1Université de Toulouse, UPS GET, 14 Avenue E. Belin, 31400
Toulouse, France
2Laboratoire d'Aérologie, Université de
Toulouse, CNRS UMR5560, 14 Av. Edouard Belin, 31400, Toulouse, France
3Centre for Regulatory and Policy Research, TERI University, New
Delhi, India
4Institut de Recherche pour le Développement (IRD),
UR234, GET; 14 Avenue E. Belin, 31400, Toulouse, France
5Departamento de Geoquimica, Universidade Federal Fluminense,
Niteroi-RJ, Brazil
6Nam Theun 2 Power Company Limited (NTPC),
Environment & Social Division, Water Quality and
Biodiversity Dept., Gnommalath Office, PO Box 5862, Vientiane,
Lao PDR
7Electricité de France, Hydro Engineering Centre,
Sustainable Development Dpt, Savoie Technolac, 73373 Le
Bourget du Lac, France
anow at: Nam Theun 2 Power Company Limited
(NTPC), Environment & Social Division, Water Quality and
Biodiversity Dept., Gnommalath Office, PO Box 5862, Vientiane, Lao PDR
bnow at: Innsbruck University, Institute of Ecology, 15
Sternwartestrasse, 6020 Innsbruck, Austria and Foundation
Edmund Mach, FOXLAB-FEM, Via E. Mach 1, 38010 San Michele all'Adige, Italy
cnow at: Arnaud Godon Company, 44 Route de Genas, Nomade Lyon, 69003
Lyon, France
1Université de Toulouse, UPS GET, 14 Avenue E. Belin, 31400
Toulouse, France
2Laboratoire d'Aérologie, Université de
Toulouse, CNRS UMR5560, 14 Av. Edouard Belin, 31400, Toulouse, France
3Centre for Regulatory and Policy Research, TERI University, New
Delhi, India
4Institut de Recherche pour le Développement (IRD),
UR234, GET; 14 Avenue E. Belin, 31400, Toulouse, France
5Departamento de Geoquimica, Universidade Federal Fluminense,
Niteroi-RJ, Brazil
6Nam Theun 2 Power Company Limited (NTPC),
Environment & Social Division, Water Quality and
Biodiversity Dept., Gnommalath Office, PO Box 5862, Vientiane,
Lao PDR
7Electricité de France, Hydro Engineering Centre,
Sustainable Development Dpt, Savoie Technolac, 73373 Le
Bourget du Lac, France
anow at: Nam Theun 2 Power Company Limited
(NTPC), Environment & Social Division, Water Quality and
Biodiversity Dept., Gnommalath Office, PO Box 5862, Vientiane, Lao PDR
bnow at: Innsbruck University, Institute of Ecology, 15
Sternwartestrasse, 6020 Innsbruck, Austria and Foundation
Edmund Mach, FOXLAB-FEM, Via E. Mach 1, 38010 San Michele all'Adige, Italy
cnow at: Arnaud Godon Company, 44 Route de Genas, Nomade Lyon, 69003
Lyon, France
Received: 05 Jun 2015 – Discussion started: 20 Jul 2015 – Revised: 03 Feb 2016 – Accepted: 12 Feb 2016 – Published: 30 Mar 2016
Abstract. Methane (CH4) emissions from hydroelectric reservoirs could represent a significant fraction of global CH4 emissions from inland waters and wetlands. Although CH4 emissions downstream of hydroelectric reservoirs are known to be potentially significant, these emissions are poorly documented in recent studies. We report the first quantification of emissions downstream of a subtropical monomictic reservoir. The Nam Theun 2 Reservoir (NT2R), located in the Lao People's Democratic Republic, was flooded in 2008 and commissioned in April 2010. This reservoir is a trans-basin diversion reservoir which releases water into two downstream streams: the Nam Theun River below the dam and an artificial channel downstream of the powerhouse and a regulating pond that diverts the water from the Nam Theun watershed to the Xe Bangfai watershed. We quantified downstream emissions during the first 4 years after impoundment (2009–2012) on the basis of a high temporal (weekly to fortnightly) and spatial (23 stations) resolution of the monitoring of CH4 concentration.
Before the commissioning of NT2R, downstream emissions were dominated by a very significant degassing at the dam site resulting from the occasional spillway discharge for controlling the water level in the reservoir. After the commissioning, downstream emissions were dominated by degassing which occurred mostly below the powerhouse. Overall, downstream emissions decreased from 10 GgCH4 yr−1 after the commissioning to 2 GgCH4 yr−1 4 years after impoundment. The downstream emissions contributed only 10 to 30 % of total CH4 emissions from the reservoir during the study.
Most of the downstream emissions (80 %) occurred within 2–4 months during the transition between the warm dry season (WD) and the warm wet season (WW) when the CH4 concentration in hypolimnic water is maximum (up to 1000 µmol L−1) and downstream emissions are negligible for the rest of the year. Emissions downstream of NT2R are also lower than expected because of the design of the water intake. A significant fraction of the CH4 that should have been transferred and emitted downstream of the powerhouse is emitted at the reservoir surface because of the artificial turbulence generated around the water intake. The positive counterpart of this artificial mixing is that it allows O2 diffusion down to the bottom of the water column, enhancing aerobic methane oxidation, and it subsequently lowered downstream emissions by at least 40 %.
Methane (CH4) emissions from hydroelectric reservoirs could represent a significant fraction of global CH4 emissions from inland waters and wetlands. The first quantification of emissions downstream of a subtropical reservoir shows that they contribute only 10 to 30 % of total CH4 emissions from the reservoir. This surprisingly low contribution is due to the seasonal reservoir overturn and the effect of the turbine on re-aerating the reservoir water column.
Methane (CH4) emissions from hydroelectric reservoirs could represent a significant fraction of...