Abstract:
Stable water isotopes have been employed as a means of challenging, validating, and
improving numerical models of the Amazon Basin since the 1980s. This paper serves as an
exemplar of how characterization of human and natural impacts on surface-atmosphere
water exchanges could beneficially exploit stable water isotope data and simulations.
Interpretations of Amazonian isotopic data and model simulations are found to be seriously
hampered by (1) poor simulation of the gross water budget (e.g., lack of surface water
conservation in models): (2) considerable model differences in the fate of precipitation (i.e.,
between reevaporation and runoff'); (3) wide ranging characterization of natural causes of
water isotopic fluctuations (especially El Nino and La Nina events); (4) isotopic landatmosphere
flux sensitivity to the prescription of boundary layer atmospheric water vapor
isotopic depletion; and (5) significantly different characterization by current land-surface
schemes of the partition of evaporation between isotopically fractionating (from lakes and
rivers) and nonfractionating (transpiration) processes. Despite these obstacles, we find
features in the recent isotopic record that might be derived from circulation and land-use
changes. ENSO events may cause decreased depletion in the dry season, because of
reduced convective precipitation, while increases in upper basin isotope depletions in the
wet season may result from relatively less nonfractionating recycling because there are
fewer trees. The promise for isotopic fingerprinting of near-surface continental water cycle
changes depends upon fixing shortcomings in current atmospheric and land-surface
models.
