Model-based analysis of solute transport and potential carbon mineralization in a permafrost catchment under seasonal variability and climate change

Hamm, Alexandra; Schytt Mannerfelt, Erik; Mohammed, Aaron A.; Painter, Scott L.; Coon, Ethan T.; Frampton, Andrew

doi:https://doi.org/10.5194/egusphere-2024-1606

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https://doi.org/10.5194/egusphere-2024-1606

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11 Jun 2024

| 11 Jun 2024

Model-based analysis of solute transport and potential carbon mineralization in a permafrost catchment under seasonal variability and climate change

Alexandra Hamm, Erik Schytt Mannerfelt, Aaron A. Mohammed, Scott L. Painter, Ethan T. Coon, and Andrew Frampton

Abstract. Permafrost carbon, stored in frozen organic matter across vast Arctic and sub-Arctic regions, represents a substantial and increasingly vulnerable carbon reservoir. As global temperatures rise, the accelerated thawing of permafrost releases greenhouse gases, exacerbating climate change. However, freshly thawed permafrost carbon may also experience lateral transport by groundwater flow to surface water recipients such as rivers and lakes, increasing the terrestrial-to-aquatic transfer of permafrost carbon. The mobilization and subsurface transport mechanisms are poorly understood and not accounted for in global climate models, leading to high uncertainties in the predictions of the permafrost carbon feedback. Here, we analyze of solute transport in the form of a non-reactive tracer representing dissolved organic carbon (DOC) using a physics-based numerical model with the objective to study governing cryotic and hydrodynamic transport mechanisms relevant for warming permafrost regions. We first analyze transport times for DOC pools at different locations within the active layer under present-day climatic conditions and proceed to study susceptibility for deeper ancient carbon release in the upper permafrost due to thaw under different warming scenarios. Results suggest that DOC in the active layer near the permafrost table experiences rapid lateral transport upon thaw due to saturated conditions and lateral flow, while DOC close to the ground surface experiences slower transport due flow in unsaturated soil. Deeper permafrost carbon release exhibits vastly different transport behaviors depending on warming and thaw rate. Gradual warming leads to small fractions of DOC being mobilized every year, while the majority moves vertically through percolation and cryosuction. Abrupt thaw resulting from a single very warm year leads to faster lateral transport times, similar to active layer DOC released in saturated conditions. Lastly, we analyze the potential susceptibility of DOC to mineralization to CO₂ prior to export due to soil moisture and temperature conditions. We find that high liquid saturation during transport coincides with very low mineralization rates and potentially inhibits mineralization into greenhouse gases before export. Overall, the results highlight the importance of subsurface hydrologic and thermal conditions on the retention and lateral export of permafrost carbon by subsurface flow.

Received: 29 May 2024 – Discussion started: 11 Jun 2024

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11 Sep 2025

Model-based analysis of solute transport and potential carbon mineralization in the active layer of a hillslope underlain by permafrost with seasonal variability and climate change

Alexandra Hamm, Erik Schytt Mannerfelt, Aaron A. Mohammed, Scott L. Painter, Ethan T. Coon, and Andrew Frampton

The Cryosphere, 19, 3693–3724, https://doi.org/10.5194/tc-19-3693-2025,https://doi.org/10.5194/tc-19-3693-2025, 2025

Short summary

Alexandra Hamm, Erik Schytt Mannerfelt, Aaron A. Mohammed, Scott L. Painter, Ethan T. Coon, and Andrew Frampton

Interactive discussion

Status: closed

RC1:
'Comment on egusphere-2024-1606', Anonymous Referee #1, 03 Jul 2024

GENERAL COMMENTS:
The manuscript deals with an important and debated problem, the permafrost carbon feedback. More specifically, it uses a numerical simulation approach for assessing solute transport in a permafrost catchment, in order to study the impacts of climate change on DOC export from the soil of the considered watershed. The mechanistic modelling of dissolved species in permaftrost affected areas is at the forefront of the current research in cryohydrogeology, and the used tool is a state of the art one. Thus this work is both novel and of great interest for cryospheric sciences.
Meanwhile, critical weaknesses affect its reliability. First of all, the study focuses on a specific site, while the claimed aim of the work is to draw general conclusions about lateral transfers in permafrost regions. This is contradictory, and if the authors want to produce a general study then the problems of upscaling and transposing to different biogeoclimatic permafrost contexts must be carefully dealt with. I have also a number of technical concerns about the set up of the performed numerical simulations, for instance the poorly justified pseudo-3D approach, the absence of any convergence study, and some problems in the definition of boundary conditions. Finally the discussion of the obtained simulation results is rather weak and should be strengthened. All these points are detailed in the specific comments below.
Overall I think that the manuscript cannot be published in its present form. A thorough effort is needed for making it more solid and meaningful, including possibly new computations and results depending on the answers to some of my concerns (e.g.: numerical convergence, relevance of top and bottom boundary conditions). So I recommend that a major revision of this work should be undertaken prior to reconsider whether or not it should be published in The Cryosphere.

