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275-1 Warming and snow loss increase reliance on old groundwater in a Colorado River headwater

Session: Mountain hydrology, glacial landscapes, and changing flow and sediment dynamics in mountain rivers

Presenting Author:

Nicholas Thiros

Authors:

Thiros, Nicholas¹, Siirila-Woodburn, Erica², Newcomer, Michelle ³, Rudisill, William ⁴, Dennedy-Frank, P. James⁵, Feldman, Daniel ⁶, Sprenger, Matthias ⁷, Carroll, Rosemary W. H.⁸, Williams, Kenneth H.⁹, Brodie, Eoin¹⁰r> (1) Energy and Resources Group, University of California, Berkeley, Berkeley, CA, USA, (2) Earth and Environmental Sciences Area, Lawrence Berkeley National Laboratory, Berkeley, CA, USA, (3) Earth and Environmental Sciences Area, Lawrence Berkeley National Laboratory, Berkeley, CA, USA, (4) Earth and Environmental Sciences Area, Lawrence Berkeley National Laboratory, Berkeley, CA, USA, (5) Department of Marine & Environmental Sciences, Northeastern University, Nahant, MA, USA, (6) Earth and Environmental Sciences Area, Lawrence Berkeley National Laboratory, Berkeley, CA, USA, (7) College of Natural Resources, North Carolina State University, Raleigh, NC, USA, (8) Division of Hydrologic Sciences, Desert Research Institute, Reno, NV, USA, (9) Earth and Environmental Sciences Area, Lawrence Berkeley National Laboratory, Berkeley, CA, USA; Rocky Mountain Biological Laboratory, Gothic, CO, USA, (10) Earth and Environmental Sciences Area, Lawrence Berkeley National Laboratory, Berkeley, CA, USA; Department of Environmental Science, Policy, and Management, University of California, Berkeley, Berkeley, CA, USA,

Abstract:

Atmospheric warming is reducing snowpack, with uncertain effects on mountainous streamflow, a crucial water resource. Despite limited historical observations of groundwater–streamflow interactions above 2,500 m, new measurements in the Upper Colorado River headwaters indicate declining groundwater storage that is dated decades to millennia old. Here we use integrated hydrologic modelling spanning water years 2015–2021 to determine whether the loss of old-age groundwater buffers streamflow during low-snow years and whether that loss is exacerbated with warming. Results show that old-groundwater contributions to streams remain relatively steady through time, unlike the more variable contributions from young groundwater. Numerical experiments of increased surface air temperatures (+2.5 °C and +4 °C) increase rain–snow fractions and evapotranspiration and decrease runoff ratio by 2–3% per degree Celsius increase. As streamflow declines with warming, the age of groundwater supporting it gets older, in part owing to intermediate-aged (1–3 year) groundwater declining twice as fast. Simulations show that water table depths at higher elevations (>3,700 m) decline disproportionately and fail to recover even during wet years. These findings suggest altered groundwater–streamflow interactions with warming and snow loss, with implications for water resources.

Warming and snow loss increase reliance on old groundwater in a Colorado River headwater

Description

Session Format: Oral

Presentation Date: 10/13/2026

Presentation Start Time: 01:35 PM

Presentation Room: CCC, 108

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