# Low methane (CH4) emissions downstream of a monomictic subtropical hydroelectric reservoir (Nam Theun 2, Lao PDR)

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

► To cite this version:

Chandrashekhar Deshmukh, Frédéric Guérin, David Labat, Sylvie Pighini, Axay Vongkhamsao, et al. Low methane (CH4) emissions downstream of a monomictic subtropical hydroelectric reservoir (Nam Theun 2, Lao PDR). Biogeosciences, 2016, 13, pp.1919-1932. 10.5194/bg-13-1919-2016. insu-03671666

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# Low methane (CH₄) emissions downstream of a monomictic subtropical hydroelectric reservoir (Nam Theun 2, Lao PDR)

Chandrashekhar Deshmukh<fnref n="1" />,<fnref n="2" />,<fnref n="3" />,ᵃ, Frédéric Guérin<fnref n="1" />,<fnref n="4" />,<fnref n="5" />, David Labat<fnref n="1" />, Sylvie Pighini<fnref n="6" />,⁸, Axay Vongkhamsao<fnref n="6" />, Pierre Guédant<fnref n="6" />, Wanidaporn Rode<fnref n="6" />, Arnaud Godon<fnref n="6" />,⁸, Vincent Chanudet<fnref n="7" />, Stéphane Descloux<fnref n="7" />, and Dominique Serça<fnref n="2" />

*Correspondence to:* Frédéric Guérin (frederic.guerin@ird.fr)

Received: 5 June 2015 – Published in Biogeosciences Discuss.: 20 July 2015

Revised: 3 February 2016 – Accepted: 12 February 2016 – Published: 30 March 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 fort-

nightly) 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⁻<fnref n="1" /> after the commissioning to 2 GgCH₄ yr⁻<fnref n="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⁻<fnref n="1" />) and downstream emissions are negligible for

<footnote n="1">¹Université de Toulouse, UPS GET, 14 Avenue E. Belin, 31400 Toulouse, France</footnote>

<footnote n="2">²Laboratoire d'Aérologie, Université de Toulouse, CNRS UMR5560, 14 Av. Edouard Belin, 31400, Toulouse, France</footnote>

<footnote n="3">³Centre for Regulatory and Policy Research, TERI University, New Delhi, India</footnote>

<footnote n="4">⁴Institut de Recherche pour le Développement (IRD), UR234, GET; 14 Avenue E. Belin, 31400, Toulouse, France</footnote>

<footnote n="5">⁵Departamento de Geoquimica, Universidade Federal Fluminense, Niterói-RJ, Brazil</footnote>

<footnote n="6">⁶Nam Theun 2 Power Company Limited (NTPC), Environment & Social Division, Water Quality and Biodiversity Dept., Gnommalath Office, PO Box 5862, Vientiane, Lao PDR</footnote>

<footnote n="7">⁷Electricité de France, Hydro Engineering Centre, Sustainable Development Dpt, Savoie Technolac, 73373 Le Bourget du Lac, France</footnote>

<footnote>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</footnote>

<footnote>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</footnote>

<footnote>ᶜnow at: Arnaud Godon Company, 44 Route de Genas, Nomade Lyon, 69003 Lyon, France</footnote>
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%.

# 1 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). This latter estimate is mostly based on CH₄ diffusion at the reservoir surface and to a lesser extent on CH₄ ebullopition, which are the two best documented pathways to the atmosphere. However, 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). Some authors attempted to include these two pathways in the global estimation of greenhouse gas emissions from reservoirs (Lima et al., 2008; Li and Zhang, 2014), and it increased drastically the emission factors of reservoirs.

The downstream emissions include the so-called degassing which occurs just below the turbines. It is attributed to the high turbulence generated by the discharge of the reservoir water into the river below the dam and the large pressure drop that the water undergoes while being transported from the bottom of the reservoir to the surface of the river below the dam. It also includes emissions by diffusion from the river below the dam. Downstream emissions were first reported at the Petit Saut Reservoir (Galy-Lacaux et al., 1997), and this pathway was later confirmed in some Brazilian reservoirs (Guérin et al., 2006; Kemenes et al., 2007). When all emission pathways from tropical or temperate hydroelectric reservoirs (disregarding the drawdown emissions) are taken into account, downstream emissions could contribute 50 to 90% of total CH₄ emissions (Abril et al., 2005; Kemenes et al., 2007; Maeck et al., 2013). At two other sites located in Canada and in the Lao People's Democratic Republic (Lao PDR) where this pathway was studied, downstream emissions were found to contribute less than 25% when it exists (Chanudet et al., 2011; Teodoru et al., 2012). 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.

In the present study, we quantified emissions downstream of the Nam Theun 2 Reservoir (NT2R) located in Lao PDR on the basis of a high temporal (weekly to fortnightly) and spatial (23 stations) resolution monitoring of CH₄ concentration. The significance of the aerobic CH₄ oxidation in the dynamics of CH₄ in the reservoir and the downstream rivers was also evaluated. We characterized the seasonal patterns of downstream emissions and evaluated the contribution of this pathway to CH₄ emissions by ebullition (Deshmukh et al., 2014) and diffusive fluxes at the surface of the reservoir (Guérin et al., 2015). We finally discuss the contribution of downstream emissions according to the reservoir hydrodynamics and the design of the water intake by comparing our results to previously published studies.

# 2 Material and methods

## 2.1 Study area

The NT2 hydroelectric Reservoir was built on the Nam Theun River located in the subtropical region of Lao PDR. The NT2 hydroelectric scheme is based on a trans-basin diversion that receives water from the Nam Theun River and releases it into the Xe Bangfai River through a 27 km long artificial downstream channel (Fig. 1; see Descloux et al., 2014, for a detailed description of the study site). Below the powerhouse, the turbinated water first reaches the tailrace channel (TRC1 in Fig. 1) and the water is then stored in an 8 Mm³ regulating pond (RD in Fig. 1) located around 3.5 km below the powerhouse. The regulating pond also receives water inputs from the Nam Kathang River (3% of its volume annually). Daily, the water discharge of the Nam Kathang River that reaches the regulating pond is returned to the downstream reach of the Nam Kathang River, below the regulating pond. The remaining water from the regulating pond is released into the artificial downstream channel. To prevent potential problems of deoxygenation in the water that passed through the turbines, an aeration weir was built midway between the turbines and the confluence to the Xe Bangfai River (AW in Fig. 1). A continuous flow of 2 m³ s⁻¹ (and occasionally spillway release) is discharged from the Nakai Dam (ND in Fig. 1) to the Nam Theun River. Annually, the NT2 Reservoir receives around 7527 Mm³ of water from the Nam Theun watershed, which is twice the volume of the reservoir (3908 Mm³), leading to a residence time of nearly 6 months.

