Stable isotope ratio measurements of atmospheric water vapor (___Ov and __Hv) are scarce relative to those in precipitation. This limitation is rapidly changing thanks to advances in absorption spectroscopy technology and the development of automatically calibrated field-deployable instrument systems. These systems allow high throughput, in situ monitoring of the temporal variability in ___Ov and __Hv . We present a robust calibration procedure for reliable, high-precision ___Ov and __Hv measurements at less than hourly intervals in this study. Our method was developed and tested using a coupled system consisting of a commercial water vapor isotopic source device and a commercial water vapor isotope analyzer (LGR model WVIA-24) based on the off-axis integrated cavity output spectroscopy (Off-Axis ICOS) technique. The isotope analyzer shows a time dependent response that varies with water vapor mixing ratio, suggesting the need of regular (hourly) calibration achievable by a single reference water source evaluated at a range of mixing ratios. By using a three-point calibration procedure with a range of user-specified water vapor mixing ratios we were able to produce hourly ___Ov and __Hv measurements with an overall accuracy (±0.2% for ___O, ±0.5% for __H) and precision (±0.3% for ___O, ±3.0% for __H) in the laboratory. The calibration procedure reliably produced data that were consistent with those collected by the conventional cryogenic method in an old-growth forest. Stable isotopic composition of water vapor (__Ha and ___O_) was measured hourly using a spectroscopy analyzer in a coniferous forest in southern Washington, U.S.A. Deuterium excess (d_ = __H_ - 8____O_) was deduced to investigate the factors that control temporal variation of d_ observed during a 37-day field campaign. We focus on providing a mechanistic interpretation of the diel d_ variation, characterized by lower values in nocturnal hours, rapid transitions in early morning and late afternoon, and highest values during midday. This diel d_ pattern (ranging from -3.1 ± 8.6 % to 20.0 ± 8.7 % at 70 m) was constantly observed in low to moderate relative humidity (Rh). During rainy hours, values of d_ are less variable (ranging from 5.8 ± 7.7 % to 12.9 ± 2.0 % at 70 m). Our results show that Rh near the evaporative surface is the first order control of d_. Using theoretical predictions originally developed for describing evaporation from ocean waters, we estimated d values of evapotranspiration (dET. Values of dET are > d_ in low to moderate Rh but become < d_ in high Rh. We demonstrate that the diel cycle of atmospheric humidity and its influence on the kinetic fractionation against the heavy water isotopologues from ET fluxes drives the pronounced diel pattern in d_. Our results suggest that d[subscript a] measured in continental locations has the potential to be used as an environmental proxy to infer local humidity in analogy to what has been known for water vapor originated in marine environments.