Analyse de la dynamique de la végétation soudano-sahélienne entre 1982 et 2006 à partir de données NDVI NOAA-AVHRR et de l'efficience de la pluie normalisée

International audienceLand 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 etween NDVI (a proxy of the Net Primary Production) and precipitation. RUE and rainfall were calculated and modeled on a .5° x 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

Can a 25-year trend in Soudano-Sahelian vegetation dynamics be interpreted in terms of land use change ? A remote sensing approach

This study is based on the premise that, in the Sahel/Sudanian belt of Africa, the main determinants of interannual variation in vegetation dynamics are rainfall and land cover type. We analyzed the spatio-temporal sensitivity of the NOAA-AVHRR 8 km-resolution vegetation index (NDVI) to (i) annual rainfall variability (0.5 degrees x 0.5 degrees resolution) acquired over a 25-year period (1982-2006); and (ii) land use changes in the different eco-climatic regions of the Bani catchment in Mali (130 000 km(2)). During the period 1982-2006, there was no clear trend in rainfall over the catchment, whereas there was a strong positive trend in the NDVI, both when the NDVI values were corrected using annual rainfall variability and when they were not. We divided the catchment into three eco-climatic regions based on the relationship between the annual NDVI and rainfall. In each region, we analyzed the observed greening in relation to changes in land use after correcting for the effect of annual rainfall on the NDVI. Results show that there is a mixed level of agreement between the land cover changes at the grid cell scale and the spatial pattern of the NDVI trend. Increased cropping does not explain the increase in the annual NDVI, except in the Sahelian part of the catchment. We hypothesize that the natural vegetation dynamics related to the non-linear rainfall patterns during the 25-year study period were responsible for these results

Can a 25-year trend in Soudano-Sahelian vegetation dynamics be interpreted in terms of land use change? A remote sensing approach

International audienceThis study is based on the premise that, in the Sahel/Sudanian belt of Africa, the main determinants of interannual variation in vegetation dynamics are rainfall and land cover type. We analyzed the spatiotemporal sensitivity of the NOAA–AVHRR 8 km-resolution vegetation index (NDVI) to (i) annual rainfall variability (0.58 _ 0.58 resolution) acquired over a 25-year period (1982–2006); and (ii) land use changes in the different eco-climatic regions of the Bani catchment in Mali (130 000 km2). During the period 1982–2006, there was no clear trend in rainfall over the catchment, whereas there was a strong positive trend in the NDVI, both when the NDVI values were corrected using annual rainfall variability and when they were not. We divided the catchment into three eco-climatic regions based on the relationship between the annual NDVI and rainfall. In each region, we analyzed the observed greening in relation to changes in land use after correcting for the effect of annual rainfall on the NDVI. Results show that there is a mixed level of agreement between the land cover changes at the grid cell scale and the spatial pattern of the NDVI trend. Increased cropping does not explain the increase in the annual NDVI, except in the Sahelian part of the catchment. We hypothesize that the natural vegetation dynamics related to the nonlinearrainfall patterns during the 25-year study period were responsible for these results