SPECIFIC COMMENTS:
- Title and l 8-9: The title refers to ‘a permafrost catchment’, while abstract draws conclusions for ‘permafrost regions’. There is no a priori reason that conclusions made from the study of a peculiar catchment should be relevant for permafrost regions as a whole.
- l 17: Why considering mineralization of organic matter to CO₂ only, and not also to CH₄? May be due to the peculiarity of the study site. This should be explained.
- l 27-41: Other questions than PCF maybe related to lateral transfer of dissolved chemical species from the active layer under climate change (e.g.: impacts on river / ocean ecosystems).
- l 71-72: The assumption that DOC may be considered as a non reactive tracer should be discussed here, with relevant references.
- l 80-82: Most likely this behavior will be site dependent! For instance not the same for a tundra hill slope with low evapotranspiration or a boreal forest hill slope with high evapotranspiration, depending on the slope itself, etc.
- l 111: Is Qs sink/source term accounting for ionic exclusion? To say, does the used model take into account the specificity of solute transport with freeze/thaw?
- l 113: Why neglecting dispersivity? The errors associated with this simplifying assumption should be discussed.
- l 118-119, « path of highest flow accumulation »: What about lateral transfers from the slopes to the thalweg? For studying the drainage of active layer waters and the associated solute fluxes the choice of focusing on the thalweg line does not seems obvious to me. This should be discussed.
- l 121, « pseudo-3D approach »: I cannot understand the usefulness of varying y-axis width of cells along the x direction. According to Fig.2, there is only one mesh along y-axis at every x position. So this seems to me a 2D mesh (as said in l 169). The Figure 3 and the associated explanations are hard to follow and should be thoroughly improved (e.g.: a large blue area appear on Fig.2, but only green and red area are mentioned in the text.) Whatever the pseudo-3D approach really is, the statement made by the authors that it « allows us to account for thermal and hydrological balances across the entire catchment area without the need for a complex and computationally intensive full 3D mesh » should be justified. How does the y-axis width varies along the thalweg? Has any comparative study between results of a full 3D approach and results of this pseudo-3D approach been done? It should have been, prior to use the pseudo 3D approach, or at least if it is not practicable due to computation time the approximation should be discussed, as well as the associated errors.
- l 126-127, « main area of interest »: Why focusing on such a tiny area of 20 m length, while simulations are done on a 1040 m large domain? This important choice should justified and discussed.
- l 129-130, « By directing precipitation from the upper slope to the lower areas, we ensure realistic hydrological conditions with flow accumulation towards the valley bottom. »: What is meant here? The physical equations solved by the numerical model do ensure that gravity exerts a vertical descendant driving force on water, so that water flows from top to bottom when gravity dominates. This sentence seems pointless, I recommend to delete it.
- l 130-131,« This division of the mesh allows for accurate modeling of the thermal-hydrological processes in the catchment. » : Such a statement should be justified.
- l 132, « Each column in this mesh area [...] varies in width in the y-direction. »: Why and how? See above the point on the pseudo-3D mesh approach.
- l 132 – 141: Here mesh cells dimensions are described, but not justified. In a modelling study relying on PDE spatio-temporal discretizations (e.g.: finite differences method, finite volume methods, finite elements methods, etc), it is mandatory to assess the truncation errors by dedicated convergence studies, designed for the simulation case under concern. The results of such a convergence study should be given here, by means of a upper bound of the truncation errors for the outputs of interest. Only in this way one can be sure that the variations discussed in the numerical results are not simply due to truncation errors. Please include here the results of such convergence study for the case under consideration.
- l 134-137: I do not understand why a buffer zone is needed. This should be explained.
- l 142-146: All of these sentences look like unjustified statements. Moreover what is exactly stated is not completely clear. For instance what means « preserving the subsurface volume representation of the catchment » and « a natural equilibrium without artificially imposed boundary conditions »? This paragraph should be either deleted or rewritten in a clearer and justified way.
- l 147-148: « The vertical sides of the model are assigned zero-flux boundaries for water » this is questionable, especially for the outlet vertical boundary. Are there any field observation that can be used to justify this choice? This should be discussed.
- l 151, « in line with borehole observation in Svalbard »: This is important, since it is likely the reason why a 40 m thickness as been chosen for the modeling domain. Please add a figure with the mentioned soil temperature profile evolution, as well as a discussion for explaining in which way these data where used for choosing not only the bottom thermal boundary conditions but also the domain thickness.
- l 159-165: I have serious concerns about the chosen methodology for building ‘present day-like’ precipitation. Why not using a day-of-the-year average like for other forcings? This is not justified. Stating that « the resulting rainfall distribution resembles the variability of natural rainfall throughout the year» is not enough for making the arbitrary, artificial precipitation forcing data set relevant. The use of arbitrary precipitation data may impair the interpretability of the simulation results, so either the ‘resemblence’ between the artificial data set and the observation should be quantitatively demonstrated, either the observation data set itself should be used as forcing data.
- l 165-166, « Soil physical properties are defined to resemble highly conductive material. »: This should be justified. Why not moderately conductive material?
- l 169-170, ‘to establish a water table at target depth »: I do not understand. What is the ‘target depth’? Are there observational evidences of a water table ‘at target depth’?
- l 173-174: Why 10 years? Please provide the criterium used for this choice.
- l 176-178: The targets of this study are simulations under climate change, while the use of a thermal boundary conditions of constant temperature equal to present day temperature at the bottom of the domain (l 150-151) is not compatible with the simulation of climate change scenarios. Or at least, it implies the assumption that the temperature at 40 m depth is not impacted by surface temperature variations at the considered time scale of 50 years. This is a major concern for the validity of the produced simulation results. In order to demonstrate that this strong approximation does not impair the discussed results, the time variations of temperature in depth, close to the bottom boundary, must be shown. If they are not negligible, then the simulations should be re-ran with an appropriate bottom boundary condition (e.g.: geothermal heat flux).
- l 184-187: Additional explanations should be added here: why this rate and total amount of injection, why this moment?
- l 204-205, «air temperature (T avg ), which is the only variable that changes over time in the respective scenarios. »: Why? Precipitation does also change along time in IPCC climate change scenario. This choice should be discussed.
- Figure 4: Why the BTCs are multimodal? For instance, 2 modes for TOL carbon in Fig4.b and 2 modes for buried carbon in Fig4.c?
- l 271-272: Is this temporal partitioning between surface and subsurface transport in agreement with field observation? At least has the ponded water before mid-June has been observed in the field? Whenever it is possible numerical results should be discussed at the light of field observations.
- l 279-281: In Figure 4.d the peak of the 13 August seems higher than the one of the 14 June, although the opposite is stated in the text. This bimodality should be better explained.
- Figure 8: Vertical peak on 30th of August, dual peaks in the 1st of September … I think these features are strange. May be due to convergence problems? The convergence study must be done for assessing it.
- l 323-324: « A substantial fraction of the initially injected tracer mass (∼ 40%) moves vertically (both upwards and downwards) within the same mesh column in which it was injected (see Fig. 9a and d and Fig. 10). » What phenomena are responsible for this vertical redistribution ? Diffusion, freeze/thaw cycles related effects? This should be explicated.
- Figure 10.h: Numerical instability ? Should be corrected, or explained if it is not an artifact.
- l 346 : « This observation highlights the importance of mesh resolution in lateral transport simulations. ». I fully agree. A convergence study must be done.
- l 352, « vertical mobilization »: once again the involved mechanism must be explicated.
- l 407-411 : « We partly address this by representing a converging slope model setup, where the cell width in transverse direction varies depending on the distance in longitudinal direction. This way, the surface area of the catchment is preserved, and it is possible to accurately represent water and energy balances as well as infiltration and evaporation rates throughout the catchment. This approach has previously been applied by Gao and Coon (2022). » I do not think that using a one cell-thick discretization in the transversal direction may allow to simulate the effect of the watershed geometry, either convergent or divergent (using the terminology of Gao and Coon 2022). Nor in the present manuscript or in Gao and Coon 2022 are presented arguments for supporting the validity/usefulness of such a ‘pseudo-3D’ meshing methodology. A proper comparative study should be done for this, between results obtained with« pseudo-3D » meshes and with full 3D meshes. Of course with only one cell no lateral fluxes may be computed.The only interest I would see would be to weight the inward fluxes through the top cell face according to the area of the cell, but then why not simply apply a spatial weighting on the incoming fluxes prescribed by the boundary conditions? Including this in the meshing seems to me inappropriate and confusing. Anyway in this case the methodology used for computing the cells widths must be explicated.
- l 465, « Under the simulated environmental and soil physical conditions in this study, »: This precision hold true not only for this point, but for all the listed conclusions. The writing of the manuscript should better reflects the fact that this is a numerical study of transport in a specific site, with no possibility of automatic generalization for permafrost regions as a whole.