Typical meteorological years are characterized by three seasons: warm wet (WW; mid-June–mid-October), cool dry (CD; mid-October–mid-February) and warm dry (WD; mid-February–mid-June). During the CD season, the reservoir water column overturns, and during the WW season, sporadic destratification occurs, allowing oxygen to diffuse
<figure><img src="image_3.png" /><figcaption>**Figure 1.** Map of the Nam Theun 2 Hydroelectric Reservoir (Lao People's Democratic Republic).</figcaption></figure>

down to the bottom of the water column (Chanudet et al., 2012; Guérin et al., 2015). Daily average air temperature varies between 12°C (CD season) and 30°C (WD season). The mean annual rainfall is about 2400 mm and occurs mainly (80%) in the WW season (NTPC, 2005).

The filling of the reservoir began in April 2008; the full water level was first reached in October 2009 and stayed nearly constant until the power plant was commissioned in April 2010. After the commissioning, during the studied period the reservoir surface varied seasonally and reached its maxima (489 km²) and minima (between 168 and 221 km², depending on the year) during the WW and WD seasons, respectively.

## 2.2 Sampling strategy

A total of 23 stations were monitored weekly to fortnightly in order to determine physico-chemical parameters and the CH₄ concentrations and emissions in pristine rivers, the reservoir, and all rivers and channels located downstream of the reservoir. In the reservoir, two stations were monitored (RES1 and

RES9, Fig. 1). Station RES1 is located 100 m upstream of the Nakai Dam and RES9 is located ~1 km upstream of the water intake which transports water to the turbines.

Below the powerhouse, the water was monitored at nine stations: in the tailrace channel (TRC1), regulating pond (REG1), artificial downstream channel (DCH1, DCH2, DCH3, and DCH4), and the Xe Bangfai River (XBF2, XBF3, and XBF4). Owing to the existence of the above-listed civil structures downstream of the powerhouse, three sections were defined in order to calculate emissions and degassing downstream of the powerhouse, the regulating pond, and the aeration weir (Fig. 1). The influence of the water released from the regulating pond on the Nam Kathang River was evaluated by the monitoring of two pristine stations (NKT1 and NKT2) upstream of the regulating pond and three stations (NKT3–NKT5) below the regulating pond (Fig. 1).

Below the Nakai Dam, four sampling stations (NTH3-NTH5 and NTH7) were used for the monitoring of the Nam Theun River. Section 4 refers to the Nam Theun River section located between stations NTH3 and NTH4 (Fig. 1).

Additionally, we monitored the pristine Xe Bangfai River (XBF1) upstream of the confluence with the artificial channel and one of its pristine tributaries (Nam Gnom River: NGM1) and a pristine tributary of the Nam Theun River (Nam Phao River: NPH1) downstream of the Nakai Dam.

During various field campaigns (March 2010, June 2010, March 2011, June 2011, and June 2013), aerobic methane oxidation rates (AMO) were determined at three stations in the reservoir (RES1, RES3, and RES7; Fig. 1). Additionally, AMO was also determined in the reservoir at the water intake (RES9) in June 2013.

## 2.3 Experimental methods

### 2.3.1 In situ water quality parameters

Oxygen and temperature were measured in situ at all sampling stations with a multi-parameter probe Quanta® (HydroLab, Austin, Texas) since January 2009. In the reservoir, the vertical resolution of the vertical profiles was 0.5 m above the oxic–anoxic limit and 1–5 m in the hypolimnion, whereas it was only measured in surface waters (0.2 m) in the tailrace channel, downstream channel, and rivers.

### 2.3.2 Methane concentration in water

The CH₄ concentrations at all stations have been monitored between May 2009 and December 2012 on a fortnightly basis. Surface and deep-water samples for CH₄ concentration were taken with a surface water sampler (Abril et al., 2007) and a Uwitec water sampler, respectively. Water samples were stored in serum glass vials, capped with butyl stoppers, sealed with aluminium crimps and poisoned (Guérin and Abril, 2007). A N₂ headspace was created and the vials were vigorously shaken to ensure an equilibration between the
liquid and gas phases prior to CH₄ concentration gas chromatography (GC) analysis. The concentration in the water was calculated using the solubility coefficient of Yamamoto et al. (1976).

### 2.3.3 Aerobic methane oxidation

In the reservoir, water samples for AMO rate measurements were collected in the epilimnion and in the metalimnion (at the oxicline). At RES9, the samples were taken in the middle of the water column since the water column was well mixed. AMO was also performed at TRC1 (immediately downstream of the powerhouse). The water was collected in 1 L HDPE bottles, homogenized, oxygenated and redistributed to twelve serum vials (160 mL). Each vial contained 60 mL of water and 100 mL of air. Vials were covered with aluminium foil to avoid the effect of light on any bacterial activity and incubated in the dark (Dumestre et al., 1999; Murase and Sugimoto, 2005) at 20 to 30 °C, depending on in situ temperatures. According to in situ concentration of CH₄ in the water, different amounts of CH₄ were added by syringe while withdrawing an equal volume of air from the headspace with a second syringe in order to obtain concentrations of dissolved CH₄ in the incubated water ranging from in situ to 4 times in situ. Incubations were performed with agitation to ensure continuous equilibrium between the gas and water phases. Total CH₄ concentrations in the vials were measured five times in a row at a 12 h interval, and oxidation rates were calculated as the total loss of CH₄ in the vial (Guérin and Abril, 2007). The oxidation rate for each concentration was the average value of the triplicates with standard deviation (±SD).

The kinetics parameters of aerobic methane oxidation obtained from the experiment were combined with the in situ CH₄ concentration profiles in order to calculate the integrated aerobic methane oxidation in the oxic water column. As the aerobic methane oxidation rates we obtained were potential, CH₄-ox were corrected for two limiting factors, the oxygen availability and the light inhibition as described in Guerin and Abril (2007). The final equation to compute in situ oxidation rates (CH₄-ox, mmol m⁻² d⁻¹) is

$$CH_{4-ox} = C_{CH_4} \cdot S_{CH_{4-ox}} \cdot C_{O_2}/(C_{O_2} + K_m(O_2)) \cdot d \cdot I(z),$$

with Cₖ₄, the CH₄ concentration; Sₖ₄-ox, the specific CH₄-ox; C₀₂, the oxygen concentration; Kₘ(O₂), the Kₘ of O₂ for CH₄ 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 CH₄ oxidation rates were integrated in the oxic water column, from the water surface to the limit of penetration of oxygen.

