Variability of greenhouse gas (CH₄ and CO₂) emissions in a subtropical hydroelectric reservoir: Nam Theun 2 (Laos PDR)

Hydroelectric reservoirs, while supporting renewable energy production, contribute notably to greenhouse gas (GHG) emissions in tropical and subtropical regions. This study presents a 14-year (2009–2022) analysis of CH4 and CO4 emissions from the Nam Theun 2 (NT2) reservoir, using a combination of discrete water sampling, bubbling funnel traps, and high-frequency eddy covariance (EC) measurements. Emission pathways assessed include diffusion, degassing, and ebullition. EC results of CO2 fluxes were consistently higher than estimates from discrete samplings due to the system’s placement in shallow, high-emission areas and its capacity to capture real-time turbulence and diurnal variability. In contrast, CH4 fluxes from eddy covariance were often lower than discrete-based calculations, particularly in later campaigns, due to spatial limitations, wind filtering, and reduced sensitivity to bubbling events. The findings highlighted the importance of integrating multiple techniques to address spatial and temporal variation and reveal the influence of reservoir stratification, carbon cycling, and hydrological operations on greenhouse gas dynamics. Over the study period, total emissions reached 10736 Gg CO2eq, with CH4 contributing 51 % (5468 Gg CO2eq) and CO2 49 % (5268 Gg CO2eq). Emissions peaked in 2010 (1276 Gg CO2eq) and declined by approximately 70 % by 2021, driven by reservoir aging and depletion of labile organic matter. Seasonally, the warm dry season accounted for 3809 Gg CO2eq (35.5 %), the cold dry for 3841 Gg CO2eq (35.8 %), and the warm wet for 3086 Gg CO2eq (28.7 %). CH4 emissions were highest in the WD season due to enhanced ebullition under stratified and anoxic conditions, while CO2 emissions peaked in the CD season due to reservoir overturn and increased respiration. Ebullition dominated CH4 emissions (77 %) and remained stable over time, whereas diffusive CH4 fluxes declined by 97 %. CO2 emissions were majorly diffusive (96 %) and showed consistent decline (-87 %). This long-term dataset improved the understanding of subtropical reservoir emissions and provided insights for global carbon budgets and improving the climate impact assessment of hydropower development.