Marshes in the drawdown area of the Three Gorges Reservoir could be a significant source of methane. Credit: Wikimedia Commons

Marshland created when China's Three Gorges Reservoir is partially drained during the summer may be a significant source of the powerful greenhouse gas methane, researchers say.

The findings, published in the Journal of Geophysical Research1, are among the latest to raise questions over the green credentials of hydropower.

Scientists have become increasingly concerned about the greenhouse gases released by submerged grass and trees when land is flooded to create dams. When such organic matter decays, it releases methane and carbon dioxide, which contribute to global warming. Methane is particularly troublesome as it has more than 20 times the warming impact of CO2.

Previous studies of methane emission have focused on dam components themselves, such as the power-generating turbines — where the gas fizzes out of solution as reservoir water is fed into them — and also on the reservoir surface, where methane is released either through slow diffusion or as bubbles. But many dams are partially drained at different times of year for maintenance purposes, such as silt removal, exposing 'drawdown' areas that have previously been underwater.

"Little attention has been paid to the drawdown regions, probably because they usually constitute a small part of the reservoir surface area," says Huai Chen, an ecologist at Chongqing University in China, who was first author of the study.

The Three Gorges Reservoir, however, has a large drawdown area of 350 square kilometres — or about one-third of the total flooded reservoir area when the dam is in full operation. That led Chen and his colleagues to suspect that methane emission from such areas might not be negligible.

Summer peak

In a pilot study, the researchers focused on an area of 37 square kilometres near the Pengxi River, a branch of the Yangtze River, that becomes marshland when the water level is lowered in the summer to clear silt. They covered a fixed area of the wetland with a plastic chamber to trap escaping gases, which they then tapped and analysed.

After taking measurements from four types of vegetation every ten days from July to September in 2008, the team found that different plants emitted varying amounts of methane at different times of the year. Plants that grow in deep water emit more methane than those that thrive in shallow water. High methane emitters also seemed to grow in soil with high levels of dissolved organic carbon, which can be used by microbes to produce methane.

The researchers report that methane emission peaks in early July, and is followed by low, steady emission before the winter flooding. The decline of methane release after the July leak may be due to low water levels during the summer, says Chen.

The researchers calculated that, on average, the newly created marshes along the Pengxi River emitted 6.7 milligrams of methane per square metre per hour — higher than the rate of methane emission from the surface of reservoirs in the tropics, such as the Petit Saut reservoir in French Guiana or the Balbina and Samuel reservoirs in Brazil.

The amount of methane emitted from the surface of the Three Gorges Reservoir has not been accurately measured, says Chen. But assuming that it is similar to that emitted by tropical reservoirs, the additional methane emitted by the marshes created by summer drainage — which account for 100 km2 and 10% of the total reservoir area — could be as much as a fifth of that emitted from the reservoir surface. Chen says that the findings might help to reduce methane emission from the reservoir by, for example, removing vegetation from marshy areas at low-water levels.

Mapping methane

"This is an important study because it is the first careful look at greenhouse-gas emission from a major Chinese dam," says David Victor, an energy-policy expert at Stanford University in California. He cautions, however, against extrapolating the findings to other reservoirs because dam construction and operation vary enormously, and calls for similar studies on reservoirs in other regions.

"The research is a welcome addition to the current efforts in estimating methane emission by reservoirs around the world," says Philip Fearnside, a conservation biologist at the National Institute for Research in the Amazon in Manaus, Brazil. The levels of methane released by flooded, decaying organic matter may fall over time, but plants growing in marshes created by periodic draining could be a long-term source of methane emission, he adds.

Chen and his colleagues are in the process of documenting methane emission from additional marshes at low-water levels as well as from the surface of the Three Gorges Reservoir. They are taking monthly measurements every 200 kilometres along the length of the reservoir, and expect to get an estimate of total emissions by the end of next year.

The team, however, will not be able to study methane release from the dam's turbines because that area is not accessible to researchers. That could be a serious drawback to the study, Fearnside warns. He believes that the turbines may be the main culprit of methane emission from the dam.