USING STABLE ISOTOPES TO DETERMINE SOURCES OF EVAPORATED WATER TO THEATMOSPHERE IN THE AMAZON BASIN

The return of water in vapor form from the land to the atmosphere, via plant transpiration and evaporation, is fundamental for the maintenan ce of the regional water cycle in the Amazon basin. Whereas transpirat ion, the dominant process, has the extensive vegetation cover as a lar ge single source, evaporation can have several sources, and their rela tive importance and location are poorly known. The isotopic compositio n (delta(18)O and delta D) of water from various sources was used to s ee whether or not specific sources of water vapor to the atmosphere co uld be determined. It is well established that natural waters fall on a line called the meteoric water line (MWL; the regression of delta(18 )O x delta D), With slope equal to eight and an intercept equal to ten . When a water body loses water via evaporation the slope become small er than eight, typically 5-6. We estimated the slope of the regression of delta(18)O x delta D for several potential sources. We analyzed 12 73 samples: 500 of rainfall, 409 of river water, 134 of lake water, 16 4 of soil water, 40 of throughfall and stemflow water, and 26 of shall ow ground-water. We found that large rivers and lakes are likely contr ibutors of evaporated water to the atmosphere. However, as they cover only a small area of the basin, other sources are needed. Probably, ev aporated water originates from several small sources that were not det ected by the isotopic composition of our data.