Subtropical reservoirs are an important source of atmospheric methane (CH₄). This study investigated the spatiotemporal variability of bubble and diffusive CH₄ emissions from a subtropical reservoir, including its upstream and downstream rivers, in eastern China. There was no obvious seasonal variation in CH₄ emissions from the main reservoir, which increased slightly from the first half year to the next half year. In the upstream river, CH₄ emissions were low from February to June and fluctuated widely from July to January due to bubble activity. In the downstream river, CH₄ emissions were lowest in February, which was possibly influenced by the low streamflow rate from the reservoir (275 m³ s⁻¹) and a short period of mixing. There was spatial variability in CH₄ emissions, where fluxes were highest from the upstream river (3.65 ± 3.24 mg CH₄ m⁻² h⁻¹) and lowest from the main reservoir (0.082 ± 0.061 mg CH₄ m⁻² h⁻¹), and emissions from the downstream river were 0.49 ± 0.20 mg CH₄ m⁻² h⁻¹. Inflow rivers are hot spots in bubble CH₄ emissions that should be examined using field-sampling strategies. This study will improve the accuracy of current and future estimations of CH₄ emissions from hydroelectric systems and will help guide mitigation strategies for greenhouse gas emissions.

Carbonaceous greenhouse gases (i.e., CO2, CH4) have often reported to be emitted from the surfaces of reservoirs, while other non-carbonaceous greenhouse gases are not available to many reservoirs. In this study, the spatiotemporal variability of N2O emissions from Xin’anjiang Reservoir was investigated at the different regions over a 13-month period using the method of the static floating chambers combined with gas chromatography. Results showed that the average N2O emission flux was the highest in the downstream river (220.24±153.68 μg m-2·s-1), followed by the upstream river (49.85±48.55 μg m-2·s-1). The reservoir’s main body had the lowest flux (26.61-39.07 μg m-2·s-1). Seasonal variability indicated that the average N2O emission fluxes before the dam reached their maximum in March, which was likely influenced by the disappearance of thermal stratification then, caused a short N2O flux peak at the waterair interface. A thorough investigation should be carried out at the different reservoir regions for a longterm basis to discover the spatiotemporal variability in N2O emission flux in a hydroelectric reservoir system, which could avoid the error of estimating the N2O emissions incorrectly.

Hydroelectricity has typically been regarded as a green energy source, but reservoirs created for its generation emit greenhouse gases (GHGs) just as natural lakes and rivers do. The role of reservoirs in GHG emissions has been overlooked. Substantial amounts of methane (CH₄) are emitted from reservoir surfaces every year, which account for about 20% of the total CH₄ emission from inland waters. GHG emissions (transferred into carbon dioxide (CO₂) equivalents) from some tropical reservoirs even exceed CO₂ emissions from thermal power plants if the same amount of electricity is generated.