Unraveling the Contribution of Turbulence and Bubbles to Air‐Water Gas Exchange in Running Waters

Klaus, Marcus; Labasque, Thierry; Botter, Gianluca; Durighetto, Nicola; Schelker, Jakob

doi:10.1029/2021jg006520

Cited by 8 publications

(11 citation statements)

References 76 publications

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“…For example, small-scale heterogeneity of channel forms promotes rippled air-water interfaces that enhance the evasion of greenhouse gases. The sequences of step-and-pools typical of mountain streams are plastic examples of small-scale geomorphic elements of rivers that control gas exchange processes at the water-air interface, eventually bearing a relevant impact on the chemical equilibrium of the atmosphere (Botter et al, 2022). Sharp discontinuities of the river bed of the types observed in steps and small cascades cause a sudden transfer of potential energy to kinetic energy of the water flow.…”

Section: Discussionmentioning

confidence: 99%

“…The simulated dynamics in the pool included a full description of the bubble entrainment associated to the water flow. The air model used in this paper describes air transport in the water column according to the standard Advection-Diffusion Equation of a scalar with known concentration C. In the simulation, the air bubbles were assumed to be are spheres with a radius, r, equal to 0.5 mm, a value congruent with the analysis of cam-derived images taken during the experiment and in line with previous experimental studies (Woolf et al, 2007;Klaus et al, 2022;Karn et al, 2016). The model allowed us to quantify the entrained and escaping bubbles fluxes across the free surface, q B .…”

Section: Numerical Simulation Of the Step And Poolmentioning

confidence: 96%

“…It should be noted the proposed scheme is suitable whether the turbulence generated at the bottom is the main driver of the overall intensity of the turbulence in the water flow. In headwater streams, instead, riverbed heterogeneity, partially emergent boulders, and small cascades can be important additional turbulence sources (Moog and Jirka, 1999a;Botter et al, 2022). In the particular setup analyzed in this study, the plunging jet of a cascade dissipates its energy by inducing high ε in the outermost fluid layer, thereby preventing the use of equation ( 4).…”

Section: Turbulence-induced Air-water Gas Exchange In the Flumementioning

confidence: 99%

“…The bubble-mediated gas exchange is here encapsulated by the bubble-mediated gas transfer velocity, k B . Therefore, in turbulent flowing waters where air entertainment takes place, the total gas transfer velocity k can be computed as the sum of velocities due to free-surface and bubble-mediated exchange (Klaus et al, 2022): The separation of the contributions of k T and k B to the total gas exchange in a step-and-pool is one of the main contributions of this paper. The following subsections describe how these terms are computed in our numerical simulation.…”

Section: Modeling Gas Exchange In Step-and-poolsmentioning

confidence: 99%

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Local processes with global impact: unraveling the dynamics of gas evasion in a step-and-pool configuration

Peruzzo

Cappozzo

Durighetto

et al. 2023

Preprint

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Abstract. Headwater streams are important sources of greenhouse gases to the atmosphere. The magnitude of gas emissions originating from such streams, however, is modulated by the characteristic microtopography of the river bed, which might promote the spatial heterogeneity of turbulence and air entrainment. In particular, recent studies have revealed that step-and-pools, usually found in close sequences along mountain streams, are important hotspots of gas evasion. Yet, the mechanisms that drive gas transfer at the water-air interface in a step and pool configuration are not fully understood. Here, we numerically simulated the hydrodynamics of an artificial step-and-pool configuration to evaluate the contribution of turbulence and air entrainment to the total gas evasion induced by the falling jet. The simulation was validated using observed hydraulic features (stage, velocity) and was then utilized to determine the patterns of energy dissipation, turbulence-induced gas exchange, and bubble-mediated transport. The results show that gas evasion is led by bubble entrainment and is mostly concentrated in a small and irregular region of a few dm2 near the cascade, where the local gas transfer velocity, k, peaks at 500 md−1. The enhanced spatial heterogeneity of k in the pool does not allow one to define a priori the region of the domain where the outgassing takes place, and makes the value of the spatial mean of k inevitably scale-dependent. Accordingly, we propose that the average mass transfer velocity could not be a meaningful metric to describe the outgassing in spatially heterogeneous flow fields, such as encountered in step-and-pool rivers.

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Section: Discussionmentioning

confidence: 99%

Section: Numerical Simulation Of the Step And Poolmentioning

confidence: 96%

Section: Turbulence-induced Air-water Gas Exchange In the Flumementioning

confidence: 99%

Section: Modeling Gas Exchange In Step-and-poolsmentioning

confidence: 99%

See 2 more Smart Citations

Local processes with global impact: unraveling the dynamics of gas evasion in a step-and-pool configuration

Peruzzo

Cappozzo

Durighetto

et al. 2023

Preprint

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show abstract

“…Crucially, the exchange coefficient K embeds the coupled effects of the mass transfer induced by the turbulence of the flow and that associated with gas transport mediated by bubbles and foams (if any, see ref. 54 ). The solution of Eq.…”

Section: Methodsmentioning

confidence: 99%

Steps dominate gas evasion from a mountain headwater stream

Botter

Carozzani²,

Peruzzo³

et al. 2022

Nat Commun

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Steps are dominant morphologic traits of high-energy streams, where climatically- and biogeochemically-relevant gases are processed, transported to downstream ecosystems or released into the atmosphere. Yet, capturing the imprint of the small-scale morphological complexity of channel forms on large-scale river outgassing represents a fundamental unresolved challenge. Here, we combine theoretical and experimental approaches to assess the contribution of localized steps to the gas evasion from river networks. The framework was applied to a representative, 1 km-long mountain reach in Italy, where carbon dioxide concentration drops across several steps and a reference segment without steps were measured under different hydrologic conditions. Our results indicate that local steps lead the reach-scale outgassing, especially for high and low discharges. These findings suggest that steps are key missing components of existing scaling laws used for the assessment of gas fluxes across water-air interfaces. Therefore, global evasion from rivers may differ substantially from previously reported estimates.

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Effect of bubble flow on radon transfer at the water–air interface: experimental studies using optical methods

Tang,

Chen,

Wang

et al. 2024

J Radioanal Nucl Chem

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Unraveling the Contribution of Turbulence and Bubbles to Air‐Water Gas Exchange in Running Waters

Abstract: Air-water gas exchange is crucial for aquatic ecosystems as it affects metabolic fluxes, elemental cycling, and the exchange of greenhouse gases (GHGs) with the atmosphere (

Cited by 8 publications

References 76 publications

Local processes with global impact: unraveling the dynamics of gas evasion in a step-and-pool configuration

Local processes with global impact: unraveling the dynamics of gas evasion in a step-and-pool configuration

Steps dominate gas evasion from a mountain headwater stream

Effect of bubble flow on radon transfer at the water–air interface: experimental studies using optical methods

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