Reducing Uncertainty in Upscaled CH4 Ebullition Estimates from Reservoirs

Methane (CH₄), a potent greenhouse gas, is produced in freshwater ecosystems by microbial activity in the sediment. Estimates of CH₄ emissions from freshwaters, including hydropower reservoirs, are highly uncertain, often reflecting annualization or upscaling from snapshot sampling events during the summer when water levels are high. Some of the uncertainty in these estimates exists because CH₄ emissions, particularly via the ebullition (bubbling) pathway, are highly variable in space and time. CH₄ ebullition occurs primarily in the shallow areas of reservoirs where lower hydrostatic pressure and oxidation potential allow CH₄-rich bubbles to escape to the surface. However, reservoir-wide estimates of CH₄ ebullition often do not account for the changing size of the ebullition zone as water levels fluctuate on multiple temporal scales. Additionally, estimates often do not account for the temporal variations in CH₄ production due to physiochemical parameters such as temperature and eutrophication. In a case study of Douglas Reservoir, TN, USA, we used a monthly sampling regime from March 2023 to February 2024 to capture temporal variation in CH₄ ebullitive fluxes and the size of the ebullition zone. We also used a comprehensive survey design that incorporated reservoir bathymetry to reduce spatial uncertainty. We found that upscaled estimates of CH₄ ebullition that accounted for spatial and temporal variation were significantly lower than estimates that assumed a constant summertime flux and full pool elevation. These results indicate that incorporating sub-annual variations in flux and reservoir bathymetry into reservoir-wide estimates of CH₄ ebullitive emissions is important to reducing the uncertainty in greenhouse gas emissions estimates from hydropower reservoirs.

Primary Presenter: Anna Cardall, Brigham Young University (anna.cardall@gmail.com)

Authors:

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

Rachel Pilla, Oak Ridge National Laboratory  (pillarm1@ornl.gov)

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