Low methane (CH<sub>4</sub>) emissions downstream of a monomictic subtropical hydroelectric reservoir (Nam Theun 2, Lao PDR)

Abstract

Abstract. Methane (CH 4 ) emissions from hydroelectric reservoirs could represent a significant fraction of global CH 4 emissions from inland waters and wetlands. Although CH 4 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)(2010)(2011)(2012) on the basis of a high temporal (weekly to fortnightly) and spatial (23 stations) resolution of the monitoring of CH 4 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 GgCH 4 yr −1 after the commissioning to 2 GgCH 4 yr −1 4 years after impoundment. The downstream emissions contributed only 10 to 30 % of total CH 4 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 CH 4 concentration in hypolimnic water is maximum (up to 1000 µmol L −1 ) and downstream emissions are negligible for Published by Copernicus Publications on behalf of the European Geosciences Union. C. Deshmukh et al.: Low methane (CH 4 ) emissions downstreamthe 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 CH 4 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 O 2 diffusion down to the bottom of the water column, enhancing aerobic methane oxidation, and it subsequently lowered downstream emissions by at least 40 %.

Effect of sporadic destratification, seasonal overturn, and artificial mixing on CH<sub>4</sub> emissions from a subtropical hydroelectric reservoir

“…At the NT2R, diffusive CH 4 fluxes covered the whole range of fluxes reported for tropical reservoirs, depending on the season. Most of the fluxes at the NT2R were around 1 order of magnitude lower than those at Petit Saut Reservoir (French Guiana) just after the impoundment (Galy-Lacaux et al, 1997), and of the same order of magnitude as reported for reservoirs 10 to 18 years older (Abril et al, 2005;Guérin et Kemenes et al, 2007;Chanudet et al, 2011). However, some diffusive fluxes at stations RES1-8 in the WW and CD seasons (up to 202 mmol m −2 d −1 ) are among the highest ever reported at the surface of a hydroelectric reservoir or a lake (Bastviken et al, 2011;Barros et al, 2011) and rivers downstream of dams (Abril et al, 2005;Guérin et al, 2006;Deshmukh et al, 2016).…”

“…with C CH 4 the CH 4 concentration, S CH 4 -ox the specific CH 4-ox from Deshmukh et al (2016), C O 2 the oxygen concentration, K m(O 2 ) the half-saturation constant (K m ) of O 2 for CH 4 oxidation, d depth of the water layer, and I (z) the inhibition of methanotrophic activity by light as defined by Dumestre et al (1999) at the Petit Saut Reservoir. Finally, the CH 4 oxidation rates were integrated in the oxic water column, from the water surface to the limit of oxygen penetration.…”

Contrasting methane emissions from upstream and downstream rivers and their associated subtropical reservoir in eastern China

“…2) are driven by bubbles (see Supplementary Table S4). In contrast to other studies 21,24 , we found that bubbles occurred in the deep-water zone (>10 m) rather than in the shallow zone (<5 m), and we suggest that the high ebullitive CH 4 emissions from deep water zones are related to heterogeneous sediment accumulation 12,13 because little or no sediment accumulates along reservoir margins 39 .…”

“…The rapid stream of water increases the water current velocity, which enhances the gas transfer velocity at the air-water interface and improves downstream CH 4 emission flux 22 . 50% of the total CH 4 emissions recorded downstream from the Balbina Reservoir in Brazil 18 represented approximately 30% of the total greenhouse gas emissions from the eight reservoirs in the dry tropical biome region of the country 23 , whereas downstream emissions accounted for 10% of the total CH 4 emissions from the Nam Theun 2 Reservoir in Laos 24 .…”

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

“…5a) compared to the CD season from 2009 to 2012. The average CO 2 emissions in 2009 and 2010 and to a lesser extend 2011 are in the same range as emissions from the Petit Saut Reservoir during the first 5 years after impoundment (Abril et al, 2005), and in the upper range of average CO 2 diffusive fluxes measured in older tropical reservoirs (dos Santos et al, 2006;Kemenes et al, 2011;Yang et al, 2013) or in young boreal reservoirs (Teodoru et al, 2011;Tadonléké et al, 2012). In 2012 and 2013, emissions from NT2R by diffusive fluxes are still higher than most of the older Asian reservoirs (Wang et al, 2011;Chanudet et al, 2011;Xiao et al, 2013;Zhao et al, 2013;Panneer Selvam et al, 2014) and other Brazilian reservoirs flooding savannah (Roland et al, 2010;Pacheco et al, 2015).…”