Temporal evolution of physical parameters over SALT/HAPEX-SAHEL site

The objective of this work is to determine the ability of remote sensing to monitor evolving ground conditions. The study relies on the observation of physical parameters required by biosphere models throughout several years. The main parameter is the albedo in two spectral bands (visible and near infrared) to determine vegetation structure constitution. The authors realised this work using a three year high resolution AVHRR data set, 1990, 1991 and 1992. After a state of the art preprocessing including accurate calibration and atmospheric corrections, they accounted for bi-directional effects using a radiative transfer model. A specific study site near Niamey (Niger) in sahelian savanna land cover was then extracted from the images. Over these three years some changes have been detected and the authors validated their recognition by the means of land cover ground data of the Banizoumbou site (SALT/HAPEX). These albedo changes were detected in both channels as well as in radiative transfer model parameters. To underline the better vegetation discrimination obtainable from this method they also applied an automatic classification. The land cover evolution is classified by a cluster analysis in order to determine the different structures of vegetation cover and their interannual evolution. This work was supported by a good ground knowledge and SPOT control data. In fact the main interest is the interfacing between a good representation of land cover by clean remote sensing data and good ground representation.