Use of water isotope ratios as tracers of environmental or climate variations extends back nearly 80 years. The utility of these water isotope ratios arises from an unequal partitioning of heavy and light isotopes during phase changes, which when integrated over the lifetime of water vapor in the atmosphere tends to yield the well-known pattern of lower heavy-to-light isotope ratios at high latitudes and altitudes and in continental interiors. In 1964, Dansgaard suggested that water isotope ratio variation in space and time can be best understood as a Rayleigh distillation process. The Rayleigh model uses three parameters to predict water isotope ratio evolution of a water mass: the initial isotope ratio of a water mass, the fractionation factor describing the partitioning between phases, and the fraction of the original mass of water vapor remaining in the parcel. Compared to the complexity of atmospheric cloud and precipitation processes, it is remarkable that the Rayleigh model captures the spatial and temporal variability of environmental water isotope ratios as well as it does. Yet, as the catalog of observations has grown, apparent deviations from the expected Rayleigh fractionation have led to an increasing number of factors suggested to have an influence of water isotope ratios, including vapor residence time, cloud type, patterns of E-P, subcloud evaporation and equilibration, and changes in vapor source origin. The relative importance of these different factors in explaining water isotope ratio variations is challenging to elucidate from standard meteorological variables since isotope ratios vary in response to the integrated history of that water mass. To address this gap, we have been developing classes of water tracers in the comprehensive Earth system models of E3SM and CESM as well as developing intermediate complexity models that provide more analytical solutions to the Rayleigh equations. We discuss how these models can be useful for advancing our understanding of both water isotope ratio variations as well as the hydrologic processes that drive these variations.