the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 17 Dec 2024
Review and Synthesis: Peatland and Wetland Models Simulating CH4 Production, CH4 Oxidation and CH4 Transport Pathways
Abstract. Peatlands play an important role in the global CH4 cycle and models are key tools to assess global change effects on CH4 processes. It remains unclear how well our existing wetland modelling frameworks are suited to peatland questions. Therefore, we reviewed 16 peatland or wetland models operating at different spatial (seconds-to-decadal) and temporal (soil core-to-global) scales, having different spin-up periods for carbon pool stabilization and various CH4 production, oxidation and transport processes. Through a literature review, model specific advantages and limitations, common and specific driving inputs of all models and critical inputs of individual models impacting CH4 plant-mediated transport, diffusion and ebullition were summarized. The 16 reviewed models were qualitatively ranked 0 to 4 (none-to-full process representations) with respect to CH4 production, oxidation and transport. The most common temporal and spatial scale for 14 models was daily time-step and field scale respectively, while the spin-up stabilization periods of different carbon pools (peat, litter, roots, exudates, microbial, humus, slow, fast) of all models ranged 7 to 90102 years. With regards to CH4 production and oxidation, 50 % of reviewed models (Ecosys, CLM-Microbe, ELM-Spruce, Peatland-VU, Wetland-DNDC, TRIPLEX-GHG, TEM, CLM4Me) exhibited full to adequate process representation. Meanwhile 44, 44 and 25 % models exhibited full to adequate process representation for plant mediated transport, diffusion and ebullition respectively. This meant there is ample scope to improve ebullition processes in the remaining 75 % models. We conclude that existing models are adequate for site-level CH4 flux assessments but may lack a predictive understanding of CH4 production pathways.
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Status: open (until 13 Feb 2025)
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RC1: 'Comment on egusphere-2024-3852', Ashley Ballantyne, 14 Jan 2025
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Summary: Here the authors provide a qualitative review of the current state-of-the-art peatland methane models and evaluate them based on the physical, chemical, and biological processes incorporated in order to simulate CH4 fluxes.
General Comments: As a researcher not directly involved in the development of methane models, but sometimes reliant on their simulations, I found this to be a helpful high-level ‘dummies’ guide to methane models. The authors identify key attributes and limitations of these models that should be considered before researchers select a particular model or rely on its output. Although I did find this qualitative review informative, the information could possibly be presented with greater clarity and a more illustrative way.
The authors do a decent job of summarizing across this diverse array of models, but I wonder if the information was better synthesized graphically it might be more useful. For instance, as much as I detest Venn-Diagrams perhaps this is a situation where differences and similarities among these models could be better illustrated. I also found some of the figures to be not that informative. For instance, maybe figure 1 could have a log-spatial scale on the x-axis using data in Table 2. Also figure 2 seems to have CH4 pathways on both axes and thus is more confusing than helpful. Perhaps a combined figure with your scores of transport (Fig. 4) vs. process (Fig. 3) or number of inputs would help researchers identify the groups of models best suited for their research questions. Alternatively, a dichotomous tree could be helpful for researchers in selecting a suitable model for their research specific research question, while providing focus in the conclusion section.
There was a detailed description of how methane is measured in the introduction using bottom-up vs. top-down approaches but this was not imperative for the rest of the paper.
There were a couple of key citations that I was expecting to find, such as (Evans et al. 2021) and others noted in PDF comments, but were not included in the references
Coupled diffusion of heat and CH4 is challenging! With my limited experience modeling CH4 in peatlands, it seemed that we spent 75% of the time trying to get the thermic properties of heat diffusion correct and then only 25% of the time actually simulating the diffusion of CH4. So these coupled diffusion processes are challenging, especially with complex hydrology and/or snowcover.
All in all, I think that this is an effective qualitative review. However, the attempts at making this review more quantitative, including some figures that were actually confusing, actually made this review less effective. I am not sure if this article is submitted as a research article or a review article but it seems to fall between the cracks- not quite a research assessment of models and not quite a review.
