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| Titel |
Semidiurnal and seasonal variations in methane (CH4) emissions from a subtropical hydroelectric reservoir (Nam Theun 2 Reservoir) measured by eddy covariance technique |
| VerfasserIn |
Chandrashekhar Deshmukh, Dominique Serça, Raphael Tardif, Maud Demarty, Stéphane Descloux, Vincent Chanudet, Pierre Guédant, Frederic Guerin |
| Konferenz |
EGU General Assembly 2013
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 15 (2013) |
| Datensatznummer |
250082008
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| Zusammenfassung |
| Hydroelectric reservoirs have globally been identified as a significant source of methane
(CH4) to the atmosphere, especially in the tropics. Assessing these emissions and their
variations at small and large time scale represent important scientific challenges. In this
context, the objectives of this work are (i) to compare different methodologies used to assess
CH4 emissions. (ii) to determine the temporal variations in these emissions at different scales
i.e. from daily to seasonal, and link these variations to environmental controlling
factors.
Measurements of CH4 emissions were made in a recently impounded (May 2008)
subtropical hydroelectric reservoir, Nam Theun 2 (NT2), in Lao PDR, Asia. The sampling
strategy included three different types of flux measurement techniques: floating chambers
(FC), submerged funnels (SF), and the eddy covariance technique (EC). Flux measurements
were carried out during four field campaigns conducted between May 2009 and
June 2011. Eddy covariance system, composed by a 3D sonic anemometer coupled
with a DLT-100 fast methane analyzer (Los Gatos Inc®), was deployed on a mast
erected in a large surface of open water. Diffusive and bubbling fluxes were measured
using respectively the FC and the SF techniques within the footprint of the EC
station.
Results from the four field campaigns show individual EC fluxes (30min) varying over 4
orders of magnitude (from 0.01 to 102 mmol.m-2.day-1). Individual 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 daily flux values varying from 0 to
102 mmol.m-2.day-1. 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 emissions from the reservoir, which is a very new and encouraging result for
further studies. To our knowledge, this is the first example of an inter-comparison
for CH4 flux measurement where EC flux data were used to compare/validate the
fluxes measured with traditional methodologies (i.e. floating chamber and funnel
measurements).
Semidiurnal variations of EC fluxes were observed with two peaks per day - one in early
morning and one in the afternoon, linked to the semi-diurnal variation of the atmospheric
pressure. Due to the larger drop in atmospheric pressure in the late morning than in the night,
the noon flux peak is higher than the night flux peak. Both funnel and EC techniques show
that ebullition of CH4 increased from 5 ± 3.5 to 28 ± 16 mmol.m-2.day-1 with the depth of
the water column increasing from 10.5 to 2 m. This result suggests that the significant
seasonal variation in CH4 emissions by ebullition was essentially driven by the water level
variations.
Keywords: Methane emissions, eddy covariance, floating chambers, submerged funnels,
methods intercomparison, semidiurnal variation, seasonal variation, subtropical hydroelectric
reservoir, Nam Theun 2 reservoir |
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