Carbon dioxide emissions from the flat bottom and shallow Nam Theun 2 Reservoir: drawdown area as a neglected pathway to the atmosphere

Chandrashekhar Deshmukh (1, 2) , Frédéric Guérin (1) , Axay Vongkhamsao , Sylvie Pighini , Phetdala Oudone , Saysoulinthone Sopraseuth , Arnaud Godon , Wanidaporn Rode , Pierre Guédant , Priscia Oliva (1) , Stéphane Audry (1) , Cyril Zouiten (1) , Corinne Galy-Lacaux (2) , Henri Robain (3) , Olivier Ribolzi (1) , Arun Kansal , Vincent Chanudet , Stéphane Descloux , Dominique Serça (2)

1 GET - Géosciences Environnement Toulouse
2 LAERO - Laboratoire d'aérologie
3 IRD - Institut de Recherche pour le Développement

Freshwater reservoirs are a significant source of CO₂ to the atmosphere. CO₂ is known to be emitted at the reservoir surface by diffusion at the air-water interface and downstream of dams or powerhouses by degassing and along the river course. In this study, we quantified total CO₂ emissions from the Nam Theun 2 Reservoir (Lao PDR) in the Mekong River watershed. The study started in May 2009, less than a year after flooding and just a few months after the maximum level was first reached and lasted until the end of 2013. We tested the hypothesis that soils from the drawdown area would be a significant contributor to the total CO₂ emissions.

Total inorganic carbon, dissolved and particulate organic carbon and CO₂ concentrations were measured in 4 pristine rivers of the Nam Theun watershed, at 9 stations in the reservoir (vertical profiles) and at 16 stations downstream of the monomictic reservoir on a weekly to monthly basis. CO₂ bubbling was estimated during five field campaigns between 2009 and 2011 and on a weekly monitoring, covering water depths ranging from 0.4 to 16 m and various types of flooded ecosystems in 2012 and 2013. Three field campaigns in 2010, 2011 and 2013 were dedicated to the soils description in 21 plots and the quantification of soil CO₂ emissions from the drawdown area. On this basis, we calculated total CO₂ emissions from the reservoir and carbon inputs from the tributaries. We confirm the importance of the flooded stock of organic matter as a source of carbon (C) fuelling emissions. We show that the drawdown area contributes, depending on the year, from 40 to 75 % of total annual gross emissions in this flat and shallow reservoir. Since the CO₂ emissions from the drawdown zone are almost constant throughout the years, the large interannual variations result from the significant decrease in diffusive fluxes and downstream emissions between 2010 and 2013. This overlooked pathway in terms of gross emissions would require an in-depth evaluation for the soil organic matter and vegetation dynamics to evaluate the actual contribution of this area in terms of net modification of gas exchange in the footprint of the reservoir, and how it could evolve in the future.

Domaines: Planète et Univers [physics]; Sciences de la Terre

Chandrashekhar Deshmukh, Frédéric Guérin, Axay Vongkhamsao, Sylvie Pighini, Phetdala Oudone, et al.. Carbon dioxide emissions from the flat bottom and shallow Nam Theun 2 Reservoir: drawdown area as a neglected pathway to the atmosphere. Biogeosciences, 2018, 15, pp.1775-1794. ⟨10.5194/bg-15-1775-2018⟩. ⟨insu-03671649⟩