the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 10 Apr 2025
Evaporation from northern latitude wetlands
Abstract. The atmospheric demand for evaporation in northern latitude ecosystems is expected to increase with increasing temperatures and a longer snow-free season. To understand how increased evaporative demand will affect ecosystems in this typically moisture-rich region, we need more knowledge about the factors that control evaporation and, furthermore, how evaporation modifies local hydrology. We used year-round evaporation estimates from four eddy-covariance wetland sites in Norway to quantify evaporation and identify its main controls along climatic gradients in temperature and precipitation. We found that ecosystem evaporation was indeed mainly controlled by atmospheric evaporative demand and spring snow-cover duration. Soil moisture remained high during the measurement period and likely never reached a level where it would impact evaporation. Annual evaporation ranged from 81 mm to 208 mm and increased with warm-season mean temperature along the spatial gradient. We found a large variation in the role of evaporation in the ecosystem water balance, with annual evaporation ranging from 9 % to 30 % of annual precipitation. In the warm season, evaporation was typically around 50 % of the seasonal precipitation, but reached a maximum of 72 %. Compared to other northern latitude sites in the FLUXNET2015-dataset, the evaporation from the Norwegian sites was lower than what would be expected from the site warm season mean temperatures. Our results show that evaporation is an important part of the northern latitude water balance, especially during the warm season and in parts of the region with low precipitation. Furthermore, our results indicate that earlier snow-cover melt-out and increased vapour pressure deficit have the potential to increase annual evaporation.
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Status: open (until 16 Oct 2025)
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RC1: 'Comment on egusphere-2025-1140', Anonymous Referee #1, 11 Jun 2025
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General Comments:
The authors present multiple years of eddy covariance data from four northern sites across Norway and compare annual evapotranspiration totals and evaporative controls to other northern FLUXNET sites. The presented data is a valuable contribution to the field due to the scarcity of evaporation measurements at northern sites, particularly over both the snow-covered and snow-free seasons, and the authors present the interannual variability of evaporative totals and controls across their sites. It is a lot of work to compile eddy covariance data from that many site-years and I commend the authors for their efforts, presentation, and gap-filling techniques. I appreciate how well they have contextualized their sites within current (albeit fairly scarce) literature and evaporation data from other northern sites. Although the writing and figures are clear, I find the abstract, discussion (mainly sections 4.2 and 4.3) and main conclusions do not entirely explain the results of the study in enough detail as the paper is currently written. This comment may be a compliment to the paper, as there were interesting results and nuances among the sites that were presented in the results, yet not fully explained in the discussion. The authors often repeat themselves, explaining that a warmer climate and increased atmospheric demand for moisture will increase ET (which is already fairly well established), yet do not fully dive into the interesting results and differences between sites. The authors reiterate that a longer snow-free period will increase overall evaporation and that their sites had low sensitivity to the phenology and soil water content. While these are important conclusions, it also does appear that there are variations among these four sites presented that could be explored in more detail with even more added scientific value (i.e. different threshold responses to VPD, varying sensitivities to soil moisture, etc.). Additionally, the Penman vs. observed ET at these sites is a useful comparison to know how to deal with surface conductance terms in the future. Overall, this paper is a useful and well-presented contribution to furthering our understanding year-round northern wetland evaporation. However, the authors may want to consider reframing the primary messaging of the paper away from describing the ET totals within the context of the water balance (which is tricky when only comparing ET and precipitation and not even presenting snow accumulation data) to focusing on highlighting the different controls/sensitivities between these sites, even if subtle.
Specific Comments:
Title:
- I appreciate the simplicity of the title and understand if the authors choose to keep it as-is. I do, however, think there may be more value in adding in key words highlighting that the paper evaluates evaporative controls across these sites, has year-round measurements, and includes tundra ecosystems. It could also mention the context of Norway, etc. It currently sounds as though this is a review paper rather than a paper that presents new data.
Abstract:
- While the paper is well written, the abstract does not capture the interesting findings that were presented throughout the paper. The abstract could emphasize both the analysis that was performed, and the dataset a bit more, particularly the fact that these are year-round measurements and the shoulder season processes are of value with variable snow-cover. I believe this paper does improve our understanding of evaporation from these sparsely studied ecosystems, but as it is written, the abstract does not really present any new findings or insights (that again, the paper results do). I do not believe the current abstract does the paper justice, and I worry those skimming it would miss the valuable contributions this work does present.
Introduction:
- The introduction needs to discuss the importance and extent of wetlands in this region more specifically. There is discussion of thaw lake basins, alpine tundra sites, etc. and the authors state that most other studies focus on forested areas and carbon, however the justification behind why we need wetland-focused studies is lacking. It would be useful to know their proportion of northern landscapes, their hydrological role in larger ecosystem functioning, unknown sensitivities to climate change, etc. The authors state these ecosystems are largely missing from the literature, but justification behind why we need this information is not currently presented. Additionally, it would be useful for the introduction to briefly discuss some of the processes/controls on evaporation in a bit more detail. For example, in the discussion you reference Liljedahl et al. (2011) and Helbig et al. (2020) and discuss VPD controls on ET and bowen ratios with varying soil moisture and the differences between snow-covered and snow-free seasons. This of course can remain in the discussion; however, some mention of these processes would be useful upfront in the introduction.
