Water availability as the driver of vegetation dynamics in the African Sahel from 1982 to 2007

In this paper we explore soil moisture as a principal determinant of Sahelian vegetation dynamics for the period 1982–2007 and compare it to the findings obtained with commonly used rainfall as the main driver. We used remotely sensed Normalized Vegetation Diference Index (NDVI) as proxy for the vegetation greenness response to water availability (rainfall and soil moisture). The association between the variables was determined for two different temporal foci: either all-season data (including the long dry period) or only the months of the growing season (JASO) were included into the analyses. Finally, NDVI residual time series, originating from regressing NDVI on rainfall were searched for significant long-term trends in vegetation greenness induced by other factors than water availability. The results show a west–east gradient of increasing moisture throughout the Sahel for the 26-year period, but not for NDVI. Large areas in the western Sahel, in particular in large parts of Senegal, underwent a strong greening (NDVI + 0.09 and more) whereas no significant increase in rainfall or soil moisture was detected in that region. Quite the contrary was observed in Sudan, where significantly more rain fell from 1982 to 2007 but NDVI decreased with − 0.03 and more. The study revealed that when using all-season data to correlate NDVI with water availability higher correlations are obtained as compared to restricting the analyses to JASO. For rainfall and soil moisture maximum r-values of 0.95 and 0.92, respectively, were achieved when using all-season data and 0.87 and 0.86, respectively, for the JASO analysis.