This paper presents the spatial distribution of land surface parameters in southwestern Niger, a region composed mainly of shrub and grassland fallows, millet crop, and tiger bush. The regional patterns of the surface albedo, the leaf area index, the fractional vegetation cover, and the fraction of absorbed photosynthetically active radiation are estimated through the growing season from airborne POLDER (Polarization and Directionality of Earth Reflectances) data acquired during the Hydrologic Atmospheric Pilot Experiment in Sahel (HAPEX-Sahel). The retrieval of these parameters is via a bidirectional reflectance model, appropriate vegetation indices, and Sun-view geometries. Comparison of the POLDER-derived surface parameters with airborne and ground measurements shows that the procedure generally performs well, enhancing the ability to constrain soil-vegetation-atmosphere transfer (SVAT) models in the Sahel area by providing spatially averaged and updated information. This will enable a more valid assessment of the role of the land surface in determining the Sahelian climate, with a better determination of the scaling effect of surface processes. Although the algorithms described in this work rely primarily on multiangular observations, such as those provided by spaceborne POLDER data sets, they should be useful in a number of remote sensing applications. ¿ 1999 American Geophysical Union