TECHNICAL CORRECTIONS :
- l 25: Missing ) at the end of the line.
- l 29, l30, and elsewhere: I would recommend to systematically use ‘organic carbon’ instead of just ‘carbon’ for naming the C part of the organic matter stored in permafrost.
- l 32: Missing s at ‘question’.
- l71-84: Part of this paragraph should be in the Methods section (e.g.: choice of distinguishing four carbon pools and using labelled tracer for identifying them).
- l 151, «bottom horizontal boundary »: According to Figure 3, the bottom boundary is not horizontal.
- l 251-260: These information should be included in the Methods section, along with a figure for quantitative locations of the injection points and of the measurement points within the modelling domain.
- l 258, « explicit », « continuous in space » : Odd vocabulary. A BTC represents the temporal evolution (not exactly continuous, since there is a time discretization) of concentration at a given location in space, while a plume vizualisation represents the spatial distribution of concentration at a given moment.
- Legend of table 2: The use of different concentration thresholds for TOL and buried carbon should be mentioned in the body of the text, in the Methods section.
- l 273: « when all runoff is occurring in the subsurface »: Then it is not run off, but groundwater flow.
- l 285: Do not refer specifically to a Figure in Supplementary material (Fig. S3) in the body of the text. Instead, refer to the supplementary material as a whole.
- l 286 – 287 : « However, given the specific solute transport patterns in this model, » What is meant here ? Unclear.
- l 299, « vertically transported upward »: Oddly said. Strictly speaking, the topographical effect mentioned here does not generate ascendant flow.
- l 302: Do not refer specifically to a Figure in Supplementary material (Fig. S4) in the body of the text.
- Caption of Fig. 6, « Note that the tracer mass is restricted to the uppermost subsurface cell in this snapshot and is difficult to visualize in this illustration » : True, then this figure has to be improved. May be that plotting the two variables separately would be an option? Besides, Fig. 6 is mentioned only very briefly once in the text. Either it should be deleted or more extensively commented.
- l 315, « some tracer is moved upwards »: see comment on l 299.
- l 317: 12 % does not obviously look negligible.
- Figure 7: I can’t see anything regarding the solute mass. This Figure should be thoroughly reworked so that it becomes informative. Besides, what is the « ponded depth » ?
- Figure 9: The annual cycles/peaks should be discussed. « The visual increase in mass above 100 mol in the injection columns in (a) and (d) is not a physical phenomenon but a result of aggregating and rounding errors during post-processing of the model output. » Then the post-processing should be improved.
- l 350-355: If the post-processing method does not allow to conclude, then it should be improved.
- Caption of Figure 11: « The visual increase in mass above 100 mol in the injection column in (b) is not a physical phenomenon but a result of aggregating and rounding errors during post-processing of the model output. » Once again the post-processing method should be improved.
- l 414-420: this should be part of the introduction, not the discussion.
- l 420-425: this should be part of the Methods section, not of the Discussion.
- l 432-447: This should be part of a perspective section, not of the Discussion.
- Section 4 Discussion: Given the parts that should not be included in the discussion (see the three comments above), the Discussion section is rather short (a bit more than one page), and lack of in depth analyses of the produced results. I recommend to strengthen this section, including for instance elements for linking the simulated behavior and the site specificity.
- l 452, « mechanical transport »: Sounds weird. Passive transport would be more relevant.

Citation: https://doi.org/10.5194/egusphere-2024-1606-RC1
- AC1: 'Reply on RC1', Alexandra Hamm, 26 Aug 2024
  
  The comment was uploaded in the form of a supplement: https://egusphere.copernicus.org/preprints/2024/egusphere-2024-1606/egusphere-2024-1606-AC1-supplement.pdf
  
  Citation: https://doi.org/10.5194/egusphere-2024-1606-AC1
RC2:
'Comment on egusphere-2024-1606', Anonymous Referee #2, 15 Jul 2024