### 2.3.4 Gas chromatography

Analysis of CH₄ concentrations were performed by gas chromatography (SRI 8610C gas chromatograph, Torrance, CA, USA) equipped with a flame ionization detector. A subsample of 0.5 mL from the headspace of water sample vials was injected. Commercial gas standards (10, 100 and 1010 ppmv, air liquid “crystal” standards and mixture of N₂ with 100 % CH₄) were injected after analysis of every 10 samples for calibration. The detection limit is 0.3 ppmv in the headspace and the accuracy is around 4 % allowing the determination of nanomolar concentrations in water samples, depending of the volume of the vials and headspace. Duplicate injection of samples showed reproducibility better than 5 %.

## 2.4 Calculations

### 2.4.1 Estimation of diffusive fluxes from surface concentrations

The diffusive CH₄ fluxes downstream of the powerhouse (sections 1–3 in Fig. 1) and downstream of the Nakai Dam (NTH3-7, Fig. 1) were calculated with the thin boundary layer (TBL) equation (Liss and Slater, 1974) from the difference between the water surface CH₄ concentrations and the average CH₄ concentration in air (1.9 ppmv) obtained during eddy covariance deployments (Deshmukh et al., 2014) combined with a gas transfer velocity (k₆₀₀) as follows:

$$F = k_T \times \Delta C, \qquad (1)$$

where F is the diffusive flux at the water-air interface, k_T the gas transfer velocity at a given temperature (T), and ΔC = Cₗ-Cₐ the concentration gradient between the water (Cₗ) and the concentration at equilibrium with the overlying atmosphere (Cₐ). Afterward, the k_T was computed from k₆₀₀ with the following equation:

$$k_T = k_{600} \times (600/Sc_T)^n, \qquad (2)$$

with Sc_T the Schmidt number of CH₄ at a given temperature (T; Wanninkhof, 1992) and n = 1/2 for turbulent water (Borges et al., 2004; Guerin et al., 2007).

The artificial channel and the Nam Theun River downstream of the dam are closed for navigation because of the potential high water level changes due to reservoir management and because of the presence of zone of very high turbulence immediately downstream of the powerhouse and downstream of the regulation pond. In the artificial channel, water current velocity never exceeds 1 m s⁻¹ and averaged 0.5 m s⁻¹. Floating chamber measurement was not possible for the accurate determination of the k₆₀₀. On a few occasions, k₆₀₀ was calculated from floating chamber measurements (Deshmukh et al., 2014) and concomitant CH₄ water surface concentrations in the turbulent waters downstream of the powerhouse (section 1 at stations TRC1 and REG1), in
the Xe Bangfai River downstream of its confluence with the artificial channel (XBF2), and in pristine rivers (XBF1, Nam On River, and Nam Noy River). The gas transfer velocity reached up to 45 cm h⁻¹ and averaged 10.5 ± 12.1 cm h⁻¹ (data not shown). This is very similar to the average k₆₀₀ value obtained using the formulation k₆₀₀–wind speed relationships from Guerin et al. (2007) obtained downstream of the Petit Saut Reservoir and in small estuaries of the same size with similar water currents like the Scheldt by Borges et al. (2004). We therefore kept 10 cm h⁻¹ as a conservative estimate of the k₆₀₀ in the artificial channel downstream of the NT2R. The gas transfer velocity for the artificial channel, the Xe Bangfai River, and downstream of the Nakai Dam (NTH3-7) was kept constant over the whole period of monitoring since the average of the results obtained by the formulations of Borges et al. (2004) and Guerin et al. (2007) was 10.06 ± 1.48 cm h⁻¹ according to the limited variation of the monthly average wind speed (1.8 ± 0.46 m s⁻¹).

### 2.4.2 Degassing

Although the so-called “degassing” usually occurs only below dams (Galy-Lacaux et al., 1997; Abril et al., 2005; Kemenes et al., 2007; Maeck et al., 2013), degassing occurs at four sites at NT2R: (1) the Nakai Dam, (2) the turbine release in the tailrace channel, (3) the regulating dam, and (4) the aeration weir using the following equation:

$$ \text{Degassing} = (C_{\text{upstream}} - C_{\text{downstream}}) \times \text{water discharge}, \quad (3) $$

where C_upstream is the CH₄ upstream of the site where degassing might occur and C_downstream is the CH₄ concentration in the water downstream of the degassing site. On each of these structures, the degassing was calculated using the water discharges and the difference of CH₄ concentration between the stations: (1) NTH3 located below the Nakai Dam and RES1, (2) TRC1 located below the turbines and RES9, (3) DCH1 below the regulating dam and REG1, and (4) DCH3 below the aeration weir and DCH2 (Fig. 1). In addition, degassing was calculated for the occasional spillway releases from the Nakai Dam.

The estimation of the concentration upstream of the degassing sites was different for the four sites. For the degassing below the turbines and below the regulating dam, the average of the vertical profile of CH₄ concentrations at RES9 and REG1 were considered as concentrations before degassing, respectively. Surface concentration at DCH2 was considered for the degassing at the aeration weir. For the degassing below the Nakai Dam, since the continuous flow of 2 m³ s⁻¹ was released from the surface water layer, we considered the average CH₄ concentration in the upper 3 m water layer at RES1 located ~100 m upstream of dam. For the spillway release of the Nakai Dam, as the spillway gate is located at 12 m below the maximum reservoir water level, the degassing due to spillway release was calculated using the

average CH₄ concentration in the upper 15 m water layer at RES1.

## 3 Results

### 3.1 Temperature, O₂ and CH₄ concentrations in the reservoir (RES1 and RES9)

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 WD (surface: 26.8 ± 2.7 °C and bottom: 18.9 ± 1.6 °C) and WW (surface: 28.0 ± 1.6 °C and bottom: 21.5 ± 1.7 °C) seasons and it overturns during the CD season (average = 23.2 ± 3.9 °C; Fig. 2). During the WD and WW season, the epilimnion was always oxygenated with surface O₂ concentrations ranging from 14 to 354 µmol L⁻¹ (5 to 137 % saturation) and the hypolimnion was anoxic. In the CD season, the reservoir water column was poorly but entirely oxygenated during a few weeks/month (127 ± 93 µmol L⁻¹). In the WD and WW seasons, the CH₄ concentrations ranged between 0.02 and 201.7 µmol L⁻¹ in the epilimnion and between 0.02 and 1000 µmol L⁻¹ in the hypolimnion. In the CD season, the CH₄ concentrations are only slightly higher in the hypolimnion than in the epilimnion. After the starting of turbines, the hydrodynamics of the water column at RES1 followed the same seasonal pattern as described before, whereas the CH₄ vertical profiles of concentrations at RES9 located upstream of the water intake were homogeneous from the surface to the bottom. At RES9 during the years 2010 to 2012, the temperature was constant from the bottom to the surface whatever the season and the water column was always oxygenated (O₂ = 166 µmol L⁻¹; Fig. 2). During this period, CH₄ concentration peaked up to 215 µmol L⁻¹ with averages of 39.8 ± 48.8, 29.9 ± 55.4 and 1.9 ± 4.3 µmol L⁻¹ during the WD, WW, and CD seasons, respectively. For the two stations, the average CH₄ concentrations over the water column were always the highest in the WD season, intermediate in the WW season and the lowest in the CD season. At the two stations, the average concentrations were significantly higher in 2009 and 2010 than they were in 2011 and 2012. The average CH₄ concentrations at NT2R were in the range reported for tropical reservoirs flooded 10–20 years ago (Abril et al., 2005; Guérin et al., 2006; Kemenes et al., 2007).
<figure><img src="image_4.png" /></figure>

