Rainfall characteristics (delta O-18, delta H-2, Delta T and Delta H-r) inwestern Africa: Regional scale and influence of irrigated areas

A summary of previous isotopic studies of rainfall in western Africa (0 deg rees-17 degrees N; 0 degrees-15 degrees E) together with the isotopic analy ses of rainfall events front 15 stations in 1989 shows that (I) the Gulf of Guinea is the nlain source of watel vapor in the SLldan-Sahelian zone (min imum of monthly mean of delta(18)O contemporaneous with the heart of the mo nsoon), (2) the reevaporated water from previous local rainfalls is an impo rtant source of water vapor of subsequent rainfalls (lack of continental ef fect, where the majority of rain events present isotopic signature either e vaporated or fed by evaporated water), and (3) no isotopic data support the Indian Ocean as a source of vapor. Isotopic ratios combined with variation s of temperature and relative humidity associated with rain events in 1989 mirror the increasing aridity From south to north and from west to east. Ho wever, a Sahelian station, Birni N'Konni, presents all the features of a hu mid station with a large contribution of continental vapor (60% of min even ts with ch d >10 parts per thousand, -2 K<Delta T<2 K and Delta H-r< 10%). This observation can be attributed to continental vapor originating from ir rigated fields 100 km upstream of the dominant monsoon currents, in the Sok oto valley of Nigeria. This is corroborated by the difference in the evolut ion of rainfall amounts and relative humidity at Niamey and Birni N'Konni s ince 1951, signaling an important effect of land use changes on regional cl imate conditions.