Is Groundwater Effective at Buffering Mountainous Watersheds in a Changing Climate?
Erica Siirila-Woodburn1* (firstname.lastname@example.org), Rosemary Carroll2, Nicholas Thiros1, P. James Dennedy-Frank1,3, Payton Gardner4, Harry Stone5, Mario Soriano5, Reed Maxwell5, Kenneth Williams1, Eoin Brodie1
1Lawrence Berkeley National Laboratory, Berkeley, CA; 2Desert Research Institute, Reno, NV; 3Northeastern University, Boston, MA; 4University of Montana, Missoula, MT; 5Princeton University, Princeton, NJ
A synthesis of western U.S. snowpack projections reveals that ongoing declines of snow are expected to continue in the coming decades, yet little is known about how groundwater buffers watershed function, including water yields, as the climate transitions to a “low-to-no snow future.” Highlighted are the results of that synthesis, as well as numerical and in situ studies examining the sensitivity of interannual variability of snowpack changes in the East River of the Upper Colorado River Basin with the aim to better understand groundwater dynamics given the feedbacks between snow, soil moisture, and groundwater. Observations of environmental tracers (e.g., dichlorodifluoromethane, sulfur hexafluoride, tritium, and helium-4) are used to estimate groundwater residence times, suggesting a mixture of modern and premodern water ages (decades to millennia), necessitating a binary mixing residence time model to represent the bedrock dynamics.
Dissolved noble gas analysis shows that groundwater recharge is preferentially generated at high watershed elevations. Additionally, results of tritium from a new hillslope weir network co-located with a soil moisture campaign suggest headwater runoff consists of water ages ranging from 10 to 70 years, highlighting the importance of groundwater flow feeding near-source surface flow. Numerical hillslope modeling suggests that long residence time bedrock groundwater can represent a considerable flux into the shallow soil and floodplain systems, and that the partitioning of young residence time water from snowmelt versus old groundwater is sensitive to the bedrock hydrogeologic parameters. These results highlight the need to better characterize bedrock systems to improve understanding of the connectivity between groundwater flow paths and shallow soil and surface waters.
Finally, watershed-scale simulations indicate that lateral subsurface flow is a critical mechanism moving snowmelt downgradient to generate streamflow, subsidize evapotranspiration, and maintain groundwater levels in water-limited portions of the basin. Simulations indicate exceptionally warm and dry water years in the last decade have initiated significant groundwater declines. Additional warming will exacerbate storage loss through increased forested water use, with streamflow reductions 20–60% larger when accounting for depleted groundwater reserves. Researchers estimate groundwater destabilization in a warming climate that is unable to rebound to historical levels with a significant impact on streamflow. Ongoing hydrologic observations (e.g., snowpack, soil moisture, groundwater levels, streamflow) coupled with a comprehensive campaign evaluating groundwater ages at the East River will help to inform model experiments to determine the role of groundwater buffering in mountainous systems.