Minor methane emissions from an Alpine hydropower reservoir based on monitoring of diel and seasonal variability

Abstract

We monitored CH₄ emissions during the ice-free period of an Alpine hydropower reservoir in the Swiss Alps, Lake Klöntal, to investigate mechanisms responsible for CH₄ variability and to estimate overall emissions to the atmosphere. A floating eddy-covariance platform yielded total CH₄ and CO₂ emission rates at high temporal resolution, while hydroacoustic surveys provided no indication of CH₄ ebullition. Higher CH₄ fluxes (2.9 ± 0.1 mg CH₄ per m² per day) occurred during the day when surface water temperatures were warmer and wind speeds higher than at night. Piston velocity estimates (k₆₀₀) showed an upper limit at high wind speeds that may be more generally valid also for other lakes and reservoirs with limited CH₄ dissolved in the water body: above 2.0 m s⁻¹ a further increase in wind speed did not lead to higher CH₄ fluxes, because under such conditions it is not the turbulent mixing and transport that limits effluxes, but the resupply of CH₄ to the lake surface. Increasing CH₄ fluxes during the warm season showed a clear spatial gradient once the reservoir started to fill up and flood additional surface area. The warm period contributed 27% of the total CH₄ emissions (2.6 t CH₄ per year) estimated for the full year and CH₄ accounted for 63% of carbonic greenhouse gas emissions. Overall, the average CH₄ emissions (1.7 to 2.2 mg CH₄ per m² per day determined independently from surface water samplings and eddy covariance, respectively) were small compared to most tropical and some temperate reservoirs. The resulting greenhouse gas (GHG) emissions in CO₂-equivalents revealed that electricity produced in the Lake Klöntal power plant was relatively climate-friendly with a low GHG-to-power output ratio of 1.24 kg CO_2,eq per MW h compared to 6.5 and 8.1 kg CO_2,eq per MW h associated with the operation of solar photovoltaics and wind energy, respectively, or about 980 kg CO_2,eq per MW h for coal-fired power plants.