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。本研究针对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），分析了其季节性二氧化碳分压（carbon dioxide partial pressure，pCO₂）及相关环境影响因子。结果显示，上述水库表层水体的二氧化碳分压具有显著的时空变化特征，且各水库中该值总体从河流区向湖泊区逐步递减。夏季时，受水体分层及水力发电用深层水泄放的影响，大坝下游的二氧化碳分压最高；而湖泊区表层水体因光合固碳作用，二氧化碳分压显著偏低。但当水温较低时，湖泊区表层水体的二氧化碳分压则因呼吸作用与有机物质分解作用而升高。在四座水库中，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报告，水库二氧化碳排放通量与水体水力停留时间呈指数负相关关系。