the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 04 Jun 2024
Does peatland rewetting mitigate extreme rainfall events?
Abstract. Pristine peatlands are believed to play an important role in regulating hydrological extremes because they can act as reservoirs for rainwater and release it gradually during dry periods. Therefore, rewetting of drained peatlands is considered an important strategy to reduce the catastrophic effects of flooding. With the anticipation of more frequent extreme rainfall events due to a changing global climate, the importance of peatland rewetting in flood mitigation becomes even more important. To date, empirical data showing that rewetting actually restores the hydrological function of drained peatlands is largely lacking, particularly in Sweden. To assess whether rewetting peatlands can mitigate extreme rainfall events and ensure water security in a future climate, we measured event-based runoff responses before and after rewetting using a BACI approach (before-after and control-impact) within a replicated, catchment scale study at the Trollberget Experimental Area in northern Sweden. High-resolution hydrological field observations, including groundwater table level, discharge, and rainfall data were collected over four years, allowing us to detect and analyze 17 rainfall-runoff events before and 30 events after rewetting. Our rainfall-runoff analysis revealed that rewetting significantly decreased peak flow, runoff coefficient, and reduced the overall flashiness of hydrographs, making the rewetted site function more like the pristine control peatland. However, “lag time” which was already similar to pristine conditions was pushed farther away from pristine conditions following rewetting. We found that the rewetted site experienced an increase in the groundwater table level following rewetting and this was consistently observed across all distances from the blocked ditch within the peatland, providing complementary data for our event-based analysis. In summary, our findings suggest that peatland rewetting has the potential to mitigate flood responses, however, further research over a longer time period is needed as peat properties and the peatland vegetation will develop and change over time.
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Status: open (until 02 Aug 2024)
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RC1: 'Comment on hess-2024-158', Anonymous Referee #1, 24 Jun 2024
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General comments
This manuscript assesses the impact of peatland rewetting on peatland flood mitigation in a nutrient-poor boreal minerogenic fen in northern Sweden. The study used an impressive four year long dataset of hourly hydrometric and GWL data, which the authors utilized with valid and clearly described scientific methods. The study was well-designed as it included data both before and after rewetting from both the treated and control sites. This is a novel study that contains important results concerning rewetting effects on peatland hydrology and it fits perfectly within the scope of HESS. However, the discussion section suffers from text bloat and loss of focus due to unnecessarily long summaries of the referenced papers, which has replaced the proper discussion of some of the results reported in this manuscript. Also, more details or at least rephrasing the descriptions of the study sites are needed. After some work, I think this manuscript will be suitable for publication in HESS.
Abstract
You don’t write that you had two catchments and that the results differed between them. Currently, you only summarize the results from the catchment where you observed significant changes and present that as the sole result of your study. I think it should be mentioned that you also had the other catchment and there you did not observe significant changes. You can briefly mention why it is not important in terms of the conclusions of the study if that is the case. My understanding is that catchment R2 was already quite similar to pristine peatland conditions and because of that, the changes were not significant there.
Introduction
The introduction addresses well the relevant scientific questions and gives a nice background for the study. However, I think the introduction is a little bit on the longer side. The first three paragraphs were really nice and clearly structured, but after that, there were parts which were dragging a bit and I think they could be written more concisely. For example, you could consider if the paragraph starting on L100 is really necessary or if it can be shortened or parts of it moved into some other part of this section.
In my opinion, the paragraph starting on L112 is not in the correct place. I think it should be much closer to the beginning of the section, for example, between the pristine peatland and rewetting paragraphs (L57). This paragraph is very Sweden-specific, so that should be led already on previous paragraphs somehow as they concentrate mostly on the European scale and suddenly having a paragraph written solely on Sweden’s point-of-view is a little too much of a contrast.
Methods
The utilized methods are well-described and scientifically valid and I have no major comments on those. However, the site descriptions need some clarification (see the “specific comments” below).
Add the company names when reporting the used instrumentation.
Results
The results were clearly presented and I enjoyed reading them.
Check that you use the hyphen and en-dash properly throughout the manuscript. For example, in 3.1. you use both of them in the same use cases (L303, L308), which does not make sense.
Fix the unit formatting: mm/h --> mm h-1.
