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

We monitored CH emissions during the ice-free period of an Alpine hydropower reservoir in the Swiss Alps, Lake Klöntal, to investigate mechanisms responsible for CH variability and to estimate overall emissions to the atmosphere. A floating eddy-covariance platform yielded total CH and CO emission rates at high temporal resolution, while hydroacoustic surveys provided no indication of CH ebullition. Higher CH fluxes (2.9 ± 0.1 mg CH per m per day) occurred during the day when surface water temperatures were warmer and wind speeds higher than at night. Piston velocity estimates (k) showed an upper limit at high wind speeds that may be more generally valid also for other lakes and reservoirs with limited CH dissolved in the water body: above 2.0 m s a further increase in wind speed did not lead to higher CH fluxes, because under such conditions it is not the turbulent mixing and transport that limits effluxes, but the resupply of CH to the lake surface. Increasing CH 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 CH emissions (2.6 t CH per year) estimated for the full year and CH accounted for 63% of carbonic greenhouse gas emissions. Overall, the average CH emissions (1.7 to 2.2 mg CH per m 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 CO-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 per MW h compared to 6.5 and 8.1 kg CO per MW h associated with the operation of solar photovoltaics and wind energy, respectively, or about 980 kg CO per MW h for coal-fired power plants.