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 (pCO2) 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 pCO2 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, pCO2 was highest downstream of the dam because of stratification and deep water discharge for hydropower generation, whereas pCO2 was much lower in the surface water of the lacustrine zone because of carbon removal by photosynthesis. When water temperature was low, however, pCO2 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.