Analysis of 1982–2006 Sudano-Sahelian vegetation dynamics using NOAA-AVHRR NDVI data and normalized rain-use efficiency

Land cover dynamic has to be taken into account to analyze changes in water resources, especially in vulnerable environment such as the Bani catchment in Mali. To study the land cover changes, we used NDVI AVHRR time series (1982-2006, 8 km spatial resolution), and monthly rainfall data from 65 stations. To interpret the NDVI trends in terms of land cover changes, we had to eliminate the inter-annual rainfall variability. We used the concept of the Rain Use Efficiency (RUE) which is the ratio between NDVI (a proxy of the Net Primary Production) and precipitation. RUE and rainfall were calculated and modeled on a 0.5° × 0.5° grid scale. For each cell we normalized the evolution of the RUE through time (RUE_cor), and calculated its trend over the 25 years period. The results indicate that RUE_cor is stable or in light increase for most of the grid cells. In areas where water is not a limiting factor of NPP, this trend is positively correlated to the fraction of cropped area changes, as determined from a couple of Landsat images acquired during a similar period. However, RUE is a complex concept and further investigations are needed to consolidate our results and conclusions.