Siirila-Woodburn et al. (2026) Warming and snow loss increase reliance on old groundwater in a Colorado River headwater

Identification

• Journal: Nature Geoscience
• Year: 2026
• Date: 2026-04-08
• Authors: Erica Siirila-Woodburn, Nicholas Thiros, Michelle Newcomer, William Rudisill, P. James Dennedy-Frank, D. Feldman, Matthias Sprenger, Rosemary Carroll, Kenneth H. Williams, Eoin Brodie
• DOI: 10.1038/s41561-026-01945-y

Research Groups

• Lawrence Berkeley National Laboratory, Earth and Environmental Sciences Area, Berkeley, CA, USA
• Departments of Marine and Environmental Sciences and Civil and Environmental Engineering, Northeastern University, Boston, MA, USA
• Department of Forestry and Environmental Resources, North Carolina State University at Raleigh, Raleigh, NC, USA
• Division of Hydrologic Sciences, Desert Research Institute, Reno, NV, USA
• Rocky Mountain Biological Laboratory, Gothic, CO, USA
• Department of Environmental Science, Policy, and Management, University of California, Berkeley, Berkeley, CA, USA

Short Summary

Integrated hydrologic modeling in the Upper Colorado River headwaters reveals that atmospheric warming and snow loss increase reliance on older groundwater for streamflow, leading to disproportionate declines in high-elevation groundwater storage and an overall aging of streamflow contributions.

Objective

• To determine whether the loss of old-age groundwater buffers streamflow during low-snow years and whether that loss is exacerbated with warming in the Upper Colorado River headwaters.

Study Configuration

• Spatial Scale: East River watershed, Upper Colorado River Basin, Colorado, USA (300 km² mountainous headwater system). Elevation ranges approximately 1,400 m with a mean of 3,266 m, with specific focus on high elevations (>3,700 m). Model resolution: 100 m horizontal.
• Temporal Scale: Water years 2015–2021 (7-year baseline simulation) with continuous numerical warming experiments applied. Groundwater age spans decades to millennia. Runoff ratio analysis includes a century-long USGS gauge record.

Methodology and Data

• Models used:
  • Integrated hydrologic model: ParFlow coupled with the Common Land Model (CLM).
  • Lagrangian particle tracking eco-hydrologic model: EcoSLIM.
• Data sources:
  • Meteorological forcing: North American Land Data Assimilation System (NLDAS-2), downscaled.
  • Topography: 2015 LiDAR Digital Elevation Model.
  • Land cover: 2018 National Ecological Observatory Network (NEON) flyover, parameterized by International Geosphere-Biosphere Program (IGBP) database.
  • Soils/Lithology: Soil Survey Geographic Database (SURGO), Gaskill et al. (1991).
  • Groundwater observations: Continuous water table depth (WTD) measurements from a well transect (since 2016). Environmental tracer observations (dissolved noble gases, chlorofluorocarbons, SF6, tritium) for groundwater age.
  • Streamflow: In-situ measurements at the watershed outlet (WF-SFA Pumphouse ISCO), USGS gauge data (Almont Gauge, 112+ years).
  • Snow: Airborne Snow Observatory (ASO) flights, SNOTEL (Snow Telemetry) stations, X-band radar disdrometer snowfall from SAIL campaign.
  • Evapotranspiration: Remotely sensed evapotranspiration, Eddy Covariance tower evapotranspiration and transpiration.
  • Stable Isotopes: Approximately biweekly stream water stable isotope (δ18O) observations (7 years), multi-year precipitation observations for δ18O.
  • Historical precipitation/temperature: Parameter-Elevation Relationships on Independent Slopes Model (PRISM).

Main Results

• Old-groundwater contributions to streams remain relatively steady through time, while young groundwater contributions are more variable.
• Numerical experiments with increased surface air temperatures (+2.5 °C and +4 °C) result in:
  • Increased rain–snow fractions.
  • Increased evapotranspiration by approximately 16–17 mm per year per degree Celsius of warming.
  • Decreased runoff ratio by 2–3% per degree Celsius increase (e.g., -2.5% per °C for +2.5 °C case).
  • Decreased streamflow by approximately 15–16 mm per year per degree Celsius of warming.
  • Decreased groundwater storage by approximately 1 mm per year per degree Celsius of warming.
• As streamflow declines with warming, the age of groundwater supporting it becomes older, partly due to intermediate-aged (1–3 year) groundwater declining twice as fast (e.g., -12% discharge fraction per decade with 4.0 °C warming).
• Water table depths at higher elevations (>3,700 m) decline disproportionately (local changes up to -7 m) and fail to recover even during wet years.
• Groundwater storage shows a steady decline across all scenarios over the 7-year period (baseline: 9.39 × 10⁶ m³ per decade; +4.0 °C: 1.25 × 10⁷ m³ per decade).
• Groundwater buffers streamflow response during low-snow years; high annual runoff ratio correlates with the greatest relative declines in groundwater storage (e.g., lowest-snow year WY2018 had a peak runoff ratio of 82.8% and a minimum groundwater storage ratio of -17.1%).
• The median age of groundwater flux to streams increases from approximately 4–6 years in the baseline to up to 8 years with 4.0 °C warming over the 7-year simulation.
• Groundwater contributes 70–75% of streamflow per year, with groundwater older than 3 years contributing 24–33% of total streamflow annually.

Contributions

• This study provides the first integrated hydrologic modeling with validation from in situ groundwater observations at high elevations in the East River watershed.
• It introduces a novel metric, the 'groundwater storage ratio' (GSR), to quantify the annual rate of change of subsurface water storage relative to annual precipitation.
• The research differentiates streamflow sources (overland flow vs. age-differentiated groundwater) across varying snow conditions and warming scenarios.
• It offers process-based insights into how groundwater, particularly older water, provides a semi-constant buffer to streamflow response.
• The findings highlight that warming leads to disproportionate groundwater declines at higher elevations and an overall aging of streamflow contributions.
• The study suggests potential unsustainable conditions and depletion of groundwater reserves in mountain headwater systems under future warming and snow loss scenarios.

Funding

• Watershed Function Science Focus Area at Lawrence Berkeley National Laboratory, funded by the US Department of Energy, Office of Science, Biological and Environmental Research (contract no. DE-AC02-05CH11231).
• US Department of Energy, Office of Science, Office of Biological and Environmental Research and the Atmospheric System Research Program (contract no. DE-AC02-05CH11231).
• National Energy Research Scientific Computing Center (NERSC), a US Department of Energy Office of Science User Facility (contract no. DE-AC02-05CH11231, awards m2398 and m4098 for 2023–2025).

Citation

@article{SiirilaWoodburn2026Warming,
  author = {Siirila-Woodburn, Erica and Thiros, Nicholas and Newcomer, Michelle and Rudisill, William and Dennedy-Frank, P. James and Feldman, D. and Sprenger, Matthias and Carroll, Rosemary and Williams, Kenneth H. and Brodie, Eoin},
  title = {Warming and snow loss increase reliance on old groundwater in a Colorado River headwater},
  journal = {Nature Geoscience},
  year = {2026},
  doi = {10.1038/s41561-026-01945-y},
  url = {https://doi.org/10.1038/s41561-026-01945-y}
}