Upstream of the water intake (RES9)

<figure><img src="image_5.png" /><figcaption>**Figure 2.** Vertical profiles of temperature, oxygen, and methane concentrations at stations RES1 and RES9 in the Nam Theun 2 Reservoir during the three seasons in 2010, 2011, and 2012.</figcaption></figure>
## 3.2 Emissions downstream of the Nakai Dam

### 3.2.1 CH₄ and O₂ concentrations below the Nakai Dam

Downstream of the Nakai Dam (NTH3) after the commissioning, the average O₂ concentration was 224 µmol L⁻¹, that is 87% saturation, and the concentration increased further downstream. When excluding the periods of spillway releases, the CH₄ concentration at NTH3 ranged from 0.03 to 6 µmol L⁻¹ (average: 0.94 ± 1.2 µmol L⁻¹), with the highest CH₄ concentrations in the WW season and the lowest in the CD season (Fig. 3a). High CH₄ concentrations (up to 69 µmol L⁻¹) were occasionally observed when CH₄-rich water was released from the spillway, especially in 2009. Ten kilometres downstream of the Nakai Dam, CH₄ concentration decreased down to 0.41 ± 0.32 µmol L⁻¹ at NTH4 and NTH5 without any clear seasonal pattern (Fig. 3a).

The concentrations observed below the Nakai Dam at stations NTH4 and NTH5 were similar to the CH₄ concentrations found in the pristine Nam Phao River (NPH1) in the watershed and 40% lower than the CH₄ concentrations at station NTH7 located 50 km downstream of the dam. They were 2 orders of magnitude lower than the concentrations observed downstream of 10–20-year old reservoirs in Brazil and in French Guiana (Guérin et al., 2006; Kemenes et al., 2007).

### 3.2.2 Diffusive fluxes below the Nakai Dam

The average diffusive flux downstream of the Nakai Dam was 3.3 ± 3.9mmol m⁻² d⁻¹ for the year 2010 and fluxes decreased down to 1.9 ± 2.5 and 1.4 ± 0.9 mmol m⁻² d⁻¹ for the years 2011 and 2012, respectively (Fig. 3b). Ten kilometres downstream from the Nakai Dam at NTH4 and at NTH5 downstream of the confluence of the Nam Phao River, the CH₄ fluxes decreased down to 1.14 ± 0.92 mmol m⁻² d⁻¹ on average (Fig. 3b). As for the concentrations, no seasonal or interannual trends were found. At station NTH4 located 10 km downstream of the dam, the CH₄ emission was similar to that found in pristine rivers of the watershed, and it was 2 orders of magnitude lower than the emissions observed downstream of 10–20-year old reservoirs (Guérin et al., 2006; Kemenes et al., 2007).

Considering that the CH₄ emissions from the Nam Theun River below the dam can be attributed to the reservoir over a maximum length of 10 km and a constant width of 30 m, annual emissions below the Nakai Dam decreased from 20 to 1 Mg–CH₄ month⁻¹ between 2009 and 2012, respectively (Fig. 3c). The very high emissions in 2009 were due to spillway releases (see below).

### 3.2.3 Degassing below the Nakai Dam

Due to the low water discharge at the Nakai Dam (2 m³ s⁻¹), CH₄ emissions by degassing reached a maximum of 0.1 MgCH₄ d⁻¹ at NTH3 (Fig. 3e). The occasional spillway

releases occurred mostly in 2009 before the commissioning of the power plant and in the CD after the commissioning. They led to very intense degassing (up to 72 Mg–CH₄ d⁻¹, August 2009, Fig. 3d). In total, 99% of the degassing below the Nakai Dam is due to the spillway releases in 2009, which represent 32% of total emissions downstream of the Nakai Dam during the study (2009–2012). Total degassing below the Nakai Dam was very significant in 2009 due to the spillway releases, and it dropped below 3 Mg–CH₄ month⁻¹ when only 2 m³ s⁻¹ were released for the years 2010 to 2012.

## 3.3 Emissions downstream of the powerhouse

### 3.3.1 CH₄ and O₂ concentrations below the powerhouse

Downstream of the turbines at station TRC1 after the commissioning, the average O₂ concentration was 174 ± 58 µmol L⁻¹, that is, 67 ± 20% saturation. After the commissioning of the power plant, the O₂ saturation downstream of station DCH4 located 30 km below the turbines was always around 100% saturation in the artificial downstream channel. Just below the regulating dam, in the Nam Kathang River (NKT3), the average O₂ concentration was 237 µmol L⁻¹, that is, 93% saturation. There was no marked interannual change in the O₂ concentration.

Surface CH₄ concentration at station TRC1, which is located below the turbines and receives water from the homogenized water column in the reservoir (RES9), varied by 4 orders of magnitude, from 0.01 µmol L⁻¹ (August–February, WW and CD seasons) to 221 µmol L⁻¹ (June, end of the WD and beginning of the WW season; Fig. 4a). The seasonal pattern of the CH₄ concentrations at TRC1 mimicked the concentrations at RES9. In 2010, the surface CH₄ concentration decreased from 117 ± 71 µmol L⁻¹ at TRC1 to 1.55 ± 1.15 µmol L⁻¹ at DCH4 in the WD season and from 88 ± 84 to 1.26 ± 1.59 µmol L⁻¹ in the WW season. In 2011 and 2012, the average CH₄ concentrations just below the turbines at TRC1 were 4-fold (33.4 ± 32.0 µmol L⁻¹) and 9-fold (9.8 ± 29.6 µmol L⁻¹) lower than in 2010 for the WD and WW seasons, respectively. At DCH4, the surface CH₄ concentration drops to 1.1 ± 2.4 µmol L⁻¹ (WD) and 0.3 ± 0.5 µmol L⁻¹ (WW) in the years 2011 and 2012, that is, similar to what was observed in 2010. Whatever the years, in the CD season, surface CH₄ concentrations were lower than 14.5 µmol L⁻¹ along the 30 km long watercourse (0.02–14.5 µmol L⁻¹).

