Preliminary report on methane emissions from the Three Gorges Reservoir in the summer drainage period

Recently reported summertime methane (CH₄) emissions (6.7 ± 13.3 mg CH₄/(m².hr)) from newly created marshes in the drawdown area of the Three Gorges Reservoir (TGR), China have triggered broad concern in academic circles and among the public. The CH₄ emissions from TGR water surfaces and drawdown areas were monitored from 3rd June to 16th October 2010 with floating and static chambers and gas chromatography. The average CH₄ emission flux from permanently flooded areas in Zigui, Wushan and Yunyang Counties was (0.33 ± 0.09) mg CH₄/(m².hr). In half of these hottest months of the year, the wilderness, cropland and deforested drawdown sites were aerobic and located above water level, and the CH₄ emissions were very small, ranging from a sink at 0.12 mg CH₄/(m².hr) to a source at 0.08 mg CH₄/(m².hr) except for one mud-covered site after flood. Mean CH₄ emission in flooded drawdown sites was 0.34 mg CH₄/(m².hr). The emissions from the rice paddy sites in the drawdown area were averaged at (4.86 ± 2.31) mg CH₄/(m².hr). Excepting the rice-paddy sites, these results show much lower emission levels than previously reported. Our results indicated considerable spatial and temporal variation in CH₄ emissions from the TGR. Human activities and occasional events, such as flood, may also affect emission levels. Long-term CH₄ measurements and modeling in a large region are necessary to accurately estimate greenhouse gas emissions from the TGR.