Discussion
This section is the one needing the most amount of work. There is a lot of summarizing of the referenced papers, which unnecessarily bloats the text. I’m not saying you should not compare your results to other studies at all, but I would like to see more answers to questions containing “why?”. Why did something happen/didn’t happen (you can use your hypotheses as a base), why your results were different/similar to other sites’ results, etc.? Now most of the discussion is summarizing what previous studies have done and then you compare your results to those sometimes with just one short sentence. This is not the point of the discussion section. See more details in "specific comments".
The last paragraph was the one I was waiting to read for the whole discussion section. This is what the discussion should contain (here: “Why the results from the two catchments differ from each other?”)!
On L220 you say :“It is noteworthy that the dipwells were also located near other side ditches, indicating a potential limitation in the study design.” I did not see any discussion about how this may have affected the results. I think it should be added.
Conclusion
Nice conclusions. I have two small suggestions.
This is the same comment I gave about the abstract as well. I think it should be briefly mentioned why the changes at R2 were not significant. Otherwise, one might get a feeling that you had two catchments and only one had experienced significant changes after rewetting, but you are making generalization based only on that catchment that experienced significant changes and ignoring/hiding the results from the other catchment.
It would be nice if you could condense the first three sentences describing what you did in this study. It currently takes about one-third of this section, which should be about the conclusions of the study. I suggest removing the last one of these three sentences and moving the mention of GWL to the second sentence.
Specific comments / Technical corrections
L4: “Agricutural” --> “Agricultural”
L14: Do you want to say that rewetting peatlands can mitigate the effects of flooding caused by extreme rainfall events? I read this sentence so that rewetting can mitigate specific weather conditions (here: when it is raining a lot). This same comment applies also to the title of the paper. Consider rephrasing.
L33: 15% of the boreal land/surface area or something else? Please clarify.
L38: “…with more than half of the peatlands estimated to have been lost…” Do you mean half of the original peatland area or what? Also, I wouldn’t say that the peatlands are lost in some of the activities you mentioned, but rather transformed as the peat is still there in the soil. I suggest using “pristine peatlands” instead of just “peatlands” to clarify what you want to say.
L40: Could you give some examples of the ecosystem services that they cannot sustain?
L75-L77: I don’t think it is an inconsistency that both studies you reference here reported a reduction in peak storm flows (+ only other observed extension in lag times), although a different amount. Or what do you mean by these “inconsistencies”, could you clarify or use different wording? You use the word “inconsistent” later in the introduction as well, check that those also make sense.
L140-L141: Is this the vegetation/tree stand situation at the site before rewetting or after? You should be more clear about the description of the site in terms of before and after rewetting.
L147: The peatland was drained for forestry purposes, so the site was an actual forest before rewetting? GWL seems to have been quite high (section 3.1) before rewetting, so it was not likely a dense forest with tall trees? Is this a typical result of drainage in Sweden? On L141 you write that there were “individual Scots Pine”, which gives the impression that there were only some pine trees there and there. Could you clarify this? Did you measure tree stand characteristics (diameter, height, basal area, etc.) at the site before rewetting? It would be nice to have that as supplementary information. Were all the trees cut during the rewetting as you say on L152? Fig 1b shows that there are some trees left in the eastern part of the site. Is that the case (then correct what you say on L152) or was the picture taken before the cuttings were finished?
L152: Were the two catchments similar in terms of the site characteristics you report in this paragraph? Please clarify.
L158: How near was Degerö to the Trollberget? Add the distance to the text.
L166: What is the source of this data? Why did you calculate the mean temperature and precipitation for a different period for Degerö and Trollberget (L144-L146) sites? Maybe the correct question is why do you report different numbers for these sites when they are located so close to each other? I think that only one set of climatological statistics is enough.
L177: Add space after GWL.
L180: “5” --> “five, “6” --> “six”
L181: How many GWL loggers were there in Degerö?
L188-L189: How frequent were those manual water level measurements?
L193: So you did not use/have discharge data from the Degerö site? If so, I think it should be mentioned explicitly.
L199: Add space between “225” and “m”.
L212: “before this date” --> “outside this period”
L232: What were these “predefined thresholds”?
Figure 1:
What kind of stream/ditch is the one at the bottom of R1? Should that have been considered in this study? Does the figure contain all the ditches at the site (related to my later comment)?
The distances between the transect lines in A and B figures do not match. What I mean is that in the lower figure, it looks like there are two groups of two transects and one sole transect, but in the upper figure there are two sole transects and a group of three transects. Why is that?
L297: Remove “has”
L300: “decreased” --> “decrease”
L303: Was the GWL significantly different on different distances from the ditch before and after the rewetting?
