Résumé
Abstract Quantitative data are needed to constrain the feedback loops between vegetation and hydroclimate. In this study, the amplitudes of variations in the triple oxygen isotope composition of water at the soil‐plant‐atmosphere interface are measured in savanna and dry forest contexts in West Africa (Benin and Senegal). Comparison of in situ data and model estimates reveals the following: (a) The value of 17 O‐excess for reconstructing climate archives is confirmed, given its small variability in precipitation (a few per meg) compared to the very large magnitude of change in the 17 O‐excess of phytoliths (hundreds of per meg) in response to RH changes. (b) At the beginning of the dry season, the 17 O‐excess in soil water is lower than that of precipitation by only 30 per meg in the sandy‐loam soils and 50 per meg in the sandy soil. This shows the limited contribution of evaporated water to bulk soil water and provides clues to constrain the complex hydrological functioning of soils; (c) The regression line connecting the triple oxygen isotope composition of water in the stems and leaves of grasses and trees can be used to determine the origin of the water absorbed by the roots. Semi‐evergreen trees draw their water from the water table during the dry season, whereas grasses and semi‐evergreen trees use surface water during the wet season. These original data open up new perspectives for the use of the triple oxygen isotope composition of water and phytoliths to better understand current and past hydrological cycles.