Dynamic simulation of CO2 flux in a hydropower reservoir in Southwest China

Abstract

• CE-QUAL-W2 is a valid module to research carbon cycle in Wujiangdu Reservoir. • CO 2 flux of reservoir in the cold season was higher than that in the warm season. • CO 2 flux varied much along the reservoir, positively correlated with water flow. • As withdrawal elevation increased 30 m, reservoir’s CO 2 emission increased by 39.31 %. • As withdrawal elevation decreased 30 m, reservoir’s carbon flux increased by 1.44 %. Carbon emissions from aquatic ecosystems, including reservoirs, play an important role in the global carbon budget, while the carbon cycle in reservoirs is still far from clearly understood. In this study, a vertical two-dimensional quality-hydrodynamic model (CE-QUAL-W2) was established to simulate CO 2 fluxes and carbon transport in the Wujiangdu Reservoir in Southwest China, verified by field measurement in 2017. Thanks to overall good accuracy of multi-indicator simulation of the model (mean RRE = 23.4 %, mean NES = 0.67), CE-QUAL-W2 is considered as a valid module to simulate carbon flux in the reservoir. Based on the model, the average CO 2 flux across the air–water interface of the reservoir was 4.37 mmol m −2 d -1 . Temporally, CO 2 fluxes in the cold season were higher than those in the warm season, indicating the important effect of seasonal temperature change on CO 2 flux, as well as the effect of other parameters, such as DO, Chla, TN, TP and pH. Spatially, CO 2 emission fluxes were lower in the water inlet area of the reservoir (2.74 mmol m −2 d -1 ) and in front of the dam (3.40 mmol m −2 d -1 ), but higher in the middle part of the reservoir (highest 5.42 mmol m −2 d -1 at Segment 54). Different discharge elevation scenarios were set up to model the influences of hydrodynamics on CO 2 emission fluxes. When the water discharge elevation increased by 30 m, CO 2 emission increased by 39.31 % and discharged downstream carbon decreased by 1.90 % compared with the actual working condition. Finally, the total mass of carbon released by the reservoir (the combination of carbon diffused into the atmosphere and carbon in released water) decreased by 1.85 % compared with the actual working condition. When the water discharge elevation decreased by 30 m, no obvious change of CO 2 emission happened, but the total mass of carbon released by the reservoir increased by 1.44 %. This research confirmed the validity of CE-QUAL-W2 in simulating carbon flux in reservoirs. Our modelling results shed light on the influence of hydrodynamics on carbon emission in reservoirs, and optimizing water discharge elevation can be helpful to mitigate carbon emission from reservoirs.