Methane and Carbon Dioxide Emissions From Reservoirs: Controls and Upscaling

Estimating carbon dioxide (CO₂) and methane (CH₄) emission rates from reservoirs is important for regional and national greenhouse gas inventories. A lack of methodologically consistent data sets for many parts of the world, including agriculturally intensive areas of the United States, poses a major challenge to the development of models for predicting emission rates. In this study, we used a systematic approach to measure CO₂ and CH₄ diffusive and ebullitive emission rates from 32 reservoirs distributed across an agricultural to forested land use gradient in the United States. We found that all reservoirs were a source of CH₄ to the atmosphere, with ebullition being the dominant emission pathway in 75% of the systems. Ebullition was a negligible emission pathway for CO₂, and 65% of sampled reservoirs were a net CO₂ sink. Boosted regression trees (BRTs), a type of machine learning algorithm, identified reservoir morphology and watershed agricultural land use as important predictors of emission rates. We used the BRT to predict CH₄ emission rates for reservoirs in the U.S. state of Ohio and estimate they are the fourth largest anthropogenic CH₄ source in the state. Our work demonstrates that CH₄ emission rates for reservoirs in our study region can be predicted from information in readily available national geodatabases. Expanded sampling campaigns could generate the data needed to train models for upscaling in other U.S. regions or nationally.