Major comments
The manuscript describe a study using a numerical model to investigate the factors that influence mobilization and transport of carbon in soils in permafrost regions. Warming and permafrost thaw are likely to cause the transport of carbon from soils through aquatic systems, so a better understanding of physical mechanisms is crucial to the development of more accurate models and climate simulations.
The study is designed as a highly idealized case for a small area in Svalbard. The grid setup is not standard. The time periods noted in the text are variable and confusing. The short time period of the abrupt that complicates interpretation. This application of a single model in this fashion makes extension of the study findings to broad regions of the Arctic questionable. The results would be more meaningful by applying the model for at least one or two other configurations based on observations of soil texture and organic carbon content, weather data, elevation gradients, which are available at select sites in Europe, Siberia, Canada, and northern Alaska. Doing so would allow the authors to have more confidence that what they are seeing represents a robust response and not an artifact of a unique model setup. The authors should also state the choice of application to the Endalen valley on Svalbard where observations are sparse. Is there a rich history of field study there?
The 0.25 km2 catchment is relatively small. Why not use a traditional uniform horizontal grid and with vertical soil layers? How many grid cells would be needed at resolution of 1 m? Would the computational expense be prohibitive? How does the setup result in “a natural equilibrium without artificially imposed boundary conditions”, and what does that phrase mean? Boundary conditions and zero-flux boundaries in a typical 3-D model setup could be the same as those in this study. In other words, I don’t see the advantages of this current grid mesh, nor the implications for the interpretation of results. While the authors refer to computational expense, more justification should be presented.
The wide variation in time steps mentioned is awkward and makes interpretation of the influence on results difficult. There is a rate for tracer injection of per second and a time step of monthly for the warming experiments. If it were implicitly daily, could mineralization in the future simulations like the present day simulation? The time step of per second is confusing in light of a monthly step in the future simulations.
The term seasonal variability in the title is odd and unnecessary, as seasonal variability in a study like this is essential. Having the term ‘sensitivity study’ in the title would be more meaningful.
Minor comments
Line 6: grammar, “Here, we analyze of solute transport…”
Line 43: Unclear. What is “they” in the statement “With permafrost acting as a largely impermeable layer between the two, they are…”
Figure 3 caption: It appears from the graphic that width (y-direction) decreases with distance in the x-direction. As x increases from 0 to 1000, the width gets smaller. Please clarify.
Line 159 states that the model is run for an average yearly cycle. Then at line 172 there is reference to year-to-year differences. How can year-to-year differences in spinup be determined? Further clarify the spinup time period and protocol.
Line 171: typo “a the”
Line 173: What ten years? Is weather data not for an average yearly cycle (climatology)? Be specific about the ten year period.
Line 186: Clarify how numerically does a tracer become injected at a rate with a time step of a second. What is the model time step? If it is daily, how can something occur every second? Does the mass enter as a total each day? Intrinsic time step of the model is not clear.
Line 235: typo, for for
Figure S3: component mass overlay is difficult to see in the graphic. Also add label for elevation for the y axis to make clearer what is being shown.
Line 18 in supplement: units of %C ¿ 10 are unclear.
Line 337: typo, “have lead to”
Line 357: Do the magnitudes (+25 and +60 cm) of the abrupt active layer deepening have any grounding in reality? That is, do any studies report abrupt thaw that is some meaningful fraction of the amount modeled here? Some perspective on the magnitudes would aid in interpretation of the results.
Line 368: “...is released from both the injection column…” grammar and unclear meaning.
Line 391: At the mention of ancient carbon, it would be helpful to remind the reader that this is frozen carbon. For readers that skim the abstract and discussion, the terms ancient and buried will have unclear meanings. Making clear that ancient means ancient currently frozen carbon would help.

Citation: https://doi.org/10.5194/egusphere-2024-1606-RC2
- AC2: 'Reply on RC2', Alexandra Hamm, 26 Aug 2024
  
  The comment was uploaded in the form of a supplement: https://egusphere.copernicus.org/preprints/2024/egusphere-2024-1606/egusphere-2024-1606-AC2-supplement.pdf
  
  Citation: https://doi.org/10.5194/egusphere-2024-1606-AC2

Interactive discussion

Status: closed

RC1:
'Comment on egusphere-2024-1606', Anonymous Referee #1, 03 Jul 2024

GENERAL COMMENTS:
The manuscript deals with an important and debated problem, the permafrost carbon feedback. More specifically, it uses a numerical simulation approach for assessing solute transport in a permafrost catchment, in order to study the impacts of climate change on DOC export from the soil of the considered watershed. The mechanistic modelling of dissolved species in permaftrost affected areas is at the forefront of the current research in cryohydrogeology, and the used tool is a state of the art one. Thus this work is both novel and of great interest for cryospheric sciences.
Meanwhile, critical weaknesses affect its reliability. First of all, the study focuses on a specific site, while the claimed aim of the work is to draw general conclusions about lateral transfers in permafrost regions. This is contradictory, and if the authors want to produce a general study then the problems of upscaling and transposing to different biogeoclimatic permafrost contexts must be carefully dealt with. I have also a number of technical concerns about the set up of the performed numerical simulations, for instance the poorly justified pseudo-3D approach, the absence of any convergence study, and some problems in the definition of boundary conditions. Finally the discussion of the obtained simulation results is rather weak and should be strengthened. All these points are detailed in the specific comments below.
Overall I think that the manuscript cannot be published in its present form. A thorough effort is needed for making it more solid and meaningful, including possibly new computations and results depending on the answers to some of my concerns (e.g.: numerical convergence, relevance of top and bottom boundary conditions). So I recommend that a major revision of this work should be undertaken prior to reconsider whether or not it should be published in The Cryosphere.

