Groundwater Age in High Elevation Basins of Northern California: Predicted Effects of Climate Change on Recharge and Stream Baseflow

Snowmelt is an important component of groundwater recharge in high elevation watersheds of the western United States.  In these watersheds, the predicted climate change impacts on snowmelt will likely alter the amount and timing of groundwater recharge, which may lead to reduced groundwater production, declining water tables, and reduced baseflow to streams.

     We apply dissolved noble gas tracers to answer questions about recharge location and aquifer residence times in two catchments of differing size in the northern Sierra Nevada: Olympic Valley near Lake Tahoe, and the Upper Merced catchment which drains through Yosemite Valley.  In both basins, deep, high capacity production wells located in the upper portion of the valley draw groundwater from over much of the sediment thickness.  These wells were sampled for tritium and dissolved noble gases in order to determine recharge temperatures (which are tied to recharge elevation in areas of high relief), and tritium-helium groundwater ages.

      Tritium-helium aquifer residence times are somewhat greater in Yosemite Valley wells (10-28 years) than in Olympic Valley wells (<1-23 years), but in both areas the results indicate rapid turnover of groundwater in the coarse alluvium of the upper valley aquifers.  The much larger Upper Merced catchment thus supplies only somewhat greater buffering to the perturbations in recharge and runoff that would be expected due to warmer temperatures.  Using the information gathered from these tracers, differing scenarios characterized by earlier snowpack melting and a higher proportion of precipitation as rain are evaluated with regard to potential impacts to recharge.  The observed rapid turnover of a significant portion of the groundwater means water managers can expect almost immediate decreases in baseflow to streams and decreased groundwater storage if infiltration decreases due to more rain on snow events, more evapotranspiration, or lower total precipitation.