A summary of previous isotopic studies of rainfall in western Africa (0°-17°N ; 0°-15°E) together with the isotopic analyses of rainfall events from 15 stations in 1989 shows that (1) the Gulf of Guinea is the main source of water vapor in the Sudan-Sahelian zone (minimum of monthly mean of delta18O contemporaneous with the heart of the monsoon), (2) the reevaporated water from previous local rainfalls is an important source of water vapor of subsequent rainfalls (lack of continental effect, where the majority of rain events present isotopic signature either evaporated or fed by evaporated water), and (3) no isotopic data support the Indian Ocean as a source of vapor. Isotopic ratios combined with variations of temperature and relative humidity associated with rain events in 1989 mirror the increasing aridity from south to north and from west to east. However, a Sahelian station, Birni N'Konni, presents all the features of a humid station with a large contribution of continental vapor (60% of rain events with d greater than 10 per thousand, -2K smaller than deltaT smaller than 2K and deltaHr smaller than 10%). This observation can be attributed to continental vapor originating from irrigated fields 100 km upstream of the dominant monsoon currents, in the Sokoto valley of Nigeria. This is corroborated by the difference in the evolution of rainfall amounts and relative humidity at Niamey and Birni N'Konni since 1951, signaling an important effect of land use changes on regional climate conditions. (Résumé d'auteur)