SPECIFIC COMMENTS:
- Title and l 8-9: The title refers to ‘a permafrost catchment’, while abstract draws conclusions for ‘permafrost regions’. There is no a priori reason that conclusions made from the study of a peculiar catchment should be relevant for permafrost regions as a whole.
- l 17: Why considering mineralization of organic matter to CO₂ only, and not also to CH₄? May be due to the peculiarity of the study site. This should be explained.
- l 27-41: Other questions than PCF maybe related to lateral transfer of dissolved chemical species from the active layer under climate change (e.g.: impacts on river / ocean ecosystems).
- l 71-72: The assumption that DOC may be considered as a non reactive tracer should be discussed here, with relevant references.
- l 80-82: Most likely this behavior will be site dependent! For instance not the same for a tundra hill slope with low evapotranspiration or a boreal forest hill slope with high evapotranspiration, depending on the slope itself, etc.
- l 111: Is Qs sink/source term accounting for ionic exclusion? To say, does the used model take into account the specificity of solute transport with freeze/thaw?
- l 113: Why neglecting dispersivity? The errors associated with this simplifying assumption should be discussed.
- l 118-119, « path of highest flow accumulation »: What about lateral transfers from the slopes to the thalweg? For studying the drainage of active layer waters and the associated solute fluxes the choice of focusing on the thalweg line does not seems obvious to me. This should be discussed.
- l 121, « pseudo-3D approach »: I cannot understand the usefulness of varying y-axis width of cells along the x direction. According to Fig.2, there is only one mesh along y-axis at every x position. So this seems to me a 2D mesh (as said in l 169). The Figure 3 and the associated explanations are hard to follow and should be thoroughly improved (e.g.: a large blue area appear on Fig.2, but only green and red area are mentioned in the text.) Whatever the pseudo-3D approach really is, the statement made by the authors that it « allows us to account for thermal and hydrological balances across the entire catchment area without the need for a complex and computationally intensive full 3D mesh » should be justified. How does the y-axis width varies along the thalweg? Has any comparative study between results of a full 3D approach and results of this pseudo-3D approach been done? It should have been, prior to use the pseudo 3D approach, or at least if it is not practicable due to computation time the approximation should be discussed, as well as the associated errors.
- l 126-127, « main area of interest »: Why focusing on such a tiny area of 20 m length, while simulations are done on a 1040 m large domain? This important choice should justified and discussed.
- l 129-130, « By directing precipitation from the upper slope to the lower areas, we ensure realistic hydrological conditions with flow accumulation towards the valley bottom. »: What is meant here? The physical equations solved by the numerical model do ensure that gravity exerts a vertical descendant driving force on water, so that water flows from top to bottom when gravity dominates. This sentence seems pointless, I recommend to delete it.
- l 130-131,« This division of the mesh allows for accurate modeling of the thermal-hydrological processes in the catchment. » : Such a statement should be justified.
- l 132, « Each column in this mesh area [...] varies in width in the y-direction. »: Why and how? See above the point on the pseudo-3D mesh approach.
- l 132 – 141: Here mesh cells dimensions are described, but not justified. In a modelling study relying on PDE spatio-temporal discretizations (e.g.: finite differences method, finite volume methods, finite elements methods, etc), it is mandatory to assess the truncation errors by dedicated convergence studies, designed for the simulation case under concern. The results of such a convergence study should be given here, by means of a upper bound of the truncation errors for the outputs of interest. Only in this way one can be sure that the variations discussed in the numerical results are not simply due to truncation errors. Please include here the results of such convergence study for the case under consideration.
- l 134-137: I do not understand why a buffer zone is needed. This should be explained.
- l 142-146: All of these sentences look like unjustified statements. Moreover what is exactly stated is not completely clear. For instance what means « preserving the subsurface volume representation of the catchment » and « a natural equilibrium without artificially imposed boundary conditions »? This paragraph should be either deleted or rewritten in a clearer and justified way.
- l 147-148: « The vertical sides of the model are assigned zero-flux boundaries for water » this is questionable, especially for the outlet vertical boundary. Are there any field observation that can be used to justify this choice? This should be discussed.
- l 151, « in line with borehole observation in Svalbard »: This is important, since it is likely the reason why a 40 m thickness as been chosen for the modeling domain. Please add a figure with the mentioned soil temperature profile evolution, as well as a discussion for explaining in which way these data where used for choosing not only the bottom thermal boundary conditions but also the domain thickness.
- l 159-165: I have serious concerns about the chosen methodology for building ‘present day-like’ precipitation. Why not using a day-of-the-year average like for other forcings? This is not justified. Stating that « the resulting rainfall distribution resembles the variability of natural rainfall throughout the year» is not enough for making the arbitrary, artificial precipitation forcing data set relevant. The use of arbitrary precipitation data may impair the interpretability of the simulation results, so either the ‘resemblence’ between the artificial data set and the observation should be quantitatively demonstrated, either the observation data set itself should be used as forcing data.
- l 165-166, « Soil physical properties are defined to resemble highly conductive material. »: This should be justified. Why not moderately conductive material?
- l 169-170, ‘to establish a water table at target depth »: I do not understand. What is the ‘target depth’? Are there observational evidences of a water table ‘at target depth’?
- l 173-174: Why 10 years? Please provide the criterium used for this choice.
- l 176-178: The targets of this study are simulations under climate change, while the use of a thermal boundary conditions of constant temperature equal to present day temperature at the bottom of the domain (l 150-151) is not compatible with the simulation of climate change scenarios. Or at least, it implies the assumption that the temperature at 40 m depth is not impacted by surface temperature variations at the considered time scale of 50 years. This is a major concern for the validity of the produced simulation results. In order to demonstrate that this strong approximation does not impair the discussed results, the time variations of temperature in depth, close to the bottom boundary, must be shown. If they are not negligible, then the simulations should be re-ran with an appropriate bottom boundary condition (e.g.: geothermal heat flux).
- l 184-187: Additional explanations should be added here: why this rate and total amount of injection, why this moment?
- l 204-205, «air temperature (T avg ), which is the only variable that changes over time in the respective scenarios. »: Why? Precipitation does also change along time in IPCC climate change scenario. This choice should be discussed.
- Figure 4: Why the BTCs are multimodal? For instance, 2 modes for TOL carbon in Fig4.b and 2 modes for buried carbon in Fig4.c?
- l 271-272: Is this temporal partitioning between surface and subsurface transport in agreement with field observation? At least has the ponded water before mid-June has been observed in the field? Whenever it is possible numerical results should be discussed at the light of field observations.
- l 279-281: In Figure 4.d the peak of the 13 August seems higher than the one of the 14 June, although the opposite is stated in the text. This bimodality should be better explained.
- Figure 8: Vertical peak on 30th of August, dual peaks in the 1st of September … I think these features are strange. May be due to convergence problems? The convergence study must be done for assessing it.
- l 323-324: « A substantial fraction of the initially injected tracer mass (∼ 40%) moves vertically (both upwards and downwards) within the same mesh column in which it was injected (see Fig. 9a and d and Fig. 10). » What phenomena are responsible for this vertical redistribution ? Diffusion, freeze/thaw cycles related effects? This should be explicated.
- Figure 10.h: Numerical instability ? Should be corrected, or explained if it is not an artifact.
- l 346 : « This observation highlights the importance of mesh resolution in lateral transport simulations. ». I fully agree. A convergence study must be done.
- l 352, « vertical mobilization »: once again the involved mechanism must be explicated.
- l 407-411 : « We partly address this by representing a converging slope model setup, where the cell width in transverse direction varies depending on the distance in longitudinal direction. This way, the surface area of the catchment is preserved, and it is possible to accurately represent water and energy balances as well as infiltration and evaporation rates throughout the catchment. This approach has previously been applied by Gao and Coon (2022). » I do not think that using a one cell-thick discretization in the transversal direction may allow to simulate the effect of the watershed geometry, either convergent or divergent (using the terminology of Gao and Coon 2022). Nor in the present manuscript or in Gao and Coon 2022 are presented arguments for supporting the validity/usefulness of such a ‘pseudo-3D’ meshing methodology. A proper comparative study should be done for this, between results obtained with« pseudo-3D » meshes and with full 3D meshes. Of course with only one cell no lateral fluxes may be computed.The only interest I would see would be to weight the inward fluxes through the top cell face according to the area of the cell, but then why not simply apply a spatial weighting on the incoming fluxes prescribed by the boundary conditions? Including this in the meshing seems to me inappropriate and confusing. Anyway in this case the methodology used for computing the cells widths must be explicated.
- l 465, « Under the simulated environmental and soil physical conditions in this study, »: This precision hold true not only for this point, but for all the listed conclusions. The writing of the manuscript should better reflects the fact that this is a numerical study of transport in a specific site, with no possibility of automatic generalization for permafrost regions as a whole.