Figure 2: Why did you combine the post-rewetting data into one in 2b and not show the years separately as you did in 2a?
Table 1: Rounding to two significant digits would be accurate enough in this table.
L360, L361, L364: Add the missing units.
L362: “… R2 showed similarities to the control site.” Add a reference to Table 2.
Figure 3: Add what “Q” is into the figure caption.
Figure 4: It would be nice to have the same y-axes for the specific discharge subplots, so their year-to-year variation would be easier to see.
Table 2:
In the caption: “4” --> “four”. Should it be “Figure 4” instead of “Figure 5”?
I’m not sure if this table is necessary and it could be combined with Fig. 4. One can see the peak flows from Fig. 4 already, so showing that again here in the table is not needed. Also, total rainfall can be added as a number within the rainfall subplots in Fig. 4. However, antecedent GWLs are a bit trickier. You could add a separate subplot for GWL similarly as you have the rainfall. Then you could add the antecedent GWL as a number somewhere within the GWL plot if you think that it needs to be highlighted separately. Alternatively, you could add GWL data as a second y-axis for the rainfall subplot.
L431: “2.3” --> “2”. I don’t think you need to be that accurate here considering that you are talking about precipitation.
L440: Remove “Moreover”.
L445: I’m confused, what do those p-values represent? R1 is p < 0.01 and R2 is p < 0.05? But you just wrote that the reduction was statistically significant solely at R1, but if the p-value was smaller than 0.05 for R2, that would mean the reduction was significant for R2 as well, right? Please fix/clarify. Also, add into the brackets a clarification of what site the p-value represents (ex. R1: p < 0.01 and R2: …).
L447: I don’t think it is worth mentioning the pre- and post-rewetting lags and their decrease in the same sentence. It should be enough to say the change in the lag time and then either pre- or post-rewetting lag times. The easiest fix is to remove the rest of the sentence after “respectively”.
L484: What makes you think you can make such a generalization based on results from just this one site? Also, your observed changes were significant only at one of the two catchments you measured.
L487-L488: I think this first sentence is missing the result, which you are comparing to other studies in the next sentence.
L493: Lower than what?
L493-L494: Related to my previous comment on the tree stand data. Here it would be nice to have some hard data about tree stand and vegetation on which to base these conclusions.
L498 & L500: “We demonstrated that the GWL increase after rewetting was spatially variable but occurred at all distances from the main ditch.” & “…, our results reveal a significant increase in GWL at all distances after rewetting.” Unnecessary repetition.
L504: The 800 mm rise in GWL in Haapalehto et al. (2014), in my opinion, was not similar to your result. You wrote on L305-L306 that the largest rise in GWL was 119 mm, which is much smaller compared to the referenced paper.
L503-L519: This paragraph contains too many details from the referenced papers and very little discussion about your results. Comparing to previous studies is ok, but there is no need to have an “extensive” summary of each referenced paper.
L520-L529: How is this related to your study? There is no mention of that. This paragraph is just a summary of three previously published papers. This is not the purpose of the discussion section.
L537: There should be a paragraph break here before “Overall”. The text before it is just another summary of a previously published study. The latter half is better as you address your hypothesis, but I think it should be done earlier in the discussion maybe at the start of the second paragraph. After doing that, you would compare your results to other studies and address why they are different/similar to your study. I’m pretty sure that you can reduce the length of the discussion about GWL change in different distances to about half of what it now is just by reordering the text and removing unnecessary extensive summarizing of the referenced papers.
L555: Why was a lower proportion of ditches blocked in R2 compared to R1? Also, looking at the blue and red lines in Fig. 1a, it seems like 99% of the ditches were blocked in R2, which is more than in R1 and conflicts with your statement. Either you meant that a lower proportion of the ditches were blocked in R1 compared to R2 or you are not showing all the ditches in Fig. 1a. If you are not showing all the ditches in the figure, you should add them.
L556: “particularly in Sweden” So there are studies outside Sweden? Why you are not mentioning those? I understand that such datasets are very limited, however from this choice of words I get a feeling that you know that there are other studies, but for some reason, you are not referencing those.
L584: “However, caution in interpreting these results…” Some words are missing here. “… caution should be taken in when…”?
L642: “the evidence suggests…” What evidence? Your study? Or some other studies (add references in that case)?
Citation: https://doi.org/10.5194/hess-2024-158-RC1
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