Specific Comments:
See PDF with editorial comments added
References
Evans, C. D., M. Peacock, A. J. Baird, R. R. E. Artz, A. Burden, N. Callaghan, P. J. Chapman, et al. 2021. “Overriding Water Table Control on Managed Peatland Greenhouse Gas Emissions.” Nature, April, 1–7.
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RC2: 'Review of egusphere-2024-3852', Anonymous Referee #2, 16 Jan 2025
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In their manuscript, Tilak and co-authors summarize features of a number of models of wetland methane production and transport. Analysis, however, remains superficial and mainly consists of summarizing the statements made by the authors of the original model description papers without deeper analysis. As a result, the authors quite often compare apples with oranges – likely unknowingly -- and the value of the manuscript is questionable.
I suggest rejecting the manuscript.
I was really looking forward to reviewing this manuscript. Having worked in both peatland and methane modelling, I believe a review and synthesis paper is overdue. However, reading the manuscript, I was quickly disappointed. It appears the authors have very little experience with modelling approaches. As a result, the paper does not go beyond the superficial analysis of the model description papers.
Unfortunately there are too many issues with the manuscript to do a complete review. Instead I will list a few of the issues I noticed as examples and to support my suggestion to not publish the manuscript.
However, I very much encourage the authors to add an experienced modeller to their team and submit a rewritten manuscript, as I believe a manuscript on this subject could be interesting to a wider audience.
Here is a collection of issues with the manuscript, ordered by order of appearance in the manuscript:
1) The discussion of top-down and bottom-up approaches (lines 51 ff) appears to be superfluous, as both measurements and the models discussed in the manuscript are usually considered as bottom-up approaches.
2) Spatial scales (lines 145 ff and table 2): I assume the authors collected what appeared in the model description papers? The scales reported (plot/field/regional etc. scale) are, however, rather arbitrary in the modelling context. Most models follow a “big-leaf” approach, where processes are calculated for a single representative specimen, usually a leaf or a plant, and then upscaled to the scale of interest. Much more interesting is whether models only run on a single point, or whether they calculate values for a grid of points, thus allowing the coverage of larger areas and not just single sites. This is completely neglected, though.
3) Spinup time (lines 155 ff): Here many apples are compared with a few oranges. Generally, carbon pools have turnover timescales of years for litter pools, decades for reactive soil carbon, centuries for non-reactive / recalcitrant soil carbon and millennia for specific peatland carbon pools. Thus, the different spinup timescales reported for the different model are determined by the carbon pools considered in the model. Unfortunately, this is not discussed in the manuscript, but the authors only report the different spinup times given by the model authors. This is highly misleading: As an example, the HIMMELI model only reported the equilibration times for the methane specific components of the model. The HIMMELI model, however, needs to be run within a land-surface / carbon cycle model, as HIMMELI doesn’t deal with any of the relevant land surface physics or carbon calculations. Spinup times will therefore be determined by the host carbon cycle model, not by HIMMELI itself.
Much more interesting is the question whether an accelerated spinup procedure is available – this is not reported, though.
4) Process representation evaluations, lines 383 – 417: The manuscript mentions “full to adequate process representation” for plant mediated transport, diffusion, and ebullition. These are, however, not clarified, and to my knowledge there is no community consensus what might constitute “full” or “adequate” process representation. Thus, the manuscript passes judgement on models (“only 25% of the models exhibited full…”) without adequately defining the criteria.
5) Advantages and disadvantages of reviewed models (lines 446 – 494): In these sections, the authors seem to summarize whatever the authors of the original model description papers gave as advantages and disadvantages, it is unclear whether these sections contain significant analysis by the manuscript authors. It is also doubtful whether the same criteria were applied to all models. One example from the disadvantages section: “… while the HIMMELI does not simulate any electron acceptors , except O2.” What is not mentioned in this sentence (or anywhere else) is that non-O2 electron acceptors are mostly handled by an unspecific bulk term, if at all, and very rarely, if ever, explicitly. Thus the “disadvantage” of the HIMMELI model is mainly that they actually reported that their treatment is not perfect, while other authors glossed over this fact. Thus this analysis is incomplete at best and potentially misleading.
Citation: https://doi.org/10.5194/egusphere-2024-3852-RC2
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