GAC NDVI images: relationship to rainfall and potential evaporation in thegrazing lands of The Gourma (northern Sahel) and in the croplands of the Niger-Nigeria border (southern Sahel)

The relationship between Normalized Difference Vegetation Index (NDVI) and rainfall is fairly well established for growing-season rainfall between 250 and 500mm. In desert border zones such as the African Sahel, with rainfall significantly lower than 250 mm y(-1), this relationship becomes unpredict able. In this paper, we first explore some causes of this unpredictability for The Gourma, an area in the northern Sahel, and find that bath the previ ous year's and current year's rainfall, as well as potential evaporation, p lay roles in determining seasonal NDVI. We next examine the NDVI-rainfall r elationship in an area of the southern Sahel on the Niger-Nigeria border wi th rainfall fluctuating between approximately 200 and 500 mm y(-1), and con firm its robustness.