21 Apr 2023
 | 21 Apr 2023
Status: this preprint is currently under review for the journal BG.

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

Paolo Peruzzo, Matteo Cappozzo, Nicola Durighetto, and Gianluca Botter

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.

Paolo Peruzzo et al.

Status: open (until 02 Jun 2023)

Comment types: AC – author | RC – referee | CC – community | EC – editor | CEC – chief editor | : Report abuse
  • RC1: 'Comment on bg-2023-68', Anonymous Referee #1, 18 May 2023 reply
    • AC1: 'Reply on RC1', Paolo Peruzzo, 26 May 2023 reply
  • RC2: 'Comment on bg-2023-68', Anonymous Referee #2, 26 May 2023 reply

Paolo Peruzzo et al.

Paolo Peruzzo et al.


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Short summary
Small cascades greatly enhance the creek's gas emissions through the turbulent energy dissipation rate and the air bubbles entrained into the water. We numerically studied the local contribution of these mechanisms on the gas transfer velocity used to quantify the outgassing. The gas evasion is primarily due to bubbles concentrated in irregular spots of limited area. Consequently, the gas exchange velocity is scale-dependent and unpredictable, posing concerns about its use in similar scenarios.