Greenhouse gas emissions (CO2, CH4 and N2O) from perialpine and alpine hydropower reservoirs

In eleven reservoirs located at different altitudes in Switzerland depth profiles of greenhouse gas (CO2, CH4, and N2O) concentrations were measured several times during spring and summer. Trace gas emissions were calculated using surface concentrations, wind speeds and transfer velocities. Additionally we assessed methane loss at the turbine and the methane input by inflowing water. All reservoirs were net emitters of CO2 with an average of 1030±780 mg m−2 d−1 and of methane with an average of 0.20±0.15 mg m−2 d−1. One reservoir (Lake Wohlen) emitted methane at a much higher rate (160±110 mg m−2 d−1), most of which (>98%) was due to ebullition. Only lowland reservoirs were sources for N2O (72±22 μg m−2 d−1), while the subalpine and alpine reservoirs seem to be in equilibrium with atmospheric concentrations. Methane loss at the turbine was as large as the diffusive flux from the surface for two subalpine reservoirs and around five times smaller for a lowland reservoir. The available data suggests greenhouse gas emissions from reservoirs in the Alps are minor contributors to the global greenhouse gas emissions.