Local processes with global impact: unraveling the dynamics of gas evasion in a step-and-pool configuration
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)
RC1: 'Comment on bg-2023-68', Anonymous Referee #1, 18 May 2023
- 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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The manuscript entitled ‘Local processes with global impact: unraveling the dynamics of gas evasion in a step-and-pool configuration’ by Peruzzo et al. focuses on the mechanisms driving high gas exchange velocity in step-pool streams. The authors used an artificial step-pool flume to evaluate gas exchange by investigating the role hydrodynamics on energy dissipation. Gas exchange is driven by diffusivity but enhanced with turbulence and gas entrainment in bubbles. Turbulence and bubble-mediated gas exchange is not easy to separate, which the authors do here and is a key component of identifying the mechanism behind high gas-exchange in mountain streams. The authors found that gas exchange in the artificial step-pool was highly heterogenous. The greatest gas-exchange was at the spout – where the water dropped from the step to the pool. Also, under steady-stream conditions, the area of bubbles created from the spout varied spatially over time. Bubble mediated gas exchange varied and was 2.5 to 5-fold greater than turbulent gas exchange. Overall, the authors found that bubble mediated gas exchange dominated in the ‘bubble’ zone whereas turbulent mediated gas exchange dominated in the ‘calmer’ zones of the flume with both types of gas exchange being highly spatially heterogenous. This is likely driven by the strong spatial heterogeneity of energy dissipation.
The authors discuss the importance of taking spatial heterogeneity into account in these types of streams, which I agree given their findings. However, I think scale is also highly dependent on how one might evaluate or estimate gas exchange. In field settings, we do rely on reach-scale gas exchange estimates in mountain streams – often from tracer gas experiments – as other methods are not feasible (i.e., night-time regression from continuous oxygen measurements, domes given the high turbulence do not often work – besides some of the work cited here, see also Hall Jr., Robert O., and Hilary L. Madinger. “Use of Argon to Measure Gas Exchange in Turbulent Mountain Streams.” Biogeosciences 15, no. 10 (May 18, 2018): 3085–92. https://doi.org/10.5194/bg-15-3085-2018). The work here illustrated how heterogenous k can be in a 120cm2 flume. At some point, averaging does occur - even within this experiment. I would caution suggesting that reach scale metrics in mountain streams are not adequete (if I understood the discussion correctly) - but rather selecting the reach with the heterogenity in mind would be a helpful step of estimating gas exchange in the field as we try to move towards some kind of scaling mechanism or even for any reach-specific studies.
L21: missing a closing paratheses after ‘such as CO2 and N2O to the atmosphere)
L36: missing ‘s’ for ‘see e.g. Cirpka et al. 1993’
L196: I appreciate quantifying the bubbles as bubble mediated gas exchange can be so much greater than turbulent driven k. The bubbles that were ‘hardly feasible’ to detect – would they not still have a significant effect on gas exchange significantly from turbulent driven k? On L264 it is discussed heterogenous size of the bubbles should not matter when estimating k. However, the diameter of the bubble is accounted for in equation 6. Perhaps as long as the bubbles are below 0.82 mm for radius, then bubble size should not matter as much?
L202: 'The decrease in [energy dissipation] with the depth was more than linear'. I don't quite follow 'more than linear' - do the authors mean the relationship was not linear? I suggest expanding what kind of relatinship was evident here for clarification.
L205: I don’t quite follow ‘the energy dissipation rate was one order (at least two orders)…’ I suggest for clarity – if it was indeed 2-fold difference in energy dissipation between the two regions – state that it was two-fold.
L266: I absolutely agree that likely in mountain streams – when bubbles are present – bubble-mediated gas exchange drives k. This work here is a great push forward to demonstrate bubble-vs-turbulent driven gas exchange by quantifying kb vs kt. However, this is not the first work to propose kb may dominate gas exchange in highly turbulent streams, which I suggest be cited appropriately.