Regional groundwater modelling studies in large semiarid regions are often hampered by field data scarcity, in both space and time. In such a case, remote sensing can offer complementary datasets and guide further investigations (e.g. Jackson 2002; Brunner et al. 2004; Schmid et al. 2005). Presented here is an example of how remote sensing and geographic information system (GIS) techniques have helped regional groundwater modelling through a better definition of groundwater recharge and discharge areas, groundwater/surface water interaction, and paleohydrological settings. The work focuses on the Quaternary unconfined aquifer covering 500,000 km2 in the central part of the Lake Chad Basin in north-central Africa. It is shared between Chad, Niger, Nigeria and Cameroon and provides fresh water for the majority of ?20 million inhabitants of the basin (Fig. 1). Quaternary sediments form a continuous layer made up of fluvio-lacustrine deposits and aeolian sands, isolated from underlying aquifers by a thick layer of Pliocene clay. The regional aridity may be illustrated by environmental conditions to the north of Lake Chad where the annual rainfall is lower than 200 mm and the population density does not exceed 0.05 inhabitants per km2. Previous reliable hydrogeological studies are few, most of them dealing with the southern half of the aquifer. Regional syntheses can be found, for instance, in Leblanc (2002) and Gaultier (2004). The Quaternary aquifer has large natural piezometric depressions, vast closed concentric sinks also called hollow aquifers. Major piezometric depressions have an amplitude of about 40 m and are found in SE Niger, central Chad, and NE Nigeria. The Quaternary aquifer interacts with the changing environment (climate, surface water and human activities). Understanding such a dynamic system and effective management of this vast groundwater resource underpins the need for a groundwater model of the Quaternary aquifer. Such a model requires good spatio-temporal definition of processes indicative of land surface and aquifer interactions.