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https://doi.org/10.5194/bg-2018-244
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Research article 15 Jun 2018

Research article | 15 Jun 2018

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This discussion paper is a preprint. It is a manuscript under review for the journal Biogeosciences (BG).

Physical-controlled CO2 effluxes from reservoir surface in the upper Mekong River Basin: a case study in the Gongguoqiao Reservoir

Lin Lin1, Xixi Lu1,2, Shaoda Liu3, and Kaidao Fu4 Lin Lin et al.
  • 1Department of geography, National University of Singapore, 117570, Singapore
  • 2Inner Mongolia Key Lab of River and Lake Ecology, School of Ecology and Environment, Inner Mongolia University, Hohhot, Inner Mongolia, 010021, China
  • 3Yale School of Forestry & Environmental Studies 195 Prospect Street New Haven, CT 06511, USA
  • 4Asian International River Center, Yunnan University, Chenggong University City, Chenggong, Kunming, Yunnan, 650500, China

Abstract. Impounding has greatly altered carbon cycle in rivers. To quantify this effect, we measured CO2 effluxes from a mountainous valley-type reservoir in the upper Mekong River (Lancang River in China) and compared with those from the pristine river channel. Evasion rates from reservoir surface was 408mgm−2d−1 and 305mgm−2d−1 in the dry season and rainy season respectively, much lower than those from river channel of 2168mgm−2d−1 and 374mgm−2d−1. Lower efflux in the rainy season deviated from the traditional theory that rainfalls can bring more organic carbon for mineralization and increase the efflux. The analysis found that efflux was closely related to the physical mixing process of inflow and reservoir water. The light overflow rich in CO2 in dry season contact the atmosphere directly and release more gases while the underflow in warm wet season leaved insufficient time for mineralization and hardly support high efflux in surface water. Evasion rate at the downstream of the dam was also limited due to surface water withdrawal. Lastly, the littoral zone was found to be a hotspot for CO2 emission despite its limited area leading to its negligible contribution in total annual emission rate. In contrast, diurnal efflux variability in the littoral zone indicates that the effluxes were significantly higher at night than in the daytime, which increased the annual emission rate to by a half.

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Damming did not significantly increase the carbon dioxide emissions from the reservoir surface, as shown in a case study in a reservoir constrained within the narrow upper Mekong River valley. This finding possibly provide some evidence for the green credits for the hydropower in China. One-year monitoring on carbon dioxide emissions from water surface found that the higher emissions was concentrating in winter and was more related to the mixing processes of incoming and reservoir water.
Damming did not significantly increase the carbon dioxide emissions from the reservoir surface,...
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