An overview of HAPEX-Sahel: a study in climate and desertification.

HAPEX-Sahel was an international experiment designed to provide the field data needed to model the climate of the Sahel and its dependence on land surface conditions. The design of the experiment was based on the study of a 1°square experimental domain in which there were three observational supersites. At each of these supersites detailed hydro-meteorological studies were made at subsites for each of the three principal vegetation types: millet, fallow savannah and tiger bush. Remote sensing from satellite and aircraft was used to scale up from the local to the regional scale. Hydrological monitoring, from 1991 to 1993, was combined with an 8-week intensive observation period that covered the end of the wet season and the beginning of the dry season in 1992. The structure and content of the HAPEX-Sahel Special Issue are described and an introduction is given to the HAPEX-Sahel information system where the data are stored.