Carbon dioxide emission from surface water in cascade reservoirs–river system on the Maotiao River, southwest of China

Recently, controversies about whether hydropower is still a clean energy have been arisen up with the studies about high CO2 emission flux from hydroelectric reservoirs in boreal and tropical regions. In this study, four subtropical reservoirs and their related reaches, draining on karstic area in southwest of China, were investigated to understand their CO2 emission, with monthly sampling strategy from July 2007 to June 2008. pCO2 values in the surface water of these reservoirs ranged from 38 to 3300 μ atm, indicating that reservoir surface could be not only source but also sink to atmosphere CO2 in different seasons. In Hongfeng reservoir, the flux of CO2 from surface water varied from −9 to 70 mmol m−2 d−2 with an average of 15 mmol m−2 d−2, and in Baihua reservoir, it had a range from −8 to 77 mmol m−2 d−2 with an average of 24 mmol m−2 d−2. Hongyan reservoir had similar average flux of CO2 to Baihua reservoir. Xiuwen had the highest average flux of CO2 with a value of 47 mmol m−2 d−2 among the studied reservoirs. Downstream the dams discharged by hydropower generation from these reservoirs generally had quite high flux of CO2, with an average of 489 ± 297 mmol m−2 d−2, which is close to those from tropical rivers. This means that water releasing from these reservoirs would be an important way for CO2 emission into atmosphere. The results showed that dam construction has significant impacts on the river water chemistry, with abrupt changes in pCO2, DO, T, pH and SIc in surface water and their outlets. In addition, with the development of thermal gradient in warm seasons, water chemistry along the water column of reservoirs also showed seasonal variations, except in Xiuwen reservoir which only has daily storage capacity.