On average, at station DCH4 (30 km below the turbines) and at station XBF4 located 90 km below the confluence of the downstream channel and the Xe Bangfai River, the CH₄ concentrations were 0.54 ± 0.95 and 0.3 ± 0.4 µmol L⁻¹, respectively. These concentrations are the same as those found in the pristine Xe Bangfai River (0.78 ± 0.86 µmol L⁻¹ at station XBF1).
<figure><img src="image_6.png" /><figcaption>**Figure 3.** Methane concentrations and emissions downstream of the Nakai Dam at the Nam Theun 2 Reservoir between 2009 and 2012. (a) Time series of CH₄ concentrations at stations NTH3 and NTH4, (b) diffusive fluxes at stations NTH3 and NTH4, (c) emissions by diffusive fluxes in section 4 (between NTH3 and NTH4), (d) degassing due to spillway release below the Nakai Dam, (e) degassing below the Nakai Dam due to the continuous water discharge of 2 m³ s⁻¹, and (f) total emissions by degassing and diffusion downstream of the Nakai Dam.</figcaption></figure>

At station NKT3 located in the Nam Kathang River just below the regulating dam, the average surface CH₄ concentration was 0.87 ± 0.77 µmol L⁻¹. At station NKT5 located 15 km downstream of the regulating dam, the average CH₄ concentration was 1.34 ± 2.09 µmol L⁻¹. These concentrations are not statistically different from the concentrations found in the pristine Nam Kathang Noy River (0.42 ± 0.49 µmol L⁻¹ at the NKT1 station), the pristine Nam Kathang Gnai River (1.01 ± 1.73 µmol L⁻¹ at the NKT2 station) and the pristine Nam Gnom River (1.08 ± 1.45 µmol L⁻¹ at NGM1), all located in the same watershed.

### 3.3.2 Diffusive fluxes below the powerhouse

In 2010, in section 1, the flux was 198 ± 230 mmol m⁻² d⁻¹, which was 2 times higher than in section 2 (94 ± 102 mmol m⁻² d⁻¹; Fig. 4b). In section 3 (below the aeration weir), fluxes were 15 times lower than the fluxes in section 1 (12.7 ± 18.6 mmol m⁻² d⁻¹). After the confluence with the Xe Bangfai River, CH₄ fluxes dropped down to 0.95 ± 0.76 mmol m⁻² d⁻¹ for the next 30 km. For the years 2011 and 2012, the average diffusive fluxes below the powerhouse decreased by a factor of 4 as compared to

2010. In 2010, most of the diffusive fluxes occurred from the middle of the WD season until the late WW season (155 ± 127 mmol m⁻² d⁻¹), whereas diffusive fluxes in the CD season were 100 times lower (1.4 ± 1.1 mmol m⁻² d⁻¹). In 2011 and 2012, most of the emissions occurred during the WD season (61.9 ± 50 mmol m⁻² d⁻¹), whereas emissions were 20-fold lower during both the WW and CD seasons (3.7 ± 3.9 mmol m⁻² d⁻¹).

As observed for the concentrations, emissions downstream of DCH4 in the downstream channel (1.5 ± 2.7 mmol m⁻² d⁻¹) and at NKT3 downstream of the regulating dam in the Nam Kathang River (2.03 ± 2.23 mmol m⁻² d⁻¹; Fig. 4b) were not significantly different from those calculated for the pristine Xe Bangfai River (2.2 ± 2.6 mmol m⁻² d⁻¹ at station XBF1), Nam Kathang Noy River (station NKT1) and Nam Kathang Gnai River (station NKT2; 1.98 ± 4.01 mmol m⁻² d⁻¹).

The average diffusive flux for sections 1 to 3 during the monitoring was 12 ± 22 mmol m⁻² d⁻¹, which is 7 times lower than the diffusive flux along the 40 km reach below the Petit Saut Dam (90 mmol m⁻² d⁻¹; Guérin and Abril, 2007) 10 years after impoundment, and 12 times lower than the diffusive flux along the 30 km reach downstream of the Balbina
<figure><img src="image_7.png" /><figcaption>**Figure 4.** Methane concentrations and emissions downstream of the powerhouse of the Nam Theun 2 Reservoir between 2009 and 2012. (a) Time series of CH₄ concentrations at stations TRC1, DCH1, DCH3, and DCH4, (b) diffusive fluxes at stations TRC1, DCH1, DCH3, and DCH4, (c) emissions by diffusive fluxes in sections 1, 2, and 3 (see Fig. 1), (d) degassing downstream of the powerhouse, the regulating dam and the aeration weir, and (e) total emissions by degassing and diffusion downstream of the Nakai Dam.</figcaption></figure>

Dam (140 mmol m⁻² d⁻¹; Kemenes et al., 2007) 18 years after impoundment.

The sum of the CH₄ emissions by diffusion from sections 1, 2, and 3 (Fig. 1) peaked at 333, 156, and 104 Mg·CH₄ month⁻¹ at the end of the WD beginning of the WW season in 2010, 2011, and 2012, respectively (Fig. 4c). Diffusion was negligible for more than half of the year. The results clearly show that emissions decrease with time within the first 4 years after flooding.

### 3.3.3 Degassing below the powerhouse

The degassing mainly occurred within 3 to 5 months around the transition between the WD and WW seasons (Fig. 4d). Below the powerhouse (TRC1), the degassing reached up to 385 Mg·CH₄ month⁻¹ at the end of the WD season and beginning of the WW season in 2010, just after the turbines were operated (Fig. 4d). Below the regulating dam, the degassing was almost 3 times higher (1240 Mg·CH₄ month⁻¹) than below the turbines, and the degassing from the release to the Nam Kathang River was 55 Mg·CH₄ month⁻¹ in the WD season. Even if CH₄ concentrations at DCH2 were 50% lower than at TRC1, up to 756 Mg·CH₄ month⁻¹ were still emitted at the aeration weir. This shows the very high degassing efficiency of the aeration weir (up to 99%), especially in the WD season (Descloux et al., 2015). Therefore,

most of the degassing emissions occurred below the regulating dam and at the aeration weir.

In 2010, most of the degassing occurred from April to August, whereas it occurred only from March to June in 2011 and 2012. The annual degassing emissions almost deceased by a factor of 4 in 2011 and 2012 compared to 2010 (Fig. 4e).

