Isotope-based studies in recent years have been crucial in the partitioning of global water fluxes including evapotranspiration, which accounts for nearly 75% of continental precipitation in the global hydrologic budget. However, long-term continuous stable water isotope measurements in the field are rare, thereby raising questions on the validity of key isotope assumptions in quantifying watershed-scale water use. In this study, I present continuous hourly values of water vapor isotopes (HDO and H218O) measured over four growing season years in the Wind River Experimental Forest, near Carson, Washington, USA. I used three separate analytical models to evaluate the isotopic composition of evapotranspiration (δET). I demonstrated that δET can be used to investigate which water sources this old-growth coniferous forest utilizes during a drying season. This study is unique in the aspect of a new approach, incorporating an isotope-enabled GCM model data with high-resolution water vapor isotope measurements to derive a solid representation of whole-canopy water use. My results suggest trees in this forest watershed mainly access mobile water sources (streams, groundwater) for sustainable productivity as the dry season progresses. This finding implies a relatively greater hydrologic connectivity between soil water and subsurface mobile water in this Mediterranean forest ecosystem, rejecting the postulation of a two-water worlds hypothesis. I conclude with the notion that hydrologic connectivity is likely to vary seasonally and from one watershed to another. Future studies should carefully design field experiments to directly quantify this important hydrologic parameter.