[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 highelevation 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 CO 2 ), 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°C.

Groundwater is a vital resource for sustaining life. Changes in the Earth's climate have the potential to affect both the quality and quantity of available groundwater, primarily through impacts on recharge, evapotranspiration and (indirectly) on pumpage and abstraction. Groundwater is a major contributor to streamflow in areas with relatively shallow water tables, so changes in groundwater systems may also impact surface-water systems. As a result, understanding how climate change could affect groundwater systems is a vital component of sound long-term management of our water supplies. However, predicting how climate change could impact groundwater systems is difficult. Part of the difficulty is rooted in uncertainties in the predictions of future climate. However, even if we were certain regarding future climate, forecasting future groundwater conditions would still be difficult because of the complex combinations of processes that affect groundwater recharge, discharge and quality. Better observations, increased understanding of processes and modeling capabilities will be needed to assess the future of this vital resource in the face of projected climate changes.

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