Spatial–temporal variations of methane emissions from the Ertan hydroelectric reservoir in southwest China

Abstract

Abstract:Methane emissions from hydroelectric reservoirs can comprise a considerable portion of anthropogenic methane. However, lack of data on CH 4 emissions in different geographical regions and high spatial-temporal variability in the emission rates of reservoirs has led to uncertainties regarding regional emission estimates of CH 4 . In the subtropical plateau climate region, we used the Ertan hydroelectric reservoir as a study area. The CH 4 flux at the air-water interface was assessed by floating chambers and factors influencing emissions, including the distance from the dam, water depth, seasonal variation in wet and dry season, air-water temperature gradient and wind speed, and was also studied through a year-long systematic sampling and monitoring experiment. The results showed that the surface of the reservoir was a source of CH 4 during the sampling period and the annual average CH 4 flux was 2Ð80 š 1Ð52 mg m 2 d 1 . CH 4 flux (and its variation) was higher in the shallow water areas than in the deep-water areas. CH 4 flux near the dam was significantly higher than that of other locations farther from the dam in the dry season. The seasonal variations of CH 4 emission in wet and dry seasons were minor and significant diurnal variations were observed in wet and dry seasons. Exponential relationships between the CH 4 flux and air-water temperature gradient were found. Air-water temperature gradient was an important factor influencing diurnal variations of CH 4 flux in the Ertan hydroelectric reservoir. These results indicate that systematic sampling is needed to better estimate CH 4 flux through coverage of the spatial variation of different water depths, measuring-point distance from the dam, seasonal variation in wet and dry seasons and changes in climate factors (such as air-water temperature gradient). Our results also provide a fundamental parameter for CH 4 emission estimation of global reservoirs.

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

“…The CH 4 emission fluxes from the water surface averaged (0.3305 ± 0.0940) mg CH 4 /(m 2 ·hr) from the beginning of June to the middle of October, which was very close to the annual average CH 4 emission fluxes of some temperate reservoirs in Canada (St. Louis et al, 2000) and the United States (Soumis et al, 2004), but about twice larger than the result of Zheng et al (2011a) in Ertan Reservoir. It should be noticed that water temperature is an important factor influencing CH 4 emissions from reservoirs (Soumis et al, 2004).…”

“…It should be noticed that water temperature is an important factor influencing CH 4 emissions from reservoirs (Soumis et al, 2004). In the study of Zheng et al (2011a), an annual average value was reported, but the average CH 4 emission flux from the TGR in the hottest months of the year was determined in the present study.…”

“…The atmospheric CH 4 concentrations have been doubted since preindustrial time (Frankenberg et al, 2005), and most of the CH 4 is produced from fossil fuel combustion, wetlands, rice paddies, waste management and ruminants (Zhuang et al, 2009). Over the past decade, researchers have suggested that CH 4 emissions from reservoirs contribute considerably to anthropogenic CH 4 (Giles, 2006;Zheng et al, 2011a). It is reported that, global CH 4 emissions from reservoirs are 70 Tg/yr, representing 18% of other anthropogenic emissions (St Louis et al, 2000).…”

“…This finding led to the assumption that CH 4 emission from the TGR surface was similar to that emitted by tropical reservoirs, and that the additional methane emitted by the marshes formed during summer drainage, which accounted for 100 km 2 and 10% of the total reservoir area, might comprise one-fifth of that emitted from the reservoir surface (Chen et al, 2009;Qiu, 2009). A subsequently published research (Zheng et al, 2011a) reported a low mean annual CH 4 emission flux of only 2.8 mg CH 4 /(m 2 ·day) (0.117 mg CH 4 /(m 2 ·hr)) from the Ertan Reservoir, located in a subtropical region of China. These low values were likely associated with biomass clearing before flooding, a low supply of organic matter, and the subsequent limitation of sediment CH 4 production (Rosa et al, 2004;Zheng et al, 2011a , fertile top soils (at least 12-16 cm in thickness) from the elevation of 139-175 m were moved to the areas at the elevation of over 182 m and a distance of more than 5 km from the bank of the TGR.…”

“…These low values were likely associated with biomass clearing before flooding, a low supply of organic matter, and the subsequent limitation of sediment CH 4 production (Rosa et al, 2004;Zheng et al, 2011a , fertile top soils (at least 12-16 cm in thickness) from the elevation of 139-175 m were moved to the areas at the elevation of over 182 m and a distance of more than 5 km from the bank of the TGR. Given that the TGR and the Ertan Reservoir were both cleared before flooding, it is necessary to establish whether CH 4 emission fluxes from the nonwetland portions of the drawdown area and from the water surfaces of continuously flooded areas in the TGR are similar to those from tropical reservoirs, as Chen et al (2009) assumed, or whether these values are lower, as reported for the Ertan Reservoir (Zheng et al, 2011a).…”