Nam Theun 2 Reservoir four years after commissioning: significance of drawdown methane emissions and other pathways

Abstract

-Hydro-reservoirs are recognized as potentially significant emitters of methane at the global scale. However, there are still large uncertainties in estimating such emissions at this scale due to the lack of comprehensive measurements of methane emissions in a wide range of climatic zones, for hydro-reservoirs with variable characteristics, and including all emissions pathways. This study presents an assessment of methane emissions for the 2013 year, that is four years after commissioning, for the Nam Theun 2 hydro-system, a 489 km² subtropical reservoir with a large drawdown area. All of the major pathways have been computed here, i.e., ebullition, diffusion, degassing, emissions from the drawdown area, and diffusion from downstream, and included in an emission budget. Emissions from the upstream area strongly dominate when compared to the downstream ones, a feature quite specific to the Nam Theun 2 reservoir. The drawdown area and the emissions by diffusion downstream represented a small contribution to the total emissions. About half of the methane is emitted during the four months of the warm dry season (from mid-February to Mid-June). Total methane emissions in NT2 have decreased from 35.6±2.6 Gg(CH 4 ) year -1 in 2010 to 24.5±1.5 Gg(CH 4 ) year -1 in 2013 though this difference was not found to be quite statistically significant.

Greenhouse gas emissions of hydropower in the Mekong River Basin

“…Our estimated gross emissions are likely to be higher than net emissions, but the inclusion of emissions from degassing would have increased our emission estimates, as it was only partially considered in our models (not all measurements underlying the regression models included CH 4 bubbles). For example, in the case of two tropical reservoirs, Balbina and Petit Saut, the degassing of CH 4 is considered to account for 35% and 60% of the total CH 4 emissions, respectively (Demarty and Bastien 2011), and in Nam Theun 2 reservoir the gross emissions were estimated to be 23%-27% larger than the net emissions (Deshmukh et al 2014, Serça et al 2016, Deshmukh et al 2016.…”

Extreme drought boosts CO2and CH4emissions from reservoir drawdown areas

“…Although previous studies have shown that GHG emissions strongly fluctuate during a drying and rewetting cycle (e.g. Yang et al 2012;Gallo et al 2014;Serça et al 2016), most assessments are based on infrequent measurements, varying from once a year along a transect (Serça et al 2016) to twice a month at specific locations (Yang et al 2012). As pulsed fluxes may be critical for the total GHG emissions, especially during a period of rewetting (Gallo et al 2014), infrequent measurements are likely to result in an underestimation of overall emissions.…”

First Assessment of Inorganic Nitrogen Deposition Budget Following the Impoundment of a Subtropical Hydroelectric Reservoir (Nam Theun 2, Lao PDR)

“…This is a transbasin hydroelectric reservoir that diverts water from the Nam Theun River (a Mekong tributary) to the Xe Bangfai River (another Mekong tributary). The reservoir was described in detail in Descloux et al (), and the carbon cycle and especially the greenhouse gas emissions were monitored since impoundment (Chanudet, Guédant, et al, ; Chanudet, Smits, et al, ; Deshmukh et al, , ; Guérin et al, ; Serça et al, ). The reservoir surface fluctuated from 140 km 2 (525.5 m above sea level) to around 490 km 2 at the full water level (538 m above sea level) between 2009 and 2013.…”