Low methane (CH&amp;lt;sub&amp;gt;4&amp;lt;/sub&amp;gt;) emissions downstream of a monomictic subtropical hydroelectric reservoir (Nam Theun 2, Lao PDR)

Chandrashekhar Deshmukh,Stéphane Descloux,Pierre Guédant,Frédéric Guérin,Axay Vongkhamsao,Sylvie Pighini,Vincent Chanudet,Dominique Serça,David Labat,Arnaud Godon,Wanidaporn Rode

doi:10.5194/bg-13-1919-2016

Abstract

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 %.

Highlights

Methane (CH4) emission from hydroelectric reservoirs at the global scale was recently revised downward, and it would represent only 1 % of anthropogenic emissions (Barros et al, 2011)
Before the commissioning of the power plant, the vertical profiles of temperature and oxygen and CH4 concentrations at stations RES1 located at the Nakai Dam and RES9 located at the water intake were similar (Fig. 2)
As already shown in Chanudet et al (2015) and Guérin et al (2015), the reservoir was thermally stratified with higher temperature at the surface than at the bottom during the warm dry season (WD) and warm wet season (WW) seasons and it overturns during the cool dry (CD) season

Summary

Introduction

Methane (CH4) emission from hydroelectric reservoirs at the global scale was recently revised downward, and it would represent only 1 % of anthropogenic emissions (Barros et al, 2011). Emissions from the drawdown area (Chen et al, 2009, 2011) and emissions downstream of dams (Galy-Lacaux et al, 1997; Abril et al, 2005; Guérin et al, 2006; Kemenes et al, 2007; Chanudet et al, 2011; Teodoru et al, 2012; Maeck et al, 2013) were poorly studied and are not taken into account in the last global estimate (Barros et al, 2011). According to the differences from one reservoir to the other, it appears that the factors controlling downstream emissions from reservoirs must be identified in order to propose realistic estimations of the global emissions from reservoirs, including downstream emissions

Methods

Results

Discussion

Conclusion