ermine important
characteristics of Amazonian hydrologic cycling by
means of testing and tuning parameters of simple models
for about 30 years. Despite a long history of isotopic
records from the Amazon, there has been, to date, no
attempt to use isotopic data to evaluate global climate
models’ (GCMs’) predictions of the possible impacts of
Salati et al. (1979) used 1-yr isotope data from pre-cipitation
and river samples and results from a sector
box model to reinforce Molion’s (1975) conclusion that
about half the Amazon basin’s water is recycled. On the
basis of 13 months of data (October 1972–October
Corresponding author address: Professor A. Henderson-Sellers,
Australian Nuclear Science and Technology Organisation, Lucas
Heights Science and Technology Centre, Private Mail Bag 1, Menai,
1973), they were able to identify that the Amazonian
inland gradient of depletion of the heavy isotope of
oxygen is surprisingly weak compared to other conti-nental
areas. This showed that a proportion of the Am-azon’s
hydrologic recycling is from nonfractionating
sources, that is, transpiration and full canopy reevapor-ation.
This moisture recycling within the Amazon basin
leads to a seasonally averaged gradient of only 1.5‰
(1000 km)21 in d 18 O going inland on an east to west
transect (Fig. 1) as compared with 2.0‰ (1000 km)21
in Europe and elsewhere (Rozanski et al. 1993).
Despite the apparent simplicity in water movement
and cycling in the Amazon, some observations seem to
point to isotope heterogeneity in originating air masses.
For example, in 1981, Leopoldo (1981) reported values
of the stable isotopes of oxygen and hydrogen as mea-sured
in samples of stemflow and throughflow at the
Duke Reserve, near Manaus, Brazil. Although his re-sults
were somewhat contradictory, the most likely
source of observed differences are believed t...