Rainfall variability and vegetation dynamics in the Mauritanian Sahel

To evaluate the state of ecosystems in Mauritania, rainfall time series and a GIMMSNDVI (global inventory modeling and mapping study-normalized difference vegetation index) data set were used for analysis of rainfall and NDVI trends and their relationships in different ecological zones. Linear regression analysis and the non-parametric Mann-Kendall test were applied to detect NDVI and rainfall trends. In addition, the interannual NDVI coefficient of variation (CV) was used to demarcate the borders of the Mauritanian Sahel, and used as an index for land degradation. The results of both parametric and non-parametric methods confirmed the presence of increasing rainfall trends in different ecological zones, except for the Saharan and coastal zones. However, NDVI time series were positive at east Sahel and southeast Senegal River zone. As concluded from trends of rainfall and NDVI, and from CV analysis, the west Mauritanian Sahel and zones west of the Senegal River were characterized by low performance and presence of degradation, while the east Sahel zone, the zone to the southeast of the Senegal River, and patchy areas in the west Sahel exhibited very good land performance and greenness during 1983–2003. The actual borders of Mauritanian Sahel rangeland approximate to the 200 mm isohyet and 0.20 mean NDVI. Land degradation in the Mauritania Sahel can be attributed principally to human activities, and the recent greenness to the increase in rainfall.