Isotope characterization of the water vapour in the Amazon region

Samples of atmospheric moisture and of precipitation have been collected daily at two stations in the Amazon Basin, that is Belem, close to the Atlantic coast, and Manaus, approximately 1200 km from the ocean. The deuterium and oxygen-18 contents have been determined in these samples. The isotopic composition of the atmospheric moisture varies generally in parallel with that of precipitation. The isotopic data show the occurrence of at least three seasons in the hydro-meteorology of the Central Amazon Basin. The period of moderate rains (usually less than 25mm/day) from June to late September show rather constant isotopic values. From October to November significantly more negative delta values occur occasionally, especially at Manaus, which are uncorrelated with events at the other station. The period January-April (occasionally extending from December to May) is characterised by periods with rains extremely depleted in heavy isotopes. In spite of the isothermy of the area and lack of clearcut frontal systems, the data suggest that many of the rainy episodes are related to events imposed on the basin from outside and that only the winter period is predominantly influenced by the internal processes within the basin. Presumably then, winter is the season whose precipitation pattern would be most influenced by any deforestation program in the basin