Many of the ''simple models'' used to study and explain the global distributions of stable water isotopes assume that the initial isotope content of an air mass can be estimated from local conditions at the oceanic source through the application of a global-scale closure equation. Using a general circulation model fitted with isotope tracer diagnostics, we show that the closure equation is generally invalid at the local scale, leading to a systematic bias in the modeled isotope contents. The quantitative results obtained in earlier simple model studies may need some adjustment in light of this bias, particularly if deuterium excess was used to characterize conditions prevailing at the evaporative source. ¿ American Geophysical Union 1996