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Sedimentation supports life-cycle CH4 production and accumulation in a river valley reservoir: A hierarchical
Qi Li1, Zhe Li2, Yuanyuan Zhang2
1CAS Key Lab of Reservoir Environment, Chongqing Institute of Green and Intelligent Technology, Chinese Academy of Sciences, Chongqing 400714, China; College of Resources and Environment, Chongqing School, University of Chinese Academy of Sciences, Chongqing 400714, China; College of River and Ocean Engineering, Chongqing Jiaotong University, Chongqing 400074, China.
Abstract:
Reservoirs have been recognized as a source of methane (CH4). With the gradual increase in the number of the world's reservoirs, predicting the long-term variation of reservoir CH4 emissions is important to understand the global change in carbon cycling due to reservoir creation and operation. Here, we first categorized the origins and transport of organic carbon (OC) by reservoir creation and operation into the following four aspects: a) the decomposition of flooded organic matter, b) the sedimentation of OC from upstream sediment inputs, c) the transition of the aquatic ecosystem from lotic to lentic type, stimulating the production of autochthonous OC; and d) reservoir as the collector of anthropogenic OC inputs from surrounding communities. It was assumed that OC from the four aspects jointly determined the production and accumulation of reservoir CH4 concentration, supporting life-cycle reservoir CH4 emissions. A hierarchical Bayesian model of reservoir CH4 concentration was established and calibrated by observed monthly datasets in 2018 in the Xiangjiaba Reservoir (XJB), a river valley dammed reservoir in the upper Yangtze River, China. The model explained the relative contributions of the four aspects to reservoir CH4 production and accumulation. Approximately 78% of the CH4 concentration was contributed by the decomposition of flooded organic matter during the first 10 years after impoundment. However, the contribution of flooding faded away after 10 years of impoundment. With the increase in reservoir age, sedimentation of OC dominantly determined the reservoir CH4 production and accumulation. Scenario analysis of the XJB's life cycle demostrated that the CH4 concentration in the XJB would reach its peak approximately 70 - 80 years after impoundment. In the cascade system, the upstream reservoir will help to reduce sediment OC input, and to mitigate downstream reservoir CH4 production and accumulation. Our effort provided a new modeling approach for long-term management strategies to reduce reservoir CH4 emissions under global change.
