Ebullition causes methane emissions in tropical reservoirs

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 dioxide1. 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 methane emissions. 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 flux 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 water levels 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 atmospheric pressure 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 methane emissions from this reservoir, and better assess their contribution to the global greenhouse effect.