Variability of greenhouse gas (CH ₄ and CO ₂ ) emissions in a subtropical hydroelectric reservoir: Nam Theun 2 (Lao PDR)

Abstract. Hydroelectric reservoirs, though fundamental to renewable energy generation, are increasingly recognized as important sources of greenhouse gases (GHGs) in tropical and subtropical regions. However, substantial uncertainty persists regarding their contributions to the GHG cycle and their overall climate impact. Here, we present a 14 year dataset (2009–2022) of CH4 and CO2 emissions from the Nam Theun 2 reservoir in Lao PDR. We report emissions through diffusion, ebullition, and downstream degassing below turbines and spillways. This work represents one of the longest continuous records of reservoir GHG emissions in a subtropical system. Complementary eddy covariance approach was also employed, the results showed that CO2 emissions were consistently higher than those estimated from discrete sampling, likely due its ability to capture real-time turbulence and hot moments, and to the location of the EC system in shallow, high-emission areas for the last two campaigns. In contrast, CH4 emissions upscaled from EC measurements were often lower than those derived from discrete sampling, particularly during later campaigns. This difference was attributed to spatial coverage limitations, meteorological influences, wind filtering, and the lower sensitivity of the EC system to episodic ebullition events. CH4 showed a clear diurnal pattern; while CO2 fluxes differed between day and night only during periods of strong stratification. In this study, discrete sampling provided broader spatial coverage and higher data availability; therefore, it was used for emission calculations. CH4 emissions peaked during the warm dry period due to lower water levels and intensified stratification that favored methanogenesis and ebullition, whereas CO2 emissions peaked during cold dry season overturn events that released accumulated hypolimnetic carbon. Across the study period, ebullition accounted for 77 % of total CH4 emissions and remained relatively stable, supported by substantial flooded organic matter reserves. In contrast, during that same period, diffusive CH4 fluxes declined by 97 %, and CO2 emissions – dominated by diffusive fluxes (96 %) – declined by 87 %, indicating reservoir aging and pro gressive depletion of labile organic matter. Over 14 years, cumulative gross emissions totaled 10 736 Gg CO2 eq., with CH4 (51 %) slightly exceeding CO2 (49 %). Annual emissions were greatest in 2010 (1276 Gg CO2 eq.), declining by ∼70 % by 2021. These findings provide new insight into long-term GHG budgets in subtropical reservoirs, refine global carbon budget estimates, and inform climate-sensitive hydropower planning.