Ebullition causes methane emissions in tropical reservoirs

Aug 14, 2014

For the first time, methane emissions by ebullition from tropical reservoirs have been accurately quantified, revealing that this emission pathway depends on both the water level in the reservoir, which is dependent on the monsoon, and on daily variations in atmospheric pressure. Although tropical reservoirs probably emit over 10% of anthropogenic methane, their emissions are still poorly quantified. In this study, a new automatic system for the continuous measurement of methane fluxes was deployed on the reservoir of the largest hydroelectric dam in Southeast Asia. The results of this work, which was carried out by researchers at the Laboratoire d'Aérologie (CNRS/Université Toulouse III Paul Sabatier) and the Laboratoire Géosciences Environnement Toulouse (CNRS/Université Toulouse III Paul Sabatier/IRD), are published in the journal Biogeosciences dated 13 August 2014.

For the past ten years, it has been known that hydroelectric reservoirs in tropical regions are an important source of methane (CH4), a greenhouse gas that is considerably more powerful than carbon dioxide. Methane is produced by bacteria that decompose organic matter in the soils and vegetation flooded when the reservoir is filled. In tropical regions, these reservoirs probably release between 1% and 18% of
anthropogenic methane into the atmosphere.

However, these emissions are still poorly quantified since only a small number of reservoirs have been studied, and not all the emission pathways have been taken into account. Methane can be released from a reservoir into the atmosphere by diffusion (when the gas dissolved in the water enters the atmosphere), by ebullition (when large gas bubbles rise up from the bottom and burst at the water surface), and,
downstream from the turbines, by degassing (turbulence speeds up transfer of dissolved methane to the atmosphere). However, until now, only diffusion had been systematically studied, unlike other pathways.

A team of researchers has been studying the Nam Theun 2 Reservoir in Laos (the largest in Southeast Asia) prior to its filling, in May 2008, to this day. In addition to conventional methods (floating chambers to capture diffusive fluxes of gas, and submerged funnels to trap bubbles), they used an innovative technique to measure total . Specifically, a micrometeorological station deployed on the 450 km2
reservoir continuously measured vertical wind speed and CH4 concentrations, which made it possible to calculate the methane flux3 from the reservoir. Unlike conventional methods, which require human presence, these stations are fully automatic and take measurements every 30 minutes, 24 hours a day.

The measurements carried out on the Nam Theun 2 reservoir enabled the scientists to show that ebullition accounted for 60% to 80% of total emissions from the reservoir in the first years following filling.

In addition, ebullition intensity varies both diurnally and seasonally. During the four months of the hot dry season (mid-February to mid-June), emissions reach their maximum, since are low. Daily variations are controlled by atmospheric pressure: during the two daily pressure drops (in the middle of the day and the middle of the night), CH4 ebullition increases. With the help of a statistical model, day-to-day
data related to and water level was thus used to reconstruct emissions by ebullition over a continuous four-year period (2009-2013).

The results obtained highlight the importance of very frequent measurements of methane fluxes. They also show that the ebullition process, and therefore the amount of methane emitted from tropical reservoirs during their first years of operation, has most certainly been underestimated until now. For the researchers, the next stage will be to quantify diffusion at the surface of the reservoir and emissions downstream from the dam to the same level of accuracy, so as to complete the assessment of from this reservoir, and better assess their contribution to the global greenhouse effect.

Explore further: Replacing coal and oil with natural gas will not help fight global warming

More information: Physical controls on CH4 emissions from a newly flooded subtropical freshwater hydroelectric reservoir: Nam Theun 2, C. Deshmukh, D. Serça, C. Delon, R Tardif, M. Demarty, C. Jarnot, Y. Meyerfeld, V. Chanudet, P. Guédant, W. Rode, S. Desclou, and F. Guérin, Biogeosciences, 13 August 2014.

Related Stories

A new global warming culprit: Dam drawdowns

Aug 08, 2012

Washington State University researchers have documented an underappreciated suite of players in global warming: dams, the water reservoirs behind them, and surges of greenhouse gases as water levels go up and down.

Figuring out methane's role in the climate puzzle

Jul 09, 2014

The U.S. may be on the verge of an economy driven by methane, the primary component of natural gas, which burns cleaner than coal and is undergoing a production boom. It has poised the country as a top fuel producer globally, ...

Recommended for you

China's struggle for water security

17 hours ago

Way back in 1999, before he became China's prime minister, Wen Jiabao warned that water scarcity posed one of the greatest threats to the "survival of the nation".

Canada revises upward CO2 emission data since 1990

18 hours ago

Canada revised its greenhouse gas emission data from 1990 to 2013 in a report Friday, showing it had higher carbon dioxide discharges each year, and a doubling of emissions from its oil sands.

Climate censorship gains steam in red states

Apr 17, 2015

While plenty of people found humor in the recent news that officials in Florida and Wisconsin are censoring state workers' ability to talk about, much less work on, climate change, other states are not necessarily laughing. ...

User comments : 0

Please sign in to add a comment. Registration is free, and takes less than a minute. Read more

Click here to reset your password.
Sign in to get notified via email when new comments are made.