ASSESSMENT OF DESERTIFICATION AROUND DEEP WELLS IN THE SAHEL USING SATELLITE IMAGERY

(1) It has been claimed that deep wells drilled in the Sahel to provide water for cattle cause vegetation degradation by concentrating animals, and may cause desertification. (2) Satellite measurements of red and near infra-red reflectance from vegetation, when combined into the normalized difference vegetation index (NDVI), are correlated with the absorbed fraction of photosynthetically active radiation, and hence to primary production. In this study, NDVI measurements from the advanced very high resolution radiometer on the NOAA satellites were used to estimate the total vegetation production during 1987 on transects from twenty deep wells in the North-Ferlo region of Senegal. (3) Regression analysis indicated no consistent relationship between primary production and proximity to a well at a resolution of 1.1 km. (4) Most of the variance in the data was explained by the differing mean productivities at the different wells correlated with a gradient of increasing rainfall southward. Distance effects fitted to model the change in productivity with distance from the wells produced both positive and negative slope coefficients which are probably associated with factors other than grazing (e.g. soil type, relief and local rainfall patterns).