected in order to investigate the main origin of these increases, which was identified in corresponding rainfall variations during the previous periods, and particularly July. On this basis, per-pixel correlation analyses were conducted to identify zones where July rainfall and August NDVI values were concurrently increasing during the study period. A rainfall/NDVI correlation map was thus obtained that highlighted areas where vegetation productivity was particularly variable from year-to-year depending on the development of the rainy season. Such 'hot spot' areas were demonstrated to correspond to those where the predictive power of NDVI data on crop (millet) yield was highest. The paper finally discusses the possible developments of the analytical methodology applied, as well as the environmental relevance of the thematic maps produced.