Greenhouse gas emissions from hydropower reservoirs: emission processes and management approaches

Hydropower reservoirs, as vital inland waters bodies of anthropogenic origin, exhibit distinct characteristics from natural waters, thereby garnering research interest in the quantification and mapping of greenhouse gas (GHG) emissions. In this review, we systematically examine studies focusing on GHG emissions from hydropower reservoirs. We identify two key primary physical mechanisms resulting from river damming, namely water impoundment and water regulation, which can significantly influence GHG emissions in hydropower reservoirs. Reservoirs vary in size, with smaller reservoirs exhibiting higher CH₄ emissions per unit area. For instance, small reservoirs have an average flux rate of 327.54 mg C-CH₄/m²/day, while medium-sized reservoirs emit 267.12 mg C-CH₄/m²/day, and large ones emit 37.34 mg C-CH₄/m²/day. This difference is potentially attributable to shorter water residence times in small reservoirs and increased susceptibility to littoral disturbance. In addition to reservoir scale, variations in GHG emissions between reservoirs are also influenced by the type of hydropower. Run-of-river and closed-loop pumped storage hydropower (PSH) systems are anticipated to exhibit lower GHG emissions (PSH: 4.2-46.5 mg C-CH₄/m²/day) in comparison to conventional impoundment hydropower, owing to their operational characteristics, facilitating mixing and oxygenation within the reservoir water column and reducing sedimentation. Nonetheless, further field measurements are warranted. Through the integration of literature insights, we propose solutions aimed at managing emissions, considering both physical mechanisms and hydropower planning. Ultimately, these findings will advance our understanding of GHG emissions from hydropower reservoirs and facilitate sustainable carbon reduction management practices.