Gas transfer velocities of CO<sub>2</sub> in subtropical monsoonal climate streams and small rivers

Li, Siyue; Mao, Rong; Ma, Yongmei; Sarma, Vedula V. S. S.

doi:https://doi.org/10.5194/bg-16-681-2019

Articles | Volume 16, issue 3

https://doi.org/10.5194/bg-16-681-2019

© Author(s) 2019. This work is distributed under
the Creative Commons Attribution 4.0 License.

Special issue:

Human impacts on carbon fluxes in Asian river systems

https://doi.org/10.5194/bg-16-681-2019

© Author(s) 2019. This work is distributed under
the Creative Commons Attribution 4.0 License.

Articles | Volume 16, issue 3

Research article

|

04 Feb 2019

Research article |

| 04 Feb 2019

Gas transfer velocities of CO₂ in subtropical monsoonal climate streams and small rivers

Siyue Li, Rong Mao, Yongmei Ma, and Vedula V. S. S. Sarma

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Final revised paper (published on 04 Feb 2019)
Supplement to the final revised paper
Preprint (discussion started on 04 Jun 2018)
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Interactive discussion

Status: closed

AC: Author comment | RC: Referee comment | SC: Short comment | EC: Editor comment

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RC1: 'Comments to bg-2018-227', Anonymous Referee #1, 29 Jun 2018
- AC1: 'Response to Anonymous Referee #1', Siyue li, 07 Sep 2018
RC2: 'Comment on “Gas transfer velocities of CO2 in subtropical monsoonal climate streams and small rivers” by Siyue Li et al.', Anonymous Referee #2, 13 Jul 2018
- AC2: 'Response to Anonymous Referee #2', Siyue li, 07 Sep 2018
RC3: 'Review bg-2018-227', Anonymous Referee #3, 17 Jul 2018
- AC3: 'Response to Anonymous Referee #3', Siyue li, 07 Sep 2018

Peer-review completion

AR: Author's response | RR: Referee report | ED: Editor decision

ED: Reconsider after major revisions (03 Oct 2018) by David Butman

AR by Siyue li on behalf of the Authors (15 Oct 2018) Author's response

ED: Referee Nomination & Report Request started (06 Nov 2018) by David Butman

RR by Anonymous Referee #2 (23 Nov 2018)

RR by Anonymous Referee #3 (24 Nov 2018)

Suggestions for revision or reasons for rejection

Review bg-2018-227_R1

(I refer to the document bg-2018-227-author_response-version1.pdf)

General comments
I can see that a lot of effort has been put in the revisions and I feel that the manuscript has definitely improved, it is clearer and much easier to follow. Changes such as restricting the upscaling to the monsoonal period is much more appropriate in my point of view, the cutoff of at delta pCO2 110 µatm for k600 calculations is well clarified, the overview map (Fig.1) gives a good impression over the sampling effort/sampling area, and the separation of the datasets for different purposes is more clear.

However, in my point of view, there are still some critical points which need to be addressed:
• Anchored vs. drifting chambers: I appreciate that you addressed this issue in the manuscript. Well, your k600 values are close to the average of Ran et al. (2015) (measured with drifting chambers) and Liu et al. (2017) (measured with static chambers in canoe shape), this indicates that your potential overestimation is limited. However, since you have a mix of anchored and drifting chamber measurements, you added a considerable amount of variability related to k600 values to your dataset. Potentially, this is part of the reason why you did not find significant correlations using the entire/complete data set? If possible, I suggest testing the relationships with chamber derived k600 values and flow velocity/depth only with the drifting chamber data. Alternatively, address this issue in the chapter where you discuss the uncertainty of the data (4.4).
• The k600 models are actually only valid for a subset of the data. Nevertheless, they are applied for the whole dataset. I would appreciate some thoughts why you think this is still meaningful. What does it mean in terms of generalization or if readers would like to apply the developed models in other regions?
• The k600 vs. flow velocity model (Fig. 4b): Sorry, but I cannot follow you there. If “extremely” values are removed (which in my opinion still needs to be justified and clearly described which ones and why), the R2 gets reduced and the p-value gets worse. Please justify and describe the strategy of removing data points. If this cannot be done in an appropriate manner, I don’t see any reason why data points should be removed.

I still think that this is a valuable study which would make a good contribution to the literature, but the above-mentioned points need to be addressed before considering publication in Biogeosciences.

Specific comments

Abstract:
Line 22: Explain the meaning of k600 already here, where you mention it for the first time. In general, I suggest not jumping between k600 and k in the abstract (i.e. stick to k600 after mentioning it for the first time).

Lines 24-26: Please make clear that the derived model for k600 is only based on a subset of the data.

Line 26: Add “e.g. lakes” after open waters.

