Physical controls on CH<sub>4</sub> emissions from a newly flooded subtropical freshwater hydroelectric reservoir: Nam Theun 2

Abstract

Abstract. In the present study, we measured independently CH4 ebullition and diffusion in the footprint of an eddy covariance system (EC) measuring CH4 emissions in the Nam Theun 2 Reservoir, a recently impounded (2008) subtropical hydroelectric reservoir located in the Lao People's Democratic Republic (PDR), Southeast Asia. The EC fluxes were very consistent with the sum of the two terms measured independently (diffusive fluxes + ebullition = EC fluxes), indicating that the EC system picked up both diffusive fluxes and ebullition from the reservoir. We showed a diurnal bimodal pattern of CH4 emissions anti-correlated with atmospheric pressure. During daytime, a large atmospheric pressure drop triggers CH4 ebullition (up to 100 mmol m−2 d−1), whereas at night, a more moderate peak of CH4 emissions was recorded. As a consequence, fluxes during daytime were twice as high as during nighttime. Additionally, more than 4800 discrete measurements of CH4 ebullition were performed at a weekly/fortnightly frequency, covering water depths ranging from 0.4 to 16 m and various types of flooded ecosystems. Methane ebullition varies significantly seasonally and depends mostly on water level change during the warm dry season, whereas no relationship was observed during the cold dry season. On average, ebullition was 8.5 ± 10.5 mmol m−2 d−1 and ranged from 0 to 201.7 mmol m−2 d−1. An artificial neural network (ANN) model could explain up to 46% of seasonal variability of ebullition by considering total static pressure (the sum of hydrostatic and atmospheric pressure), variations in the total static pressure, and bottom temperature as controlling factors. This model allowed extrapolation of CH4 ebullition on the reservoir scale and performance of gap filling over four years. Our results clearly showed a very high seasonality: 50% of the yearly CH4 ebullition occurs within four months of the warm dry season. Overall, ebullition contributed 60–80% of total emissions from the surface of the reservoir (disregarding downstream emissions), suggesting that ebullition is a major pathway in young hydroelectric reservoirs in the tropics.

A simple calculation algorithm to separate high-resolution CH<sub>4</sub> flux measurements into ebullition- and diffusion-derived components

“…The obtained diurnal trend is in accordance with findings of Sahlée et al (2014) and Lai et al (2012), who reported higher nighttime and lower daytime CH 4 emissions for a lake site in Sweden and an ombrotrophic bog in Canada, respectively. However, an opposing tendency was found by Deshmukh et al (2014), who reported higher daytime and lower nighttime CH 4 emissions from a newly flooded subtropical freshwater hydroelectric reservoir within the Nam Theun river valley, Laos. In contrast to diurnal trends obtained for CH 4 total and CH 4 diffusion , estimated ebullition events occurred erratically and showed neither clear seasonal nor diurnal dynamics.…”

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

“…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 CH 4 concentrations and the average CH 4 concentration in air (1.9 ppmv) obtained during eddy covariance deployments (Deshmukh et al, 2014) combined with a gas transfer velocity (k 600 ) as follows:…”

“…Floating chamber measurement was not possible for the accurate determination of the k 600 . On a few occasions, k 600 was calculated from floating chamber measurements (Deshmukh et al, 2014) and concomitant CH 4 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).…”

“…Therefore, emissions were probably underestimated since the peak of downstream emissions at the end of the WD seasonbeginning of the WW season was missed. Table 2 reports CH 4 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 surface and the variations of depth.…”

“…The significance of the aerobic CH 4 oxidation in the dynamics of CH 4 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 CH 4 emissions by ebullition (Deshmukh et al, 2014) and diffusive fluxes at the surface of the reservoir . 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.…”