FRIEND 2002—Regional Hydrology: Bridging the Gap between Researcli and Practice (Proceedings of tile l-'ourth International Rlll-ND Conference held al Cape Town. South Africa. March 2002). IAHS Publ. no. 274. 2002. 483 Space-time rainfall variability in West Africa derived from observations and GCMs HUBERT ONIBON, THIERRY LEBEL LTHE/1RD, 1025 BP 53 Domaine Universitaire, F-38041 Grenoble Cedex 9, France e-mail: onibon@hmg.inpa.lT ABEL AFOUDA Université Nationale dit Bénin, BP 526, Colonon, Benin Abstract The CATCH project quantifies the impact of climate variability on water resources in West Africa by using observed and simulated rainfall scenarios as input for hydrological models. The reliability of the rainfall of a global circulation model (GCM) (LMD-6) was investigated over the CATH region (0° to 5°E and 6° to 15°N). Observed daily rainfall data between 1960 and 1990, spatially averaged over the five selected GCM cells, were compared with the GCM rainfall to identify which variability is reproduced. This includes the seasonal cycle, the inter-annual variability and daily rainfall characteristics. Substantial discrepancies between the observed and simulated rainfall regime of West Africa were found. Key words global circulation model (GCM); rainfall; simulation; observation; variability; season; climate; West Africa INTRODUCTION The rainfall regime of West Africa is characterized by multiscale variability (e.g. Lebel et al, 2000; Le Barbé et al, 2002) that has great impact on the water resources. Rainfall regimes need to be considered to understand the links between rainfall and water resources variability in various climatic regions. One special concern is the possible modification of these rainfall regimes due to climate change. Observations of present rainfall regimes and plausible scenarios for future rainfall regimes derived from general circulation models (GCMs) have to be jointly considered to characterize the possible impact of climate variability on water resources. There are, however, good reasons to check the reliability of GCM simulations since their coarse resolution has involved a simplified parameterization of rainfall-producing processes and an inadequate scale for representation of rain fields. Therefore various intercomparison studies have been launched recently (e.g. Phillips et al., 2000). This study contributes to the validation of the GCMs representation of the rainfall regime in West Africa as a preliminary step for coupled climate and water resources studies. SYNTHESIS OF WEST-AFRICAN RAINFALL VARIABILITY High resolution data, such as those collected in HAPEX-Sahel in the 1990s and within the framework of the CATCH project (Couplage de l'Atmosphère Tropicale au Cycle Hydrologique) since 1997, are needed to characterize the West African variability at the