Substantial amounts of the greenhouse gas methane can be released from run-of-the-river reservoirs in Switzerland, researchers have discovered.

The finding by scientists at Eawag, the Swiss Federal Institute of Aquatic Science and Technology, tarnishes the reputation of hydropower as a climate-neutral way of generating electricity.

Research at Lake Wohlen near Bern showed that on average, daily emissions of methane amount to more than 150 mg per square metre surface area – by far the highest emission rate so far recorded for a temperate reservoir. When the water temperature rises to 17 degrees Celsius the rate is twice as high, which makes it comparable with the emission rates observed for tropical reservoirs.

Overall, the reservoir on the River Aare produces 150 tonnes of methane a year. This is about the same amount as is emitted annually by around 2,000 cows. In terms of global warming potential, it is equivalent to the carbon dioxide emissions from 25 million car kilometres, since methane is about 25 times as potent a greenhouse gas as carbon dioxide (CO2).

However, the researchers do not wish to dramatise their findings: even if the Aare hydropower plant is taken to be responsible for all the methane emissions from the reservoir, and these are expressed in CO2-equivalents, a coal-fired power station with the same output produces 40 times as much CO2.

The researchers now plan to study other run-of-the-river reservoirs on the Central Plateau to determine whether Lake Wohlen is a special case, or whether methane budgets need to be revised for Switzerland as a whole.

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