### 3.4 Aerobic CH₄ oxidation in the reservoir and downstream of the powerhouse and the Nakai Dam

In the reservoir, the potential AMO rates increased linearly with the CH₄ concentration (Fig. 5a, b, c) in both epilimnetic and metalimnic waters at stations RES1, RES3, and RES7. The AMO rates in the middle of the well-mixed water column at station RES9 were not statistically different from the AMO rates in the metalimnion at the other stations of the reservoirs. Therefore, the AMO rates from RES9 were plotted versus the initial CH₄ concentration together with AMO rates from the metalimnion. The slope of the linear correlation, or the so-called specific oxidation rate (SOR, d⁻¹) in the metalimnion, was similar for the CD and WD seasons (SOR = 0.88 ± 0.03 d⁻¹; Fig. 5a). In the epilimnion the SOR was twice higher in the WD season (5.28 ± 0.43 d⁻¹) than in the CD season (2.24 ± 0.41 d⁻¹; Fig. 5b, c). Overall, the SOR in the epilimnion was 2- to 4-fold higher than the SOR in the metalimnion. Downstream of the powerhouse,
<figure><img src="image_8.png" /><figcaption>**Figure 5.** Linear relationships between methane (CH₄) concentrations and aerobic methane oxidation in the (a) metalimnion, (b) the epilimnion in the cool dry season, and (c) the epilimnion in the warm dry season at the Nam Theun 2 Reservoir.</figcaption></figure>

the SOR was 1.47 ± 0.07 d⁻¹, that is, intermediately between the observation in the epilimnion and the metalimnion (data not shown). The values of SOR observed at the NT2R are in the same range as those reported at the Petit Saut Reservoir (2.64–4.13 d⁻¹; Dumestre et al., 1999; Guérin and Abril, 2007) and boreal experimental reservoirs during the summer period (0.36–2.4 d⁻¹; Venkiteswaran and Schiff, 2005).

The depth-integrated oxidation rates ranged from 0.16 to 931 mmol m⁻² d⁻¹ at RES9 and from 0.13 to 310 mmol m⁻² d⁻¹ at RES1 upstream of the Nakai Dam. Overall, for the years 2010, 2011, and 2012, the average integrated oxidation rate at RES9 is 122 mmol m⁻² d⁻¹, that is, more than 3 times higher than the average integrated oxidation rate at RES1 (35 mmol m⁻² d⁻¹). Since oxidation occurs from the surface to the bottom of the water column at RES9 and mostly around the oxicline at RES1, the depth-integrated oxidation rates were 5–20 times higher at RES9 than at RES1 during the WD season, and no clear tendency can be drawn for the WW and CD seasons (Table 1). At RES9, the total amount of oxidized CH₄ decreased from 5 to 1 Gg(CH₄) yr⁻¹ between 2010 and 2012, whereas it ranged between 0.4 and 0.7 Gg(CH₄) yr⁻¹ without a clear trend at RES1 (Table 1).

# 4 Discussion

## 4.1 Spatial and temporal variations of downstream emissions

Before the power plant was commissioned in April 2010, only a few m³ of water was discharged at the powerhouse for testing the turbines and most of the water was discharged at the Nakai Dam. The continuous water discharge at the Nakai Dam was about 2 m³ s⁻¹ and occasionally, water was spilled in order to prevent dam overflow. The continuous discharge at the Nakai Dam mimics the lowest annual water flow in the Nam Theun River before it was dammed. Since it expels CH₄-poor water (0.95 µmol L⁻¹) from the surface associated with a very low discharge, subsequent degassing and diffusive emissions below the Nakai Dam were lower than 4 Mg–CH₄ month⁻¹ in 2010 just after the commissioning and lower than 1 Mg–CH₄ month⁻¹ in 2012 (Fig. 3e). Degassing was 4-fold higher in 2010 than in 2012 because of the very high CH₄ concentrations in the water column resulting from the long residence time of water in the reservoir before the first water releases. In 2011, the concentrations were lower than in 2012 due to the high water discharges from the inflows that decreased the CH₄ concentrations by dilution (Guérin et al., 2015). The spillway releases reached up to 5309 m³ s⁻¹ and water from the top 15 m of the water column having an average concentration around 100 µmol L⁻¹ at RES1 were released at these occasions. During these short releases, up to 3000 Mg–CH₄ month⁻¹ were released in 2009 (Fig. 3d). After the commissioning, the spillways were used only twice in October 2010 and September 2011. The diffusive fluxes in the Nam Theun River below the Nakai Dam were only highly significant during the spillway releases, when it reached up to 20 Mg month⁻¹ in 2009. After the commissioning, the diffusion ranged between 0.2 and 1.5 Mg–CH₄ month⁻¹ (Fig. 3c) and contributed only a few percent of total downstream emissions below the Nakai Dam (Fig. 3f). Emissions below the Nakai Dam are low compared to emissions below the powerhouse because, except during spillway releases, only a small amount of water is discharged downstream, and this water has a low CH₄ concentration since surface water is released. However, we show here that short spillway releases with high water discharge and moderate CH₄ concentrations could contribute up to 30 % of downstream emissions in 4 years.

Downstream of the powerhouse, maximum yearly emissions were dominated by degassing (Fig. 4e). They ranged between 1 and 3 Gg month⁻¹ and had a clear seasonal pattern. Emissions below the powerhouse peaked during the WD season until the beginning of the WW season, when the CH₄ concentration in the hypolimnion of the reservoir is up to 1000 µmol L⁻¹ (Guérin et al., 2015) and concentration at RES9 higher than 100 µmol L⁻¹. Emissions were negligible in the late WW and during the CD seasons when hypolimnic concentration in the reservoir and concentration at RES9 decreased down to 5 µmol L⁻¹ (Guérin et al., 2015). Due to
<table><caption>**Table 1.** Depth-integrated methane oxidation rates (mmol m⁻² d⁻¹) and annual amount of oxidized CH₄ (Gg(CH₄) yr⁻¹) at stations RES9 and RES1 of the Nam Theun 2 Reservoir. The depth-integrated CH₄ oxidation rates are given for each season: cold dry (CD), warm dry (WD), and warm wet (WW) for each year.</caption><thead><tr><th rowspan="2">Year</th><th rowspan="2">Season</th><th colspan="2">RES9</th><th colspan="2">RES1</th></tr><tr><th>mmol m<sup>-2</sup> d<sup>-1</sup></th><th>Gg(CH<sub>4</sub>) yr<sup>-1</sup></th><th>mmol m<sup>-2</sup> d<sup>-1</sup></th><th>Gg(CH<sub>4</sub>) yr<sup>-1</sup></th></tr></thead><tbody><tr><td rowspan="3">2010</td><td>CD</td><td>11.6 ± 5.5</td><td></td><td>2.8 ± 1.0</td><td></td></tr><tr><td>WD</td><td>444.1 ± 106.1</td><td>5.2 ± 1.2</td><td>18.2 ± 6.5</td><td>0.7 ± 0.2</td></tr><tr><td>WW</td><td>442.3 ± 93.6</td><td></td><td>96.3 ± 29.8</td><td></td></tr><tr><td rowspan="3">2011</td><td>CD</td><td>1.0 ± 0.2</td><td></td><td>7.5 ± 2.7</td><td></td></tr><tr><td>WD</td><td>128.2 ± 46.2</td><td>1.0 ± 0.5</td><td>5.3 ± 2.4</td><td>0.4 ± 0.2</td></tr><tr><td>WW</td><td>46.9 ± 31.8</td><td></td><td>50.2 ± 26.3</td><td></td></tr><tr><td rowspan="3">2012</td><td>CD</td><td>33.9 ± 9.6</td><td></td><td>34.7 ± 11.3</td><td></td></tr><tr><td>WD</td><td>94.1 ± 19.4</td><td>1.2 ± 0.3</td><td>41.9 ± 21.8</td><td>0.6 ± 0.2</td></tr><tr><td>WW</td><td>80.7 ± 24.2</td><td></td><td>26.13 ± 5.3</td><td></td></tr></tbody></table>

