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| Titel |
Assessment of methane (CH4) dynamics in a sub-tropical hydroelectric reservoir: Nam Theun 2, Lao PDR |
| VerfasserIn |
Chandrashekhar Deshmukh, Frederic Guerin, Fabien Becerra, Pierre Guédant, Stéphane Descloux, Sylvie Pighini, Vincent Chanudet, Dominique Serça |
| Konferenz |
EGU General Assembly 2011
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 13 (2011) |
| Datensatznummer |
250052943
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| Zusammenfassung |
| In the 90’s, hydroelectric reservoirs have been identified as significant CH4 contributors to the
atmosphere, especially in tropics. It has been shown than the degradation of organic matter
from the flooded soil and vegetation and from the watershed leads to the production of CH4.
Part of the CH4 which is not consumed by aerobic CH4 oxidation is emitted to
the atmosphere by different pathways i.e. diffusion from the water surface or the
surrounding soils, ebullition and by emissions downstream of the dam. Since then,
most of the debate regarding emissions of this potent greenhouse gas resulting
from the anthropogenic flooding of large land surfaces is based on results from
hydroelectric reservoirs flooding primary forest in South America. We present here the first
detailed study on the CH4 dynamics in a subtropical reservoir located in Southeastern
Asia.
The Nam Theun 2 Reservoir (NT2) located in Lao PDR was flooded in 2008. At high
water stage, it floods 450 km2 of tropical forests, wetlands and rice paddies whereas the area
of the water body is only 80 km2 during the dry season. Based on fortnightly water sampling,
laboratory and field works, we studied the CH4 dynamics (production and oxidation) and
atmospheric emissions from NT2 with a description of the temporal and spatial
variation.
Vertical profiles of CH4 concentration were measured in the water column. Methane
concentration varied at the spatial and temporal scale. Results show that surface CH4
concentration ranged from 0.09 μmol L-1 to 2.09 μmol L-1 with bottom concentration of
methane ranged from 51 μmol L-1 to 693 μmol L-1, concentrations being higher
during the dry season than during the wet season. Soils were sampled from the
drawdown area and reservoir’s surrounding. They have been incubated in the laboratory
to determine the kinetics of potential GHG production in anaerobic conditions.
CH4 production rates range from 0.003 to 8389 nmol h-1gsoil-1 depending on the
soil types. In the laboratory, kinetics of potential aerobic CH4 oxidation have been
determined for a varied range of methane concentrations (water sampled from epilimlion
water column, oxycline and water from the downstream rivers). Methane oxidation
rates ranged from 0.011 to 27.16 μmol.L-1day-1. Relationships between CH4
concentrations and oxidation rates show different kinetics at the oxicline, in the
epilimnion and in the river downstream of the dam but are not dependant on the season.
The diffusive fluxes of CH4 measured by floating chambers are in the order of
1.2±1.35 mmol m-2day-1 with very high spatial and temporal variations. Bubbling
fluxes were measured with funnels and found to highly depend on the water depth
(decreasing with water depth). Mean values of bubbling CH4 fluxes were 2.5±2.7
mmol m-2day-1(EGU2011-6175). Methane fluxes measured from drawdown area
of NT2 reservoir were well correlated with soil moisture and varied from -0.4 to
124 mmol m-2 day -1 with an average value of 8.8±23.3(39) mmol m-2 day
-1.
Total CH4 emissions from the NT2 are at least two times lower than reported emissions
from hydroelectric reservoirs from South America at the same “age”. The final goal of the
project is to integrate the empirical relationships for processes into a coupled physical
biogeochemical model which would be useful to predict impact on the CH4 emissions from
existing and new dam projects.
Corresponding author: Chandrashekhar DESHMUKH, Ph.D Student, Laboratoire
d’Aérologie, Observatoire Midi-Pyrénées, 14 Avanue Edouard Belin, 31400, Toulouse
Tel : +033(0)5 61 33 27 15 E-MAIL : desc@aero.obs-mip.fr |
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