TECHNICAL CORRECTIONS :
- l 25: Missing ) at the end of the line.
- l 29, l30, and elsewhere: I would recommend to systematically use ‘organic carbon’ instead of just ‘carbon’ for naming the C part of the organic matter stored in permafrost.
- l 32: Missing s at ‘question’.
- l71-84: Part of this paragraph should be in the Methods section (e.g.: choice of distinguishing four carbon pools and using labelled tracer for identifying them).
- l 151, «bottom horizontal boundary »: According to Figure 3, the bottom boundary is not horizontal.
- l 251-260: These information should be included in the Methods section, along with a figure for quantitative locations of the injection points and of the measurement points within the modelling domain.
- l 258, « explicit », « continuous in space » : Odd vocabulary. A BTC represents the temporal evolution (not exactly continuous, since there is a time discretization) of concentration at a given location in space, while a plume vizualisation represents the spatial distribution of concentration at a given moment.
- Legend of table 2: The use of different concentration thresholds for TOL and buried carbon should be mentioned in the body of the text, in the Methods section.
- l 273: « when all runoff is occurring in the subsurface »: Then it is not run off, but groundwater flow.
- l 285: Do not refer specifically to a Figure in Supplementary material (Fig. S3) in the body of the text. Instead, refer to the supplementary material as a whole.
- l 286 – 287 : « However, given the specific solute transport patterns in this model, » What is meant here ? Unclear.
- l 299, « vertically transported upward »: Oddly said. Strictly speaking, the topographical effect mentioned here does not generate ascendant flow.
- l 302: Do not refer specifically to a Figure in Supplementary material (Fig. S4) in the body of the text.
- Caption of Fig. 6, « Note that the tracer mass is restricted to the uppermost subsurface cell in this snapshot and is difficult to visualize in this illustration » : True, then this figure has to be improved. May be that plotting the two variables separately would be an option? Besides, Fig. 6 is mentioned only very briefly once in the text. Either it should be deleted or more extensively commented.
- l 315, « some tracer is moved upwards »: see comment on l 299.
- l 317: 12 % does not obviously look negligible.
- Figure 7: I can’t see anything regarding the solute mass. This Figure should be thoroughly reworked so that it becomes informative. Besides, what is the « ponded depth » ?
- Figure 9: The annual cycles/peaks should be discussed. « The visual increase in mass above 100 mol in the injection columns in (a) and (d) is not a physical phenomenon but a result of aggregating and rounding errors during post-processing of the model output. » Then the post-processing should be improved.
- l 350-355: If the post-processing method does not allow to conclude, then it should be improved.
- Caption of Figure 11: « The visual increase in mass above 100 mol in the injection column in (b) is not a physical phenomenon but a result of aggregating and rounding errors during post-processing of the model output. » Once again the post-processing method should be improved.
- l 414-420: this should be part of the introduction, not the discussion.
- l 420-425: this should be part of the Methods section, not of the Discussion.
- l 432-447: This should be part of a perspective section, not of the Discussion.
- Section 4 Discussion: Given the parts that should not be included in the discussion (see the three comments above), the Discussion section is rather short (a bit more than one page), and lack of in depth analyses of the produced results. I recommend to strengthen this section, including for instance elements for linking the simulated behavior and the site specificity.
- l 452, « mechanical transport »: Sounds weird. Passive transport would be more relevant.

Citation: https://doi.org/10.5194/egusphere-2024-1606-RC1
- AC1: 'Reply on RC1', Alexandra Hamm, 26 Aug 2024
  
  The comment was uploaded in the form of a supplement: https://egusphere.copernicus.org/preprints/2024/egusphere-2024-1606/egusphere-2024-1606-AC1-supplement.pdf
  
  Citation: https://doi.org/10.5194/egusphere-2024-1606-AC1
RC2:
'Comment on egusphere-2024-1606', Anonymous Referee #2, 15 Jul 2024

Major comments
The manuscript describe a study using a numerical model to investigate the factors that influence mobilization and transport of carbon in soils in permafrost regions. Warming and permafrost thaw are likely to cause the transport of carbon from soils through aquatic systems, so a better understanding of physical mechanisms is crucial to the development of more accurate models and climate simulations.
The study is designed as a highly idealized case for a small area in Svalbard. The grid setup is not standard. The time periods noted in the text are variable and confusing. The short time period of the abrupt that complicates interpretation. This application of a single model in this fashion makes extension of the study findings to broad regions of the Arctic questionable. The results would be more meaningful by applying the model for at least one or two other configurations based on observations of soil texture and organic carbon content, weather data, elevation gradients, which are available at select sites in Europe, Siberia, Canada, and northern Alaska. Doing so would allow the authors to have more confidence that what they are seeing represents a robust response and not an artifact of a unique model setup. The authors should also state the choice of application to the Endalen valley on Svalbard where observations are sparse. Is there a rich history of field study there?
The 0.25 km2 catchment is relatively small. Why not use a traditional uniform horizontal grid and with vertical soil layers? How many grid cells would be needed at resolution of 1 m? Would the computational expense be prohibitive? How does the setup result in “a natural equilibrium without artificially imposed boundary conditions”, and what does that phrase mean? Boundary conditions and zero-flux boundaries in a typical 3-D model setup could be the same as those in this study. In other words, I don’t see the advantages of this current grid mesh, nor the implications for the interpretation of results. While the authors refer to computational expense, more justification should be presented.
The wide variation in time steps mentioned is awkward and makes interpretation of the influence on results difficult. There is a rate for tracer injection of per second and a time step of monthly for the warming experiments. If it were implicitly daily, could mineralization in the future simulations like the present day simulation? The time step of per second is confusing in light of a monthly step in the future simulations.
The term seasonal variability in the title is odd and unnecessary, as seasonal variability in a study like this is essential. Having the term ‘sensitivity study’ in the title would be more meaningful.
Minor comments
Line 6: grammar, “Here, we analyze of solute transport…”
Line 43: Unclear. What is “they” in the statement “With permafrost acting as a largely impermeable layer between the two, they are…”
Figure 3 caption: It appears from the graphic that width (y-direction) decreases with distance in the x-direction. As x increases from 0 to 1000, the width gets smaller. Please clarify.
Line 159 states that the model is run for an average yearly cycle. Then at line 172 there is reference to year-to-year differences. How can year-to-year differences in spinup be determined? Further clarify the spinup time period and protocol.
Line 171: typo “a the”
Line 173: What ten years? Is weather data not for an average yearly cycle (climatology)? Be specific about the ten year period.
Line 186: Clarify how numerically does a tracer become injected at a rate with a time step of a second. What is the model time step? If it is daily, how can something occur every second? Does the mass enter as a total each day? Intrinsic time step of the model is not clear.
Line 235: typo, for for
Figure S3: component mass overlay is difficult to see in the graphic. Also add label for elevation for the y axis to make clearer what is being shown.
Line 18 in supplement: units of %C ¿ 10 are unclear.
Line 337: typo, “have lead to”
Line 357: Do the magnitudes (+25 and +60 cm) of the abrupt active layer deepening have any grounding in reality? That is, do any studies report abrupt thaw that is some meaningful fraction of the amount modeled here? Some perspective on the magnitudes would aid in interpretation of the results.
Line 368: “...is released from both the injection column…” grammar and unclear meaning.
Line 391: At the mention of ancient carbon, it would be helpful to remind the reader that this is frozen carbon. For readers that skim the abstract and discussion, the terms ancient and buried will have unclear meanings. Making clear that ancient means ancient currently frozen carbon would help.