Methods:
- I understand why the authors use time since rain as a proxy for soil moisture availability and I do think this is fine for their presented analysis, however it would be useful to know how their time since rain variable compared to the point measurements they also took. This warrants a bit more discussion or explanation (even one or two sentences), as precipitation alone of course does not describe soil moisture dynamics. There is a large difference in soil moisture if it rained 0.1 mm (their threshold for a rain event) vs. 20 mm over a short period. Particularly without the context of vegetation structure or soil properties. Additionally, I initially assumed the authors would discuss changing precipitation patterns with climate change as rationale for using time since rain – are there sensitivities to drought or more frequent precipitation? This is not discussed. Is there any further information on soil properties?
- Is it reasonable to use time since rain = 0 during the snow-covered season?
- In my experience, evaporative fluxes measured with eddy covariance in the snow-covered and shoulder seasons are challenging in terms of poor data quality. It would be useful for the reader to have an idea of how much gap-filling was done per month of measurement once the QAQC on the data was performed.
- In the site descriptions, you mention specific vegetation for some types and not others.
- The energy balance closure is presented in the appendix but more thorough explanation of the differences between the snow-free and full year energy balance closures could improve the paper. It is discussed in relation to the surface conductance, but I feel it could use further explanation.
Results:
- Line 279: Is there a physical reason why there are higher relative errors at high values of soil water content at Finse and Hisaasen?
- Line 320-324: The year with the lowest annual evaporation corresponded to the longest lasting snow cover – Can you discuss whether this was due to similar ET rates that just lasted longer/had more days to evaporate? Or perhaps the year with the earlier snowmelt had warmer/sunnier conditions or higher soil moisture? Higher VPD? Was the total ET higher simply because it was a longer season or were there other factors? Later in the discussion, the authors seem to indicate this could be due to lower albedo with no snow cover, however I think some of the above factors could also be investigated in a dataset like this.
Discussion:
- Line 346-359: You discuss the role of vegetation here and in the following paragraph, but I think this could be expanded a bit more to explain other results (i.e. perhaps why Finse has higher sensitivity of Bowen Ratio to mid-range soil moistures? Fig C9. Is this vegetation influence?). The general messaging in the paragraph ending in Line 380 is that the sites are well-watered and vegetation plays a very minor role. This may be true, but I think more information on the vegetation structure of the sites (even photos) may help.
- Line 389: You found sites with lower interannual variation in snow-cover duration also have lower interannual variation in evaporation, but can you comment on the amount of accumulated snow in these years? How is this related to the amount of snow? Of course, increased snow depth will also likely result in later snow cover, but is it simply the presence/absence of snow, or the amount of added moisture to the system? Likely just the presence, as these systems are not moisture-limited, but perhaps worth including some snow data if possible.
- Line 401-421 discussing the magnitude of ET and influence of temperature: This is a lengthy paragraph to explain that warmer temperatures and humidity influence ET. I am not sure there is much added value here. I recommend that the discussion focus more on differences between the studied sites and physical processes that may be causing these, rather than explaining these general relationships that are already well understood.
- Line 422: Be careful with stating the “local water balance” as this is simply two components of the water balance.
- The last paragraph of section 4.2 does not read as a discussion.
- Line 446: References here too (i.e. Helbig et al., 2020, etc)
- Section 4.3: This section reads more like an introduction than a discussion. The first paragraph states what we already predict – a longer snow-free season will increase evaporation. These results are not adequately discussed here. The second paragraph discusses VPD as a constraining factor for evaporation, which is a point related to some interesting results you present for each site. However, you do not really discuss your sites or results specifically at all. It ends with discussion of vascular plants limiting transpiration, which you mention above likely is not a large control at your sites.
- Why was VPD the only significant control at Adventdalen in the snow-covered season? (Line 253?) It seems to be behaving differently and any physical justification for this is not adequately discussed.
Additional specific comments:
- Line 20: Do you have any references for evaporation being a minor component of the annual water balance?
- Line 24-27: Reference Penman, etc? There are older references here that summarized these processes before the references you listed.
- Line 27: Could use VPD instead of writing it out each time
- Line 67: boreal is not a Koppen classification – This needs to be fixed.
- Line 74: It is redundant to mention Nature in Norway again here, as it is in the first paragraph.
- It would be useful to know the percent rain vs. snow for each of these sites in the site descriptions.
- Figure 2 and 6: I appreciate the consistency of color throughout the figures for each site. However, for Figures 2 and 6, since the purpose is comparing years within each plot rather than sites, I would recommend each year in a different color (and line style) to help distinguish the years. This is not critical but would improve readability.
- Line 167: “according to its usual definition” – Please explain this or at least put a reference to how this is calculated.
- Line 199: Why was a KMO value of 0.5 used - Reference?
- Line 217: What are the various vegetation heights?
- Figure C7: Hourly evaporation vs. cumulative growing degree day – is this a needed figure? Not sure what value it adds and it is not adequately discussed.
Technical Corrections:
- Line 69: It is referenced several times, but can you put a year for the NGU reference?
- Line 73: Why mention the two towers if only one is used? I thought maybe the other tower was perhaps used for meteorological data gap-filling later, but that does not seem to be the case?
- Some Figures (i.e. C6) have units in mm/h and others have mm h-1
- Figure C10 Caption – need space on y-axis label for mm h-1
- In Figure C1 and C2 captions, the x and y axis are flipped. “The energy balance is estimated from the slope of the linear regression of the sum of latent and sensible heat on the x-axis against available energy (difference between net radiation and soil heat flux) on the y-axis”
- Line 304 onwards: I would use the term “vertical water balance” rather than local water balance, as you have no other information on the watershed runoff, etc.
- Line 352: Is a comma needed after deficit?
- Line 411: Avoid contractions
- Line 439: Recommend formatting dates within the text differently
Citation: https://doi.org/10.5194/egusphere-2025-1140-RC1
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