Low and contrasting impacts of vegetation CO<sub>2</sub> fertilization on global terrestrial runoff over 1982–2010: accounting for aboveground and belowground vegetation–CO<sub>2</sub> effects

Yang, Yuting; McVicar, Tim R.; Yang, Dawen; Zhang, Yongqiang; Piao, Shilong; Peng, Shushi; Beck, Hylke E.

doi:https://doi.org/10.5194/hess-25-3411-2021

Articles | Volume 25, issue 6

https://doi.org/10.5194/hess-25-3411-2021

© Author(s) 2021. This work is distributed under
the Creative Commons Attribution 4.0 License.

https://doi.org/10.5194/hess-25-3411-2021

© Author(s) 2021. This work is distributed under
the Creative Commons Attribution 4.0 License.

Articles | Volume 25, issue 6

Research article

|

17 Jun 2021

Research article |

| 17 Jun 2021

Low and contrasting impacts of vegetation CO₂ fertilization on global terrestrial runoff over 1982–2010: accounting for aboveground and belowground vegetation–CO₂ effects

Yuting Yang, Tim R. McVicar, Dawen Yang, Yongqiang Zhang, Shilong Piao, Shushi Peng, and Hylke E. Beck

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Final revised paper (published on 17 Jun 2021)
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Preprint (discussion started on 14 Dec 2020)
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Status: closed

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

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RC1: 'Review of the manuscript by Yang et al. “Low and contrasting impacts of vegetation CO2 fertilization on terrestrial runoff over the past three decades”', Anonymous Referee #1, 05 Jan 2021
- AC1: 'Reply on RC1', Yuting Yang, 14 Feb 2021
RC2: 'Review of Yang et al.', Anonymous Referee #2, 15 Jan 2021
- AC1: 'Reply on RC1', Yuting Yang, 14 Feb 2021
RC3: 'Review of Yang et al.', Anonymous Referee #3, 18 Jan 2021
- AC1: 'Reply on RC1', Yuting Yang, 14 Feb 2021

Peer-review completion

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

ED: Reconsider after major revisions (further review by editor and referees) (22 Feb 2021) by Anke Hildebrandt

AR by Yuting Yang on behalf of the Authors (15 Mar 2021) Author's response Author's tracked changes Manuscript

ED: Referee Nomination & Report Request started (22 Mar 2021) by Anke Hildebrandt

RR by Anonymous Referee #1 (07 Apr 2021)

Suggestions for revision or reasons for rejection

The authors responded to my comments in the first round of review, and amended the text accordingly, but I am not convinced their approach is theoretically sound (though it might work in practice). Mixing and matching analytical models with different assumptions and re-parameterizing an analytical solution using an empirical (albeit simpler) equation makes the overall approach of this contribution not very appealing nor elegant. I realize that formal elegance might not be the goal here, but there are objective shortcomings too. For example, approximating Porporato’s analytical solution for the Budyko curve with Choudhury’s empirical formulation results in two types of errors: first, the empirical formulation is not necessarily a good approximation, and second, the relationship between the ‘n’ parameter and physically-based parameters in Porporato’s analytical solution also implies approximations and errors. I realize these errors are probably small, but still, it is not a ‘clean’ approach in my view.

The authors explain that “it is almost impossible to derive an explicit solution for Zr” for Guswa 2010 model, and that “Porporato’s model is much more complex than the Budyko model and can only be solved numerically”. However, the change in Q (Eq. 23) and the sensitivity S (Eq. 24) are calculated numerically anyway (L209), so there is no apparent disadvantage having a more complex solution for the Budyko curve. I do see the point that the approach adopted here has been also used in many previous papers, but that does not imply that the approach is set in stone - it can indeed be improved. Instead I agree there is a clear advantage in having an explicit solution for the rooting depth.

To sum up these thoughts, I would suggest trying the following:
1) Use Guswa 2008 model (as done in the original manuscript and in the current revision)
2) Use the Budyko curve obtained from the long-term mean water balance from the same.hydrologic model as in Guswa 2008. Essentially, it’s a simpler version of Porporato 2004 already derived previously (Milly, 1993), and was also compared to Porporato 2004 in Rodriguez-Iturbe and Porporato (2004, Section 2.6.2 Minimalistic models of soil moisture dynamics); this would allow a consistent model approach throughout.
3) With this simpler Budyko curve solution, it would not be necessary to approximate the analytical solution with Choudhury’s empirical formulation.

My impression is that this approach would make this work more theoretically sound, even though the main results might not be severely impacted.

Regarding my comment on seasonality, the authors commented that “The determined effective rooting depth during growing season is then used to determine the Porporato’s parameter and further, the Budyko parameter”. I understand that the rooting depth is calculated using growing season data, but the analytical solution of the Budyko curve by Porporato 2004 is then applied to determine annual discharge. Porporato’s model is not suitable to calculate annual averages if interpreted strictly - it assumes constant evaporative demand and precipitation regimes, and does not account for snow accumulation. To be strict, it should be applied to a-seasonal climates only. This requires some clarifications and discussions.

Minor comments
L82: I would mention that by “resource” it is actually meant water, since as explained elsewhere light- and nutrient-limited environments have high values of resource availability according to Fig. 2.
L95: acronym WUE is not defined.
L193: “factors… encode” (plural)
L205: “factors driving…”
L357: “factors… dominate changes…”

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RR by Anonymous Referee #3 (16 Apr 2021)

ED: Publish subject to minor revisions (review by editor) (05 May 2021) by Anke Hildebrandt

AR by Yuting Yang on behalf of the Authors (12 May 2021) Author's response Author's tracked changes Manuscript

ED: Publish as is (22 May 2021) by Anke Hildebrandt

Short summary

This study developed an analytical ecohydrological model that considers three aspects of vegetation response to eCO₂ (i.e., stomatal response, LAI response, and rooting depth response) to detect the impact of eCO₂ on continental runoff over the past 3 decades globally. Our findings suggest a minor role of eCO₂ on the global runoff changes, yet highlight the negative runoff–eCO₂ response in semiarid and arid regions which may further threaten the limited water resource there.

Low and contrasting impacts of vegetation CO2 fertilization on global terrestrial runoff over 1982–2010: accounting for aboveground and belowground vegetation–CO2 effects

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Low and contrasting impacts of vegetation CO₂ fertilization on global terrestrial runoff over 1982–2010: accounting for aboveground and belowground vegetation–CO₂ effects