Isotope fractionation of water across mid-latitude mountain rangess

As moist air passes over a mountain range, the ratio of heavy to light stable isotopes (D/H and 18O/16O) in the remaining water vapor decreases due to precipitation and Rayleigh fractionation. The change in isotope ratio is directly related to the Drying Ratio (DR): the fraction of water vapor removed from the air flow in orographic precipitation. In this report, we compare recent isotope measurements in stream water collected across six major mid-latitude mountain ranges: Oregon Cascades, southern Andes, northern California Sierras, Alaska coastal range, Scottish Highlands and the Southern Alps of New Zealand. Data is compared with a theoretical upper bound formula for Drying Ratio; DR(x)=1-exp⁡(h_MAX (x)/H_(SAT)) where hmax is the highest terrain height and Hsat is the saturation water vapor scale height. Each case shows the strongest isotope and DR gradients just inland of the on-shore coast. The isotopes of hydrogen and oxygen fractionate proportionately following the expected meteoric water line (MWL) with a slope of about eight. The data fits a conceptual model of progressive down-wind fractionation rather than an “isotope lapse rate” model. Total isotope-derived DR values across these major ranges vary from 30% to 50%. Isotope data from extracted tree stem water agrees well with stream water isotope ratios. The largest uncertainties in the fractionation process relate to questions of vertical mixing and sensitivity to ice or liquid cloud phase.