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

Abstract

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 (Sv_max), 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 Sv_max 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.