Abstract
Abstract. Inland waters in general and freshwater reservoirs specifically are recognized as a source of CH4 into the atmosphere. Although the diffusion at the air–water interface is the most studied pathway, its spatial and temporal variations are poorly documented. We measured temperature and O2 and CH4 concentrations every 2 weeks for 3.5 years at nine stations in a subtropical monomictic reservoir which was flooded in 2008 (Nam Theun 2 Reservoir, Lao PDR). Based on these results, we quantified CH4 storage in the water column and diffusive fluxes from June 2009 to December 2012. We compared diffusive emissions with ebullition from Deshmukh et al. (2014) and aerobic methane oxidation and downstream emissions from Deshmukh et al. (2016). In this monomictic reservoir, the seasonal variations of CH4 concentration and storage were highly dependent on the thermal stratification. Hypolimnic CH4 concentration and CH4 storage reached their maximum in the warm dry season (WD) when the reservoir was stratified. Concentration and storage decreased during the warm wet (WW) season and reached its minimum after the reservoir overturned in the cool dry (CD) season. The sharp decreases in CH4 storage were concomitant with extreme diffusive fluxes (up to 200 mmol m−2 d−1). These sporadic emissions occurred mostly in the inflow region in the WW season and during overturn in the CD season in the area of the reservoir that has the highest CH4 storage. Although they corresponded to less than 10 % of the observations, these extreme CH4 emissions (> 5 mmol m−2 d−1) contributed up to 50 % of total annual emissions by diffusion. During the transition between the WD and WW seasons, a new emission hotspot was identified upstream of the water intake where diffusive fluxes peaked at 600 mmol m−2 d−1 in 2010 down to 200 mmol m−2 d−1 in 2012. The hotspot was attributed to the mixing induced by the water intakes (artificial mixing). Emissions from this area contributed 15–25 % to total annual emissions, although they occur in a surface area representative of less than 1 % of the total reservoir surface. We highly recommend measurements of diffusive fluxes around water intakes in order to evaluate whether such results can be generalized.
Highlights
Since the 1990s, hydroelectric reservoirs are known to be a source of methane (CH4) into the atmosphere
In tropical amictic and wellstratified reservoirs with CH4-rich hypolimnion, the highest diffusive fluxes are usually observed during dry periods and when the stratification weakens at the beginning of the rainy season (Guérin and Abril, 2007)
The Nam Theun 2 Reservoir (NT2R) receives around 7527 Mm3 of water from the Nam Theun watershed, which is more than twice the volume of the reservoir (3908 Mm3)
Summary
Since the 1990s, hydroelectric reservoirs are known to be a source of methane (CH4) into the atmosphere Their contribution to total CH4 emissions still needs refinement since the discrepancies among estimates is large, ranging from 1 to 12 % of total CH4 emissions (St Louis et al, 2000; Barros et al, 2011). A study of CH4 emissions from a dimictic reservoir suggests a potential large outgassing of CH4 during the overturn (Utsumi et al, 1998b), as is the case in natural monomictic and dimictic lakes (Kankaala et al, 2007; López Bellido et al, 2009; Schubert et al, 2010; Schubert et al, 2012; Fernández et al, 2014) Such hot moments of emissions (McClain et al, 2003) could contribute 45–80 % of annual CH4 emissions by diffusion (Schubert et al, 2012; Fernández et al, 2014). As was shown for CO2 emissions from a tropical hydroelectric reservoir, taking into account both spatial and temporal variability of emissions significantly affects positively or negatively carbon budgets and emission factors (Pacheco et al, 2015)
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