Mountainous Groundwater Response to Warming and Low-To-No Snow Conditions
Authors
Nicholas Thiros1* (nthiros@lbl.gov), Erica Siirila-Woodburn1, Rosemary Carroll2, Charuleka Varadharajan1, Curtis Beutler3, Craig Ulrich1, Michelle Newcomer1, Kenneth Williams1, Eoin Brodie1 (elbrodie@lbl.gov)
Institutions
1Lawrence Berkeley National Laboratory, Berkeley, CA; 2Desert Research Institute, Reno, NV; 3Rocky Mountain Biological Laboratory, Gothic, CO
URLs
Abstract
Groundwater is among the least understood components of the coupled hydrologic and biogeochemical cycles in mountainous watersheds, requiring further insights into its dynamics, connectivity to surface water, and watershed traits. To evaluate the effectiveness of groundwater storage in buffering streamflow and evapotranspiration during press and pulse disturbances (such as projections of low-to-no snow years in the coming decades), researchers develop integrated hydrologic models of the East River watershed that couple groundwater with surface water and energy fluxes. Researchers compare model predictions with in situ groundwater level and environmental age tracer observations.
At a lower montane hillslope, a high-resolution ParFlow-CLM hydrologic model and EcoSLIM particle tracking model suggest groundwater flowpaths contribute >60% of the water mass flux to the hillslope floodplain. These groundwater flowpaths persist all year and are characterized by decadal to century-scale water ages, which are consistent with mean ages from observed environmental tracers. Extending the particle tracking technique to account for matrix diffusion due to high fractured bedrock traits improves predictions of mixing between young and old-aged groundwater environmental tracers (3H and 4He, respectively). The hillslope models that best match observed 3H and 4He suggest groundwater storage has declined over the past decade. This result is consistent with observed groundwater level declines of >1 m since 2016 and potentially highlights the importance of old-aged groundwater storage in streamflow generation. To further interrogate connections between groundwater storage losses and streamflow generation researchers develop watershed-scale 3-D integrated hydrologic models. Simulations show annual runoff efficiency is inversely correlated with groundwater storage efficiency (ratio of annual groundwater storage change to precipitation), suggesting groundwater storage losses buffer runoff efficiencies during low precipitation years. This hypothesis is supported by a majority of simulated groundwater fluxes to streams consistently having water ages > 1 year. In a warmer climate, modeling suggests increased forest water use reduces groundwater recharge and results in groundwater storage loss. Losses are most severe during dry years and do not recover to historical levels during wet periods. Groundwater depletion will significantly reduce annual streamflow and force the basin toward intermittent conditions when precipitation is low. Expanding results across the region suggests groundwater decline will be highest in the Colorado Headwater and Gunnison basins. Ongoing work to validate model predictions with diverse observation datasets will continue to shed light on the role of groundwater on integrated hydrologic processes in mountain systems experiencing changing snowpack conditions.