Navigating Greenhouse Gas Emission Unknowns: A Hydroacoustic Examination of Mediterranean Climate Reservoirs.

Inland aquatic systems, such as reservoirs, contribute substantially to global methane (CH4) emissions; yet they are among the most uncertain contributors to the total global carbon budget. Reservoirs generate significant amounts of CH4 within their bottom sediment, where the gas is stored and can easily escape via ebullition. Due to the large spatial and temporal variability associated with ebullition, CH4 fluxes from these aquatic systems are challenging to quantify. To address these uncertainties, six different water storage reservoirs, with average flux rates ranging between 20 and 678 mg CH4 m−2 d−1, were hydro‐acoustically surveyed using a previously established technique to investigate the spatial variability of free gas stored at the sediment surface that could be released as bubbles. Sediment samples and vertical profiles of temperature and dissolved oxygen were also collected to understand their respective influences on sediment gas formation. We found that the established relation used to determine sediment gas storage via the sonar technique, which relied solely on acoustic backscatter (Svmax), tended to underestimate gas storage in shallower, siltier sediment zones and overestimate gas storage in coarser (>2 mm) sediment zones. In response, we introduce an improved model, incorporating gas and sediment type as correction factors for gas attenuation effects on Svmax values. The extended model is able to elucidate patterns within the gas volume data, revealing clearer trends across different sediment types. This research provides valuable data and methodological insights that can enhance the accuracy of greenhouse gas modeling and budget assessments for reservoirs. Plain Language Summary: Inland aquatic systems, like reservoirs, contribute substantially to greenhouse gas emissions, but these systems comprise the most uncertain components of the CH4 budget. Reservoirs can produce significant amounts of CH4 in submerged sediments, which escape slowly through diffusion and quickly through bubbling. However, accurately measuring CH4 emissions from reservoirs is difficult due to significant variability in bubbling patterns over space and time. To better understand these patterns, we surveyed six reservoirs using an established underwater sonar technique to explore the spatial variability of free gas stored in sediments, which could bubble up. We also collected sediment samples and measured dissolved oxygen and temperature at various water depths to study their influence on sediment gas formation. We found that the previously established sonar model used to obtain the amount of gas in the sediments tended to underestimate gas storage in shallower, siltier areas and overestimate it in larger‐grained sediment zones. In response, we introduced a refined model that incorporates gas fraction and sediment type to correct for gas attenuation effects on the sonar output. This extended model clarifies gas volume patterns across sediment types. Our study offers valuable data and methodological insights to refine greenhouse gas modeling and budget assessments for reservoirs. Key Points: Hydroacoustic models relying solely on backscatter underestimate gas in shallow, silty sediments and overestimate in larger (>2 mm) onesAn extension to the current model incorporates gas fraction and sediment type to improve gas predictionsBubble plume mapping reveals ebullition hotspots are tied to gas storage, bottom temperature, dissolved oxygen, and sediment type [ABSTRACT FROM AUTHOR]