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.