DEGASSING EMISSIONS IN A TEMPERATE HYDROPOWER RESERVOIR: EXPLORING SUB-ANNUAL VARIABILITY AND SENSITIVITY

Globally, human-made reservoirs are important ecosystems in inland water carbon processing. Reservoirs can be significant sources of methane (CH₄) emissions, though temporal variability in emissions is uncertain given limited empirical studies. In many hydropower reservoirs, one pathway of CH₄ emissions is via degassing, where deepwater intakes can pull water with high dissolved CH₄ concentrations through turbines. When this water is removed from deepwater pressure and circulated during power generation, much of the dissolved CH₄ is released (“degassed”) into the atmosphere. While this emissions pathway is understudied, a global modelling study suggested it can account for over one-third of greenhouse gas emissions from hydropower reservoirs. In 2023-2024, we conducted monthly sampling at Douglas Reservoir (Tennessee, USA) to quantify sub-annual temporal patterns in CH₄ degassing and sensitivity to sampling date. Over this year, CH₄ degassing emissions were highest in summertime due to CH₄ accumulation in deep waters during thermal stratification. However, discharge was highly variable at daily to weekly time scales, meaning that degassing flux estimates were very sensitive to sampling date. For instance, discharge from a different date within one week of the original sampling date could have resulted in 85% lower to 82% higher CH₄ degassing estimates. Understanding the variability in degassing fluxes at sub-annual scales is necessary to improve estimates of CH₄ emissions from hydropower reservoirs and understand their role in global carbon processing.

Presentation Preference: Oral

Primary Presenter: Rachel Pilla, Oak Ridge National Laboratory (pillarm@ornl.gov)

Authors:

Rachel Pilla, Oak Ridge National Laboratory  (pillarm@ornl.gov)

Natalie Griffiths, Oak Ridge National Laboratory  (griffithsna@ornl.gov)

Carly Hansen, Oak Ridge National Laboratory  (hansench@ornl.gov)

R Trent Jett, Oak Ridge National Laboratory  (jettrt@ornl.gov)

Michael Jones, Oak Ridge National Laboratory  (jonesmw@ornl.gov)

Nikki Jones, Oak Ridge National Laboratory  (jonesnj@ornl.gov)

Trystan Bordeau, Oak Ridge National Laboratory  (bordeauta@ornl.gov)