the accumulation of CH₄ in the reservoir in the absence of turbining until commissioning, emissions downstream of the powerhouse in 2010 were higher than in 2011 and 2012, and lasted from the commissioning to the beginning of the next CD season in 2010. After the commissioning, the high emissions downstream of the powerhouse occurred within 3–5 months in the WD season and the very beginning of the WW season. During the wet 2011 year, emissions became negligible after the first rainfalls. For all years, downstream emissions were negligible in the CD season. These results show the very high seasonal variations over 3–4 orders of magnitude for downstream emissions as already observed in tropical reservoirs flooding primary forest (Abril et al., 2005; Kemenes et al., 2007). However, we show in this monomic- tic reservoir that downstream emissions are negligible most of the year, and this is mostly due to the seasonal overturn in the CD and some sporadic destratification events and di- lution of the hypolimnoion in the WW season. Overall, these results highlight the fact that the precise determination of downstream emissions cannot be done on the basis of dis- crete sampling one to four times in a year. It requires weekly to monthly monitoring in order to (1) capture the hot mo- ment(s) of emissions and (2) determine their duration. For instance, downstream emissions reported for the Nam Ngum and Nam Leuk reservoirs located in the same region were obtained at the beginning of the WD season when down- stream emissions are moderate and during the CD and WW seasons, when no emissions occur (Chanudet et al., 2011). Therefore, emissions were probably underestimated since the peak of downstream emissions at the end of the WD season– beginning of the WW season was missed.

## 4.2 Contribution of downstream emissions to CH₄ gross emissions

Table 2 reports CH₄ emissions by ebullition and diffusion at the surface of the reservoir from Deshmukh et al. (2014) and

Guérin et al. (2015), respectively. These estimates take into account the seasonal variations of the reservoir water sur- face and the variations of depth. Between June and Decem- ber 2009, the spillway releases contributed to 30 % of total gross emissions from the NT2R. In 2010, downstream emis- sions (degassing + diffusive fluxes) contributed to more than 30 % of total gross emissions (disregarding drawdown emis- sions). In 2011 and 2012, downstream emissions contributed to about 10 % of total gross emissions. This contribution of downstream emissions to total emissions is low compared to tropical reservoirs located in South America (Abril et al., 2005; Kemenes et al., 2007). Disregarding the first 2 years of monitoring (2009 and 2010) during which the quantifica- tion highly depends on the management of the reservoir, the contribution of downstream emissions to total emissions is even lower than in boreal reservoirs (Teodoru et al., 2012). The low downstream emissions arise from the fact that the reservoir is monomictic. Each time the reservoir overturns in the CD season, 1–3 Gg of CH₄ are emitted to the atmo- sphere within a few days and up to a month, which purge the reservoir water column (Guérin et al., 2015). As a con- sequence, bottom concentrations decrease from 500 to less than 5 µmol L⁻¹ during these events, and the amount of CH₄ transferred from the reservoir to the downstream reaches de- creases by 2 orders of magnitude and stays low during 8 to 9 months, before the CH₄ concentration in the reservoir increases again. Monomictic reservoirs like Nam Theun 2, Nam Leuk, Nam Ngum in Lao PDR (Chanudet et al., 2011), the Three Gorges Dam in China (Li et al., 2014) and the Cointzio Reservoir in Mexico (Némery et al., 2015) are com- mon in the subtropics and especially in Asia, where 60 % of the worldwide hydroelectric reservoirs are. Although CH₄ emissions below amictic reservoirs like Petit Saut and Bal- bina are high and very significant in the total emissions (Abril et al., 2005; Kemenes et al., 2007), low emission downstream of monomictic/dimictic/polymictic reservoirs is likely to be a general feature. The thermal stratification of hydroelectric
<table><caption>**Table 2.** Methane emissions from the Nam Theun 2 Reservoir between 2009 and 2012.</caption><thead><tr><th>Gg(CH<sub>4</sub>) yr<sup>-1</sup></th><th>2009</th><th>2010</th><th>2011</th><th>2012</th></tr></thead><tbody><tr><td>Emission from reservoir</td><td></td><td></td><td></td><td></td></tr><tr><td>Ebullition<sup>1</sup></td><td>11.21 ± 0.16</td><td>14.39 ± 0.11</td><td>14.68 ± 0.10</td><td>12.29 ± 0.09</td></tr><tr><td>Diffusion at RES9 only<sup>2</sup></td><td>0.02 ± 0.01</td><td>2.33 ± 0.21</td><td>0.86 ± 0.12</td><td>0.66 ± 0.11</td></tr><tr><td>Diffusion at RES1 only<sup>2</sup></td><td>0.06 ± 0.03</td><td>0.09 ± 0.07</td><td>0.01 ± 0.00</td><td>0.01 ± 0.00</td></tr><tr><td>Total diffusion<sup>2</sup></td><td>4.45 ± 1.01</td><td>9.34 ± 2.32</td><td>3.71 ± 0.81</td><td>4.95 ± 1.09</td></tr><tr><td>Total emissions from reservoir</td><td>15.66 ± 1.02</td><td>23.73 ± 2.32</td><td>18.39 ± 0.82</td><td>17.25 ± 1.09</td></tr><tr><td>Emissions from downstream</td><td></td><td></td><td></td><td></td></tr><tr><td>Degassing (continuous release)</td><td>0.49 ± 0.03</td><td>8.48 ± 0.74</td><td>1.83 ± 0.41</td><td>1.67 ± 0.31</td></tr><tr><td>Degassing (spillway release)</td><td>7.20 ± 0.90</td><td>0.92 ± 0.39</td><td>0.14 ± 0.00</td><td>0.00 ± 0.00</td></tr><tr><td>Diffusion</td><td>0.10 ± 0.02</td><td>1.33 ± 0.03</td><td>0.32 ± 0.02</td><td>0.33 ± 0.03</td></tr><tr><td>Total downstream emissions</td><td>7.79 ± 0.90</td><td>10.73 ± 0.83</td><td>2.29 ± 0.41</td><td>2.00 ± 0.32</td></tr><tr><td>Total emissions (reservoir + downstream)</td><td>23.45 ± 1.36</td><td>34.46 ± 2.46</td><td>20.67 ± 0.92</td><td>19.24 ± 1.14</td></tr><tr><td>Downstream emissions (%)</td><td>33</td><td>31</td><td>11</td><td>10</td></tr></tbody></table>

reservoirs has to be taken into account for the estimation of global downstream emissions from hydroelectric reservoirs. Therefore, global estimates of CH₄ emissions from hydroelectric reservoirs that include downstream emissions (Lima et al., 2008; Li and Zhang, 2014) calculated on the basis of the results from Amazonian reservoirs (Abril et al., 2005; Guérin et al., 2006; Kemenes et al., 2007) must be considered with caution as also pointed out by Narvenkar et al. (2013).

