QUANTIFICATION OF GROUNDWATER RECHARGE FROM SNOWMELT IN THE SOUTHWESTERN USA USING STABLE ISOTOPE MIXING MODELS

In many parts of the world, precipitation falls primarily as rain in the summer and snow in the winter.  Spring melt after accumulation of snowpack during the winter represents a significant opportunity for recharge, as the volume of water released during melt can be many times that of an individual rain event.  Thus, the temporal distribution of recharge in snow-accumulating areas may be heavily weighted to the spring.  The contribution of snowmelt to groundwater recharge at four sites in the southwestern USA was evaluated using stable isotopes of O and H.  Paired precipitation collectors were installed at the study sites; data show that: a) 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 b) 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 to 70% of groundwater recharge at the study sites, although only 25 to 50% of average annual precipitation falls as snow.  Based on these results and presently-accepted scenarios for alterations in precipitation in the western USA over the next 50 yr (significantly decreased snowpack due to increased atmospheric CO₂), significant reduction in groundwater recharge appears likely.  Further research is needed to assess the impact on water resources in the Southwest.