Eddy covariance flux measurements confirm extreme CH4 emissions from a Swiss hydropower reservoir and resolve their short-term variability
1ETH Zurich, Institute of Agricultural Sciences, 8092 Zurich, Switzerland
2Eawag, Swiss Federal Institute of Aquatic Science and Technology, Kastanienbaum, Switzerland
3ETH Zurich, Institute for Biogeochemistry and Pollutant Dynamics, 8092 Zurich, Switzerland
4Uppsala University, Department of Ecology and Evolution, Limnology, Uppsala, Sweden
Abstract. Greenhouse gas budgets quantified via land-surface eddy covariance (EC) flux sites differ significantly from those obtained via inverse modeling. A possible reason for the discrepancy between methods may be our gap in quantitative knowledge of methane CH4 fluxes. In this study we carried out EC flux measurements during two intensive campaigns in summer 2008 to quantify methane flux from a hydropower reservoir and link its temporal variability to environmental driving forces: water temperature and pressure changes (atmospheric and due to changes in lake level). Methane fluxes were extremely high and highly variable, but consistently showed gas efflux from the lake when the wind was approaching the EC sensors across the open water, as confirmed by floating chamber flux measurements. The average flux was 3.76 ± 0.39 μg C m−2 s−1 (mean ± SE) with a median of 1.42 μg C m−2 s−1, which is quite high even compared to tropical reservoirs. Fluxes increased exponentially with increasing temperatures, but were decreasing exponentially with increasing atmospheric and/or lake level pressure. A multiple regression using lake surface temperatures (0.1 m depth), temperature at depth (10 m deep in front of the dam), atmospheric pressure, and lake level was able to explain 35.4 % of the overall variance. This best fit included each variable averaged over a 9-h moving window, plus the respective short-term residuals thereof. We estimate that an annual average of 3 % of the particulate organic matter (POM) input via the river is sufficient to sustain these large CH4 fluxes. To compensate the global warming potential associated with the CH4 effluxes from this hydropower reservoir a 1.3 to 3.7 times larger terrestrial area with net carbon dioxide uptake is needed, which indicates the potential relevance of temperate reservoirs and lakes in local and regional greenhouse gas budgets.