## 4.3 Consequence of outgassing and aerobic CH₄ oxidation at the water intake for the emissions below the powerhouse

In addition to the dynamics of the thermal stratification of the NT2R, the design of the water intake contributes to lowering the emissions downstream of the powerhouse. After the power plant was commissioned, the water column at station RES9 was always completely mixed from the top to the bottom, as revealed by the vertical profiles of temperature. Consequently, O₂ penetrated down to the bottom of the water column and CH₄ concentration were higher than 100 µmol L⁻<fnref n="1" /> from the top to the bottom of the water column in the WD season and at the beginning of the WW season. The overturn of the water column at RES9 results from the artificial mixing due to the advection of water caused by the water current generated by the water intake localized around 11–20 m under the water surface, depending on the water level. The water intake is responsible for the mixing of the whole water column over an area of 3 km² according to the hydrodynamic model of Chanudet et al. (2012). This mixing has a strong effect on both the outgassing (Guérin et al., 2015) and the aerobic oxidation of CH₄ around the water intake and on the oxidation of CH₄ below the powerhouse.

In the area of influence of the water intake where RES9 is, large amounts of CH₄ (up to 600 mmol m⁻² d⁻<fnref n="1" />) are emitted by diffusive fluxes at the end of the WD season–beginning

of the WW (Guérin et al., 2015). The artificial mixing at RES9 generated a hotspot of CH₄ emissions where diffusive fluxes are 15 to 150 times higher than at other stations in the reservoir for the years 2010 to 2012 (Guérin et al., 2015). The emissions at RES9 correspond to 20 to 40 % of the total downstream emissions (Table 2). Therefore, a very significant amount of CH₄ that could be emitted downstream is emitted at the reservoir surface, and this contributes to lower downstream emissions.

However, the mixing at the water intake has a strong impact on aerobic CH₄ oxidation. The vertical mixing allows O₂ to penetrate down to the bottom in the vicinity of the water intake and enhances both oxidation at the water intake and downstream of the powerhouse. On average, depth-integrated CH₄ oxidation at RES9 upstream of the water intake is 1 order of magnitude higher than at station RES1 upstream of the Nakai Dam, where the water column is thermally stratified. Over the 3 km² area representative of RES9 between 2010 and 2012, aerobic CH₄ oxidation consumed an amount of CH₄ that is equivalent to 50 % of total CH₄ downstream emissions (Tables 1 and 2). In the absence of artificial mixing, aerobic CH₄ oxidation would only remove an amount of CH₄ that is equivalent to the amount of CH₄ removed by oxidation at RES1 that is average, that is, 11 % of total downstream emissions over the 3 years of monitoring (Tables 1 and 2). Total downstream emissions were therefore lowered by 20 % due to the enhancement of aerobic CH₄ oxidation at RES9 if we compare total downstream emissions to total downstream emissions plus the amount of CH₄ that would not be oxidized in the absence of mixing (oxidation at RES9 minus oxidation at RES1). In addition, aerobic methane oxidation in the downstream channel might be enhanced too since water from RES9 being transferred to the artificial downstream channel is better oxygenated than it would be in the absence of artificial mixing.

<footnote n="1">¹ Deshmukh et al. (2014); ² Guérin et al. (2015).</footnote>
Overall, the design of the water intake that mixes the whole water column decreases virtually downstream emissions since part of the CH₄ is outgassed at the reservoir surface instead of being transported and emitted downstream. The very positive consequence of this artificial mixing at the water intake is that the mixing allows O₂ to penetrate down to the bottom of the water column, enhancing aerobic methane oxidation both at the water intake and in the river/channel downstream of the powerhouse. Roughly, CH₄ emissions from the NT2 Reservoir are lowered by 40% or more due to the artificial mixing of the water column at the water intake.

## 5 Conclusions

This first quantification of CH₄ emissions downstream of a subtropical monomictic hydroelectric reservoir shows that emissions are negligible during most of the year due to low CH₄ concentrations in the hypolimnion. They occurred only during 2-4 months at the end of the warm season-beginning of the wet season and globally contribute 10% of total emissions as observed during normal reservoir operation years (2011 and 2012). The monitoring of downstream emissions before and just after the commissioning (2009 and 2010) after a period with long water residence time in the reservoir (up to 5 years) with occasional use of spillways stresses that reservoir management can have very significant impact on emissions by enhancing diffusive emissions and downstream emissions resulting from the use of spillways.

Emissions downstream of the Nam Theun 2 Reservoir have a low contribution to total emissions also because a very significant amount of CH₄ that could be emitted downstream of the reservoir is (1) emitted upstream of the water intake and (2) is oxidized in the vicinity of the water intake because of the artificial mixing it generates. This artificial mixing contributes to improving the water quality downstream of the turbines since the water that passes through is well oxygenated (70% saturation). The other positive consequence is that it generates a hotspot of aerobic methane oxidation that contributes to the oxidation of 20% of the CH₄ that would potentially be emitted at the water intake or downstream of the turbines. This study shows that downstream emissions from future or existing reservoirs could be significantly mitigated by the adoption of water intake design or the installation of devices enhancing artificial water column destratification and oxygenation upstream of the turbines.

On the basis of these results, different from those previously published, we recommend that estimates at the global scale of emissions below dams take into account the mixing status of reservoirs, the water residence time and depth of the water intake, and its impact on the oxygenation of the water column immediately upstream of the turbines.

*Acknowledgements.* The authors thank everyone who contributed to the NT2 monitoring programme, especially the Nam Theun 2 Power Company (NTPC) and Electricité de France (EDF) for providing financial, technical, and logistic support. We are also grateful to the Aquatic Environment Laboratory of the Nam Theun 2 Power Company whose Shareholders are EDF, Lao Holding State Enterprise and Electricity Generating Public Company Limited of Thailand. C. Deshmukh benefited from a PhD grant by EDF. The authors want to thank an anonymous reviewer and D. Maher and W. Naqvi for their constructive comments.

Edited by: S. W. A. Naqvi

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