Semidiurnal and seasonal variations in methane emissions from a sub-tropical hydroelectric reservoir (Nam Theun 2, Laos) measured by eddy covariance technique
Abstract
The quantification of sources and sinks of greenhouse gases (GHG) have become an important scientific issue. Hydroelectric reservoirs have been identified as considerable methane (CH4) sources to the atmosphere, especially in the tropics. Assessing these emissions and their variations on small and large time scale represent important challenges in our understanding of water-atmosphere exchange. In this context, objectives of this study are (i) to quantify the CH4 emissions, (ii) to determine the variations in the emissions on daily and seasonal time scale, and link these variations to environmental driving forces (iii) to compare different methodologies to assess CH4 emissions. Measurements of CH4 emissions were made in a recently impounded (in 2009) subtropical hydroelectric reservoir, Nam Theun 2 (NT2), in Lao PDR, Asia. The sampling strategy included three different types of flux measurement techniques: floating chambers, submerged funnels, and a micrometeorological station allowing for flux determination based on the eddy covariance technique (EC). We carried out flux measurements during four intensive field campaigns conducted in between May 2009 and June 2011. Eddy covariance system, composed by a 3D sonic anemometer coupled with a cavity ring-down spectroscopy (CRDS) analyzer, was deployed on the mast in a large surface of open water corresponding to an homogeneous ecosystem (floodplain). Diffusive and bubbling fluxes were measured using respectively floating chambers and submerged funnel techniques around the mast. Our results from the all four field campaigns show that individual 30-min EC fluxes varied by 4 order of magnitude (from 0.01 to 102 mmol.m-2.day-1). Average EC fluxes of individual campaigns varied inversely with water depth, from 5±3.5 to 28±16 mmol.m-2.day-1 for respectively from 10.5 to 2 m of water depths. Diffusive fluxes measured by floating chambers ranged between 0.2 and 3.2 mmol.m-2.day-1. Bubbling fluxes were found to be highly sporadic, with individual flux values varying from 0 to 102 mmol.m-2.day-1, with bubbles made by up to 80% of CH4. For all field campaigns, EC fluxes were very consistent with the sum of the two terms measured independently (diffusive fluxes + bubbling fluxes = EC fluxes), indicating that the eddy covariance system picked-up both diffusive and bubbling fluxes from the reservoir, which is a very new and encouraging result for further studies. An opposite behavior was found between fluxes and atmospheric pressure. Furthermore, we observed a semidiurnal variation of EC fluxes during all four campaigns, that is two peaks per day - one in early morning and one in the afternoon, linked to the semi-diurnal atmospheric variation. Due to the larger drop in atmospheric pressure in the late morning than in the night, the afternoon flux peak is comparatively higher than in the early morning flux peak. On a seasonal basis, monitoring suggested that the significant seasonal variation in CH4 emissions was strongly correlated with associated changes in the water depth.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2012
- Bibcode:
- 2012AGUFM.B13E0584D
- Keywords:
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- 0328 ATMOSPHERIC COMPOSITION AND STRUCTURE / Exosphere;
- 0414 BIOGEOSCIENCES / Biogeochemical cycles;
- processes;
- and modeling;
- 0458 BIOGEOSCIENCES / Limnology;
- 1890 HYDROLOGY / Wetlands