Citation: https://doi.org/10.5194/egusphere-2024-1606-RC2
- AC2: 'Reply on RC2', Alexandra Hamm, 26 Aug 2024
  
  The comment was uploaded in the form of a supplement: https://egusphere.copernicus.org/preprints/2024/egusphere-2024-1606/egusphere-2024-1606-AC2-supplement.pdf
  
  Citation: https://doi.org/10.5194/egusphere-2024-1606-AC2

Peer review completion

AR: Author's response | RR: Referee report | ED: Editor decision | EF: Editorial file upload

ED: Reconsider after major revisions (further review by editor and referees) (09 Sep 2024) by Philipp de Vrese

AR by Alexandra Hamm on behalf of the Authors (29 Nov 2024) Author's response Author's tracked changes Manuscript

ED: Referee Nomination & Report Request started (09 Dec 2024) by Philipp de Vrese

RR by Anonymous Referee #1 (09 Jan 2025)

Suggestions for revision or reasons for rejection

The authors have significantly enhanced their manuscript, providing additional information, reorganizing the paper a bit and clarifying the aims and limits of their work. The newly included Appendixes are of high interest.

Meanwhile, some of my methodological concerns are not fully alleviated. First, the novelty of the proposed pseudo-3D approach for the simulation of coupled water, heat and solute transfer in a real watershed is still not acknowledged, and consequently the need of testing it is still overlooked. Second, the proposed convergence study is only partial, and especially it does not include the main area of interest. Additional numerical experiments must be undertaken for consolidating the methodological aspect of this work.

So I recommend a supplementary revision step for this manuscript.

COMMENTS :

L 515-523 (problem with lines numbering, I take the manuscript version with apparent modifications as reference): “similar systems are widespread across the Arctic” Please quantify the % of surface coverage of ‘high Arctic hillslope systems underlain by continuous permafrost’ over the total permafrost area, over the continuous permafrost area and over the Arctic region. Please also include the ecotype (bare soil, tundra …?) in the site characteristics.

L 76: In my opinion it should be at least stated here that considering non-reactive tracer is a simplifying assumption.

L 126, Qs in Eq. (1) : OK, then to which physical process is associated Qs? Is Qs set to 0 in your simulations? The reader should be able to understand this after this paragraph.

L 188-190: “This strategy, together with variable-width elements that preserve flow convergence, provides an accurate representation of catchment-scale processes without the computational burden of a full 3D model (Appendix A and D).” Please find hereafter my comments on Appendix A and D. Although things are a bit clearer now, I think that there is still some work to be done. Stated here that the representation of catchment-scale processes is accurate still look like an unjustified statement, see my comments on lines 602-606.

L 592: Why 1040 m?

L 600 : “dominant topographic controls on flow (Dunne and Black, 1970; Anderson and Burt, 1978) “ Why citing experimental studies made in peculiar hillslopes in non-permafrost areas, as relevant for characterising the flow in this peculiar permafrost watershed? Obviously topography is important for flow in continental surfaces, but why using these references here for justifying this is not clear to me.

L 602-606 :
Fan and Bras 1998 proposed 1D analytical solutions for flow in and over a hillside with a Darcy-type equation for either divergent, convergent or uniform hillslopes. Then, for applying these analytical solutions at the catchment scale, they suggested to divide the complex topographical surface of the catchment in elementary hillslopes of one of these three basic types. Troch et al. 2003 then extended this approach to nine basic plan shape / Curvature profile types hillslopes, and by using a numerical resolution of Boussinesq equation for flow on the obtained 1D domains. Paniconi et al., 2003, made a comparative study of the results of this approach with the one obtained with fully 3D Richards equation-based flow simulation, once again only for basic hillslope types. Finally Hazenberg et al., 2015 apply this approach for LSM simulations.
On the other hand If I understand correctly what you do is to approximate the whole catchment as a kind of convergent hillslope with the thalweg topographical line as longitudinal profile, then to approximate this 3D convergent hillslope as a 2D transect with variable width of the unique y-axis cell, and then to apply a numerical resolution of variably saturated flow and transport with freeze/thaw to the obtained pseudo-3D representation of the watershed. This would be significantly different from the developments cited above, although directly inspired by them. I would recommend this new approach to be explained and tested. If I correctly understood, in Gao and Coon 2022, a basic geometry in the style of Troch et al. 2003 was dealt with, without studying whether or not such a simplified geometry allows to catch the dynamics in real, complex watersheds, but rather for giving a theoretical test case for evaluating relative importance of various processes in the numerical simulation of permafrost dynamics. And no comparison with full 3D results was proposed in this later paper.
So I think that this new approach is potentially promising, but should be carefully assessed, from at least two points of view :
- Is the simplified approach developed in Fan and Bras 1998, Troch et al., 2003, Paniconi 2003 and Hazenberg et al., 2003 for Boussinesq equation applicable to the modelling of permafrost dynamics + solute transport, i.e. for 3D simulations of coupled flow, heat transfer and solute transfer?
- Is this simplified approach applicable to the considered watershed, given its complex morphological structure ? I Guess that this approach could not always be successfully applied, for peculiar watershed topologies for instance. Furthermore, I wonder whether or not the proposed methodology for building the y-axis width along slope does conserve the properties of the hypsometric curve of the watershed? I think this point could be important for properly handle the altitudinal distribution of contributing areas, and thus for the time of concentration.
Such an assessment would likely request large computational means, which should not be a problem given the High Performance Computing capabilities of ATS.

