Using stable water isotopes to evaluate basin-scale simulations of surface water budgets

Two rare but naturally occurring isotopes of water, ¹H ₂ ¹⁸O and ¹H²H ¹⁶O, 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: ¹H¹H¹⁶O) 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 plant-respired 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 ¹⁸O and ¹H²H ¹⁶O 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.