Spatial and seasonal variability of CO2 flux at the air-water interface of the Three Gorges Reservoir

Abstract

Diffusive carbon dioxide (CO 2 ) emissions from the water surface of the Three Gorges Reservoir, currently the largest hydroelectric reservoir in the world, were measured using floating static chambers over the course of a yearlong survey. The results showed that the average annual CO 2 flux was (163.3 ± 117.4) mg CO 2 /(m 2 ·hr) at the reservoir surface, which was larger than the CO 2 flux in most boreal and temperate reservoirs but lower than that in tropical reservoirs. Significant spatial variations in CO 2 flux were observed at four measured sites, with the largest flux measured at Wushan (221.9 mg CO 2 /(m 2 ·hr)) and the smallest flux measured at Zigui (88.6 mg CO 2 /(m 2 ·hr)); these differences were probably related to the average water velocities at different sites. Seasonal variations in CO 2 flux were also observed at four sites, starting to increase in January, continuously rising until peaking in the summer (JuneAugust) and gradually decreasing thereafter. Seasonal variations in CO 2 flux could reflect seasonal dynamics in pH, water velocity, and temperature. Since the spatial and temporal variations in CO 2 flux were significant and dependent on multiple physical, chemical, and hydrological factors, it is suggested that long-term measurements should be made on a large spatial scale to assess the climatic influence of hydropower in China, as well as the rest of the world.

Gas transfer velocities of CO<sub>2</sub> in subtropical monsoonal climate streams and small rivers

“…4b) largely overestimated the CO 2 fluxes, i.e., 1.66 ± 1.55 (1.08-2.23) Tg CO 2 was 2.3-3-fold higher than other estimations (Table 3b) and our earlier evasion using TBL on the TGR river networks (Li et al, 2018). Moreover, our estimated CO 2 emission during the monsoonal period also suggests that CO 2 annual emissions from rivers and streams in this area were previously underestimated, i.e., 0.03 Tg CO 2 yr −1 (Li et al, 2017) and 0.37-0.44 Tg CO 2 yr −1 (Yang et al, 2013), as the former used a TBL model with a lower k level; the latter employed floating chambers, but they both sampled very limited tributaries (i.e., two to three rivers). Therefore, measurements of k must be made mandatory along with pCO 2 measurement in river and stream studies.…”

“…Larger uncertainty of riverine CO 2 emission in China was anticipated by the use of k 600 from other continents or climate zones. For instance, k 600 for CO 2 emission from tributaries in the Yellow River and karst rivers originated from the model in the Mekong (Zhang et al, 2017), Pearl (Yao et al, 2007), Longchuan (Li et al, 2012) and Metropolitan rivers (Wang et al, 2017), which are mostly from temperate regions. Our k 600 values will therefore largely improve the estimation of CO 2 evasion from subtropical streams and small rivers and improve the refinement of the riverine carbon budget.…”

“…4) and other studies (Alin et al, 2011) (Tables 2 and 3). CO 2 evasion was estimated for rivers in China with k values ranging between 8 and 15 cm h −1 (Li et al, 2012;Yao et al, 2007;Wang et al, 2011) (Table S2). These estimates of CO 2 evasion rate were considerably lower than using present k 600 values (48.4 ± 53.2 cm h −1 ).…”

Carbon dioxide emissions from the flat bottom and shallow Nam Theun 2 Reservoir: drawdown area as a neglected pathway to the atmosphere

“…As the amount of labile OM originating from the flooded soils and biomass decreases with time due to the progressive mineralization of the carbon stock, emissions decrease progressively with reservoir ageing (Abril et al, 2005;Barros et al, 2011). CO 2 emissions are higher in tropical reservoirs than in temperate and boreal ones, a latitudinal difference attributed to the enhancement of OM degradation with temperature (Barros et al, 2011;Marotta et al, 2014;Yvon-Durocher et al, 2014). Emissions occur through diffusion at the air-water interface of the reservoir and from rivers downstream of dams (Abril et al, 2005;Guérin et al, 2006;Kemenes et al, 2011).…”

Carbon dioxide emission from drawdown areas of a Brazilian reservoir is linked to surrounding land cover

“…The most up-to-date review indicates that greenhouse-gas emission from reservoirs-predominantly as methane (CH 4 ) and carbon dioxide (CO 2 )-is responsible for ~ 1.5% of the global anthropogenic CO 2 -equivalent emissions (Deemer et al 2016). The importance of understanding spatial and temporal variability in order to reliably assess total carbon emission from reservoirs is getting increasingly evident (Descloux et al 2017;Paranaíba et al 2018;Teodoru et al 2012;Roland et al 2010;Yang et al 2013). Nevertheless, existing studies on reservoir emissions focus almost exclusively on emission from the water surface.…”