Modelling of sedimentation-regulated methane ebullition from reservoirs

Ebullition is a primary pathway of CH₄ emission from water reservoirs, which is poorly constrained due to its episodic nature, especially in large reservoirs. Here, by inducing the intrinsic link between sedimentation and CH₄ production, and hence ebullition, a novel mechanistic reservoir CH₄ model is developed for quantifying CH₄ emissions from reservoirs. The model is first validated and applied in the Saar River reservoirs, effectively reproducing the seasonal patterns of ebullitive fluxes and the enhanced effects of sedimentation on ebullition. Through analyzing modelling CH₄ production, bubble formation, dissolved CH₄ concentration in sediments, and their interplay at different sedimentation rates, the regulatory mechanisms of sedimentation on ebullition are investigated. The results indicate that the increase in sedimentation rate augments sediment CH₄ production. Excessive CH₄ production beyond diffusive transport in the sediment is required to induce porewater CH₄ super-saturation and trigger bubble generation. Under porewater super-saturation, the majority of the increased sediment CH₄ production, resulting from an elevated sedimentation rate, is released via ebullition. Thus, sedimentation can regulate reservoir CH₄ bubble formation and ebullition by influencing CH₄ production. Then, the proposed model is applied to estimating CH₄ emissions from the Three Gorges Reservoir (TGR). The TGR have trapped 80% sediments derived from the upstream Yangtze River since dam closure, potentially fueling CH₄ production and ebullition, which has not been quantified yet. We find that the CH₄ flux of the TGR is 6.71 (4.12-11.41) mg CH₄-C m^-2d^-1 with 3.07 (0.90-6.06) mg CH₄-C m^-2d^-1 via ebullition and 3.64 (1.88-5.35) mg CH₄-C m^-2d^-1 via diffusion. This flux is 36% larger than other estimates in the literature. By trapping most of the sediments from upstream, the TGR experiences a surge in CH₄ emission from 0.17 to 1.38 Gg CH₄-C yr^-1 after impoundment, mainly via ebullition (0.63 Gg CH₄-C yr^-1). Ebullition is highly related to the sedimentation along the channel. The proposed model provides a mechanistic approach for estimating reservoirs ebullition and introduces a new aspect for the effects of reservoir sedimentation on river carbon budget.