Heterogeneity Matters: Aggregation Bias of Gas Transfer Velocity Versus Energy Dissipation Rate Relations in Streams

The gas transfer velocity, k , modulates gas fluxes across air-water interfaces in rivers. While the theory postulates a local scaling law between k and the turbulent kinetic energy dissipation rate ε , empirical studies usually interpret this relation at the reach-scale. Here, we investigate how local k ( ε ) laws can be integrated along heterogeneous reaches exploiting a simple hydrodynamic model, which links stage and velocity to the local slope. The model is used to quantify the relative difference between the gas transfer velocity of a heterogeneous stream and that of an equivalent homogeneous system. We show that this aggregation bias depends on the exponent of the local scaling law, b , and internal slope variations. In high-energy streams, where b > 1 , spatial heterogeneity of ε significantly enhances reach-scale values of k as compared to homogeneous settings. We conclude that small-scale hydro-morphological traits bear a profound impact on gas evasion from inland waters.

[Impact of Rainfall-Runoff Events on Methane Emission from Xiangxi Bay of the Three Gorges Reservoir]

Methane is an important greenhouse gas and whether reservoirs act as a source or sink of methane has attracted great attention worldwide. However, unrepresentative sampling periods and a lack of consideration of unfavorable weather conditions have limited the accurate estimation of CH₄ emission from reservoirs. This study focused on the middle reach of Xiangxi Bay in the Three Gorges Reservoir to track an entire rainfall-runoff event via on-site measurements in the summer of 2019, and initiatively investigated the impact of rainfall and inflow processes on methane concentration and emission. Results showed that from before to after the rainfall event, methane flux at the air-water interface ranged between 0.011 and 0.326 mg·(m²·h)^-1, indicating a net source of methane to the atmosphere. Both wind velocity and rainfall affected methane evasion from the surface by altering the gas transfer velocity, with the effect of wind being more prominent. Methane concentrations at the bottom layer significantly increased when rainfall-induced density flow from the watershed arrived at the sampling section. This was likely due to methane export from upstream and along the flow path. During this event, discharge was too small to destratify the water column, and methane was strongly oxidized as it diffused upwards, having little impact on surface methane concentrations and air-water methane flux.

[Gas Transfer Velocity of CH4 at Extremely Low Wind Speeds]

Thin boundary theory equation (TBL) is widely used to determine gas fluxes across water-air interfaces, and the gas transfer velocity (k₆₀₀) is the key environmental factor in the equation. A monthly field campaign was carried out during one year to measure CH₄ flux and to probe its exchange rate across the air-water interface in a drinking reservoir and 5 adjacent ponds. The ranges of wind speed and surface water temperature were 0-0.75 m·s^-1 and 6.3-30.9℃respectively, and their average values were 0.19 m·s^-1 and 19.3℃ respectively. The gas transfer velocity of CH₄ varied from 0.20 to 1.99 cm·h^-1 with an average of 0.50 cm·h^-1. Correlation functions between the gas transfer velocity and the wind speed at 10 m height (U₁₀) and surface water temperature (T_w) were given here to quantify k₆₀₀. There were significant correlations between the fitted values and actual values both for original and bin-averaged data.