Isotopic exchange between snow and atmospheric water vapor: Estimation of the snowmelt component of groundwater recharge in the southwestern United States

[1] The contribution of snowmelt to groundwater recharge at four sites in the southwestern United States was evaluated using stable isotopes of O and H. Paired precipitation collectors were installed at the study sites; data show that ( 1) there is often a significant difference between the stable isotope composition of fresh snow and the bulk meltwater derived from it ( this suggests that using the isotope composition of high-elevation springs as a proxy for precipitation may not be sound if snow is a recharge source) and ( 2) collector design can significantly influence the stable isotope composition of collected snow. Because the isotope composition of snow from a given location becomes heavier (i.e., more rain-like) with increased exposure, using bulk snowmelt compositions to calculate input to groundwater recharge results in significantly increased estimates of snowmelt contributions to recharge ( compared to estimates derived from fresh snow signatures). Snowmelt provides at least 40 - 70% of groundwater recharge at the study sites, although only 25 - 50% of average annual precipitation falls as snow. On the basis of these results and presently accepted scenarios for alterations in precipitation in the western United States over the next 50 years ( significantly decreased snowpack due to increased atmospheric CO2), investigations of how climate change may affect groundwater resources are needed. We also investigated the potential for snow/ atmospheric water vapor isotope exchange to influence the isotope signature of snow ( which has been a subject of debate); the results of a laboratory experiment suggest that it can drive significant shifts in the isotope signature of snow, even at temperatures below 0 degrees C.