Abstract:
Two rare but naturally occurring isotopes of water. 1H2 18O and 1H2H16O, are becoming of practical use in
diagnosis of climate and earth system model performance. Their value as tracers and validation tools in hydrological
subsystems derives from the systematic and different (from each other and from the most abundant water
isotope: 1H1H160) paths and residence times they exhibit as a result of phase change. chemical exchange, and
diffusive differentiation. Applications of the simulation of stable isotopic behavior to resolving uncertainty in
global climate or earth system models, including river isotopic characterization of basin changes and plantrespired
oxygen isotope "tagging." are limited until more basic criteria such as conservation. current mean
climate. and capture of observed variability are demonstrated. Here the authors assess the simulation of isotopic
fluxes in basin-scale hydrology. focusing on the representation of land surfaces in numerical models as the
current mechanism for incorporating water isotopes. They find that surface water budgets are still rather poorly
simulated and inadequately constrained at the scale of large basins, yet surface energy partition can be apparently
well captured by models with inadequate land surface parameterization. Despite this. simulations of fluxes and
reservoirs of the isotopes H2 180 and 'H 'H'60 are demonstrated here to have diagnostic utility in evaluating
surface energy and water budgets. The hypothesis that aspects of basin water budgets and fluxes are explained
and improved by isotopic investigation is demonstrated.
