Physical-controlled CO<sub>2</sub> effluxes from reservoir surface in the upper Mekong River Basin: a case study in the Gongguoqiao Reservoir

Abstract

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 408 mg m 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 20 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.

Reply to Comments #2 on bg-2018-244 Physical controlled CO2 effluxes from reservoir surface

“…1&2 in this letter. The average water residence time was 0.01 year (Shi et al, 2017) or 1.4 days (See Page 3 Line 9).The inflow of the mainstem and tributary could be found in the figure below (the left panel is the water discharge in the mainstem while the right panel is the inflow from tributary). Noticed that the inflow of the tributary was extrapolated with the instantaneous water discharge during the sampling at Point R2.…”