Carbon dioxide emissions from cascade hydropower reservoirs along the Wujiang River, China

Currently, CO₂ emissions from cascade hydropower reservoirs are not well understood. In this study, we investigated the seasonal carbon dioxide partial pressure (<i>p</i>CO₂) and related environmental factors in 4 cascading reservoirs (Hong Jia Du, Dong Feng, Suo Feng Ying, and Wu Jiang Du) of Wujiang River, southwest China. The results showed that <i>p</i>CO₂ in the surface water of these reservoirs had obvious spatiotemporal changes and generally decreased from the riverine zone to the lacustrine zone in each reservoir. In summer, <i>p</i>CO₂ was highest downstream of the dam because of stratification and deep water discharge for hydropower generation, whereas <i>p</i>CO₂ was much lower in the surface water of the lacustrine zone because of carbon removal by photosynthesis. When water temperature was low, however, <i>p</i>CO₂ was higher in the surface water of the lacustrine zone because of respiration and organic decomposition. Among these reservoirs, only Suo Feng Ying had CO₂ emissions higher than the average value of natural lakes. In addition, CO₂ emission flux showed an exponentially negative relationship with hydraulic retention time of reservoirs, based on this work and other reports of reservoirs in the Yangtze River basin.

目前，梯级水电水库的二氧化碳排放尚未得到充分认知。本研究以中国西南乌江流域的4座梯级水库（洪家渡、东风、索风营、乌江渡）为研究对象，对其季节性二氧化碳分压（pCO₂, carbon dioxide partial pressure）及相关环境因子展开了调查。研究结果显示，各水库表层水体中的pCO₂存在显著的时空变化特征，且整体呈现从河流区向湖泊区逐渐降低的趋势。夏季时，受水体分层与水电开发所需的深层水泄放影响，大坝下游区域的pCO₂达到峰值；而湖泊区表层水体因光合作用固碳，pCO₂显著降低。但当水温偏低时，湖泊区表层水体的pCO₂反而更高，这主要源于水体呼吸作用与有机质分解过程。在上述水库中，仅索风营水库的二氧化碳排放量高于天然湖泊的平均水平。此外，结合本研究与长江流域其他水库的相关研究成果，二氧化碳排放通量与水库的水力停留时间呈指数负相关关系。