L 630-643: Hard to follow. A schematic figure presenting which slope is dealt with etc would probably be helpful.

L 646-647: “At the top boundary (surface), a surface energy balance, which serves as a source and sink for water and energy in the subsurface, is derived from site-specific weather data.” How is this SEB derived? This should be explained.

L 659-661: “By using a frequency-density function, the resulting distribution more closely resembles natural rainfall variability, ensuring a realistic representation of precipitation events. This precipitation model plays a critical role in shaping the hydrological and thermal balance within the soil.” Interesting. If I understand correctly the used precipitation distribution is a realization of a random process with prescribed statistical moments? Then I think that it would be interesting to test the variability of the output results of this study when using different precipitation distributions obtained by different realization of this random process.

L 662-665: Table 1 should be inserted here, or this paragraph moved in the body of the text as a comment of this table. This should be also more precise : why these peculiar values has been chosen ? Saying ‘to resemble highly water-conductive material’ is not specific enough. You did not invent these value I guess, the abaqus from where they were extracted or whatever should be mentioned.

L 703-709: Figure D3 should be cited here.

L 716: Figure D4 is not displayed in the Appendix.

L 716-728: The convergence study, showing dependency of the results on mesh refinement, is done only for the upper part of the domain. It should be done for the whole domain, especially in the Main Area of Interest. Another comment related to the answer of the authors regarding my comments 32 : “energy- and mass conservation apply to these cells in the same way that it does in the contemporary active layer cells. The mesh resolution is therefore adequate.” Please do not forget that energy and mass conservation is necessary but not sufficient for obtaining converged numerical results ; truncation errors must also be assessed.

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ED: Reconsider after major revisions (further review by editor and referees) (23 Jan 2025) by Philipp de Vrese

AR by Andrew Frampton on behalf of the Authors (05 Mar 2025)

EF by Vitaly Muravyev (14 Apr 2025) Manuscript Author's response Author's tracked changes

ED: Referee Nomination & Report Request started (23 Apr 2025) by Philipp de Vrese

RR by Anonymous Referee #1 (02 May 2025)

Suggestions for revision or reasons for rejection

The additional work done by the authors enables to better defined the scope and the limits of their study. The authors propose a modelling work somewhere in between the study of an idealized, generic case and the study of a site-specific case, which in my opinion makes difficult to draw the line between the conclusions that are generic and those that are site-specific. Nevertheless that is the explicit aim of the authors, and as such I think that it can be presented to the community. I have still some minor concerns regarding the presentation of the study site and of the simulation methodology that should be dealt with prior to publication, see my specific comments below. But overall I think that now the manuscript is in a sufficiently good shape for publication, so at this stage I recommend acceptance with minor revisions.

Specific comments:

l99-111 (line numbering from the final manuscript version): in this paragraph the ecotype of the site (tundra, bare soil, ... ?) must be given.

l115-116: “a synthetic domain that is broadly representative of hillslopes in continuous permafrost regions.”
I am still not convinced that the chosen hillslope maybe considered as ‘broadly representative’ of continuous permafrost region. This is a N-W slope in (probably) high arctic tundra, most likely it is not representative of S-E slopes, or of boreal forest environments. This sentence could be rephrased as follow : “a synthetic domain that is representative of the site conditions, in a high arctic tundra environment (?) of the continuous permafrost area.”

l 154-156: “The variable-width mesh approach preserves flow convergence, enables a reasonable representation of the surface-energy balance, and allows hydrological processes to be well represented without the expense of a fully three-dimensional model (e.g., Fan and Bras, 1998; Troch et al., 2003; Hazenberg et al., 2015).”
Concerning the ‘reasonable representation of surface-energy balance’ and the ‘well represented hydrological processes’, this sentence presents assumptions as demonstrated facts, at least in permafrost contexts. Moreover, the cited literature does not deal with coupled water and heat transfers with freeze/thaw, nor with solute transport. The sentence could be rephrased in the following way: ”The variable-width mesh approach preserves flow convergence, and enables a reasonable representation of the hydrological processes in non-permafrost contexts without the expense of a fully three-dimensional model (e.g., Fan and Bras, 1998; Troch et al., 2003; Hazenberg et al., 2015). In this study we assume that this modelling approach is also suitable for the simulation of coupled water and heat transfers with freeze/thaw, and to waterborne solute transport in such a permafrost context.”

l 675-676: “Simulated temperature and x-velocity using the fine and superfine meshes are
nearly indistinguishable (Fig. D5 and D6).” Indistinguishable by the human eye on a figure is not a relevant criterium of comparison for a convergence study. Here a quantitative comparison must be done, for instance by giving the L1 and L2 norms of the values of the differences between the temperature fields obtained with the two meshes.

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ED: Publish subject to minor revisions (review by editor) (17 May 2025) by Philipp de Vrese

AR by Andrew Frampton on behalf of the Authors (23 May 2025) Author's response Author's tracked changes Manuscript

ED: Publish as is (11 Jun 2025) by Philipp de Vrese

AR by Andrew Frampton on behalf of the Authors (17 Jun 2025)

Journal article(s) based on this preprint

11 Sep 2025

Model-based analysis of solute transport and potential carbon mineralization in the active layer of a hillslope underlain by permafrost with seasonal variability and climate change

Alexandra Hamm, Erik Schytt Mannerfelt, Aaron A. Mohammed, Scott L. Painter, Ethan T. Coon, and Andrew Frampton

The Cryosphere, 19, 3693–3724, https://doi.org/10.5194/tc-19-3693-2025,https://doi.org/10.5194/tc-19-3693-2025, 2025

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Alexandra Hamm, Erik Schytt Mannerfelt, Aaron A. Mohammed, Scott L. Painter, Ethan T. Coon, and Andrew Frampton

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https://doi.org/10.5194/egusphere-2024-1606-supplement

Alexandra Hamm, Erik Schytt Mannerfelt, Aaron A. Mohammed, Scott L. Painter, Ethan T. Coon, and Andrew Frampton

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The fate of thawing permafrost carbon is essential to our understanding of the permafrost-climate feedback and projections of future climate. Here, we modeled the transport of carbon in the groundwater within the active layer. We find that carbon transport velocities and potential microbial mineralization rates are strongly dependent on liquid saturation in the seasonally thawed active layer. In a warming climate, the rate at which permafrost thaws determines how fast carbon can be transported.


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