Large hydropower reservoirs in Russia can act as net anthropogenic sinks of carbon-based greenhouse gases.

This study presents a methodology for assessing the anthropogenic component of the carbon-based greenhouse gases (GHG) balance of hydropower reservoirs in accordance with the Intergovernmental Panel on Climate Change (IPCC) guidelines. The methodology utilizes carbon balance calculations and the difference between the parameters of the reservoir water and the river located upstream. It integrates the results of field measurements in 2021–2023 and process-based modeling. Measurements of methane fluxes from the surface of reservoirs were carried out from a vessel using the floating chamber method without bubble emission shield in the shallow and deep-water parts of eight large reservoirs located in different bioclimatic zones of Russia (Kolymskoye, Bureiskoye, Volgogradskoye, Zeyskoye, Kuibyshevskoye, Rybinskoe, Chirkeyskoe, Sayano-Shushenskoe). Using this combined measurement and modeling data, we compared the obtained methane emission factors against those outlined in the IPCC methodological reports. The revised Tier 1 national factors for the boreal and cool temperate climate zones – 5.6 (± 4.3) and 33.2 (± 16.8) kg CH4 ha− 1 year− 1, respectively – are 59% and 39% lower than the corresponding default IPCC factors of 13.6 ± 6.3 and 54.0 ± 5.5 kg CH4 ha− 1 year− 1. According to the Tier 3 approach, the anthropogenic component of the GHG flux of all investigated reservoirs corresponds to negative net emissions, indicating an anthropogenic carbon sink, although the permanence of that accumulated carbon is uncertain. This suggests that the hydroelectric power of the studied reservoirs has a negative carbon footprint when direct fluxes considered.