Line 34: There are k600 models for streams (see e.g. Raymond et al., 2012). Do you mean for the specific regions/watersheds? Please be more specific here.

Introduction:
Line 59: Add “in situ” before “temperature”.

Lines 65-66: Please add the information that the standardized k600 is valid for freshwaters.

Lines 97-98: This sentence is not clear to me, please rephrase.

Line 108: Please rephrase “diffusive models from other continents”, it sounds very vague.

Lines 105-115: This is a good and important paragraph, but in my opinion it breaks the flow of the introduction. I suggest implementing/moving it before line 98 (i.e. the new contributions to the literature).

Materials and methods
Lines 134-135: So if I understood correctly, according to the definition you use, the Daning and Qijiang are rivers and the rest are TGR streams and small rivers? If that is the case, unify the terminology in the complete MS (text, figures, tables), ev. define it already in the introduction, and stick to it. Otherwise, this is confusing.

Line 151: As far as I understood, the nutrients were excluded. Please clarify.

Lines 158-159: Please rephrase/revise the last part of the sentence (i.e. “with an accuracy is better than 0.2%”).

Line 167: Sorry, but I still don’t really understand what you mean with this sentence. Do you intend to describe the quality of the used solvents and reagents?

Line 170: How was flow velocity measured? It plays a major role later on, and I think it is important to know how it was measured.

Line 182: Not sure what you mean/refer to with this sentence, please clarify.

Lines 197-201: How many AC and how many DC measurements were done?

Lines 200-201: Please add the range of overestimation here.

Lines 212-213: The manufacturer of the EGM specified the 0.95 R2 threshold?

Lines 246-247: I think this aspect should be addressed in the discussion section in which you discuss the uncertainty of the data (4.4).

Line 256: Do you mean the TBL model? Please be consistent with the terminology.

Lines 262-264: So the data of the other large river (Daning) could not be used at all? Please make this clear.

Results:
For results in general: Please indicate the absolute value of p, i.e. not only < 0.05.

Line 274: Significantly lower? Please be precise.

Lines 286-290: To me is not clear, where DOC is significantly higher. Please rephrase this sentence.

Lines 314-320: This seems contradictory to me: No significant relationship with current velocity using the “entire” data set, but significant relationship of flow velocity using “combined” data. What is the difference between current velocity and flow velocity, and between “entire” and “combined” data? Please make this clear.

Discussion:
For the discussion: In terms of the desired funnel shape (from detailed to broad), I suggest starting the discussion with 4.4 (Uncertainty assessment of pCO2 and flux-derived k600 values).

Lines 351-359: Thanks for adding a paragraph discussing the chemical enhancement. Nevertheless, I got a bit confused: did you actually calculate the chemical enhancement? (“…of sampling sites that were strongly affected by chemical enhancement…”) And how did you come up with 1.7%? Where did u make the cut-off in terms of pH? Please make this clearer by e.g. having a look at Alshboul Z, Lorke A (2015) Carbon Dioxide Emissions from Reservoirs in the Lower Jordan Watershed. PLoS ONE 10(11): e0143381. doi:10.1371/journal.pone.0143381.

Lines 457-461: Please revise this sentence, to me it is quite hard to understand.

Tables
Table 2: I thought if combined data are used then the models do not work? This seems contradictory with the table caption. Furthermore, I do not understand the meaning of the first header in relation to the categories below.
Table footnote c: Revised how? By taking out extreme values? Please add some additional information here.

Table 3a: Please add also the standard deviation/CI for the annual emission to be transparent in terms of uncertainty.

Table 3b: Please add the information that you also present emission data here, I guess in Tg CO2/y.

Technical corrections:
Lines 124-125: Replace “concentrated in April through September” with “concentrated between April and September”.

Line 128: Add “data of“ before 48.

Line 208: Delete P0 and T0.

Line 342: Change “lower than one third” to “one third lower”.

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ED: Publish subject to minor revisions (review by editor) (17 Dec 2018) by David Butman

AR by Siyue li on behalf of the Authors (27 Dec 2018) Author's response Manuscript

ED: Publish subject to technical corrections (16 Jan 2019) by David Butman

AR by Siyue li on behalf of the Authors (18 Jan 2019) Author's response Manuscript

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Short summary

We provide a first determination of k in human-impacted subtropical streams and small rivers. High and highly variable k values reflect different controls on water turbulence. New models of k are developed using water depth and flow velocity. We show that previous estimates of riverine CO₂ evasion from subtropical streams and small rivers are conservative and highlight the importance of incorporating scale-appropriate k measurements into extensive pCO₂ investigations for accurate C budgets.

Gas transfer velocities of CO2 in subtropical monsoonal climate streams and small rivers

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Interactive discussion

Peer-review completion

Gas transfer velocities of CO₂ in subtropical monsoonal climate streams and small rivers