the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 17 Dec 2024
High-resolution InSAR Regional Soil Water Storage Mapping Above Permafrost
Abstract. The hydrology of thawing permafrost affects the fate of the vast amount of permafrost carbon due to its controls on waterlogging, redox status, and transport. However, regional mapping of soil water storage in the soil layer that experiences the annual freeze-thaw cycle above permafrost, known as the active layer, remains a formidable challenge over remote arctic regions. This study shows that Interferometric Synthetic Aperture Radar (InSAR) observations can be used to estimate the amount of soil water originating from the active layer seasonal thaw. Our ALOS InSAR results, validated by in situ observations, show that the thickness of the soil water that experiences the annual freeze-thaw cycle ranges from 0 to 75 cm in a 60-by-100-km area near the Toolik Field Station on the North Slope of Alaska. Notably, the spatial distribution of the soil water correlates with surface topography and land vegetation cover types. We found that pixel-mismatching of the topographic map and radar images is the primary error source in the Toolik ALOS InSAR data. The amount of pixel misregistration, the local slope, and the InSAR perpendicular baseline influence the observed errors in InSAR Line-Of-Sight (LOS) distance measurements non-linearly. For most of the study area with a percent slope of less than 5%, the LOS error from pixel misregistration is less than 1 cm, translating to less than 14 cm of error in the soil water estimates.
- Preprint
(40443 KB) - Metadata XML
- BibTeX
- EndNote
Status: open (until 28 Jan 2025)
-
RC1: 'Comment on hess-2024-362', Anonymous Referee #1, 09 Jan 2025
reply
This manuscript presents a methodology for estimating the volumetric storage of soil water within the active layer of periglacial landscapes using InSAR observations. The method from a prior proof of concept publication is re-introduced and applied to a larger area of undisturbed tundra in the vicinity of Toolik Field Station on the Alaskan North Slope using L-band ALOS data acquired between 2006 and 2010. The InSAR-estimated equivalent water depth estimates are compared with in situ estimates, showing strong overall agreement. Lastly, there is a discussion of sources of interferometric error due to DEM errors and SAR image to DEM pixel misregistration.
Overall, I think that this manuscript presents several relevant contributions. However, I would first recommend several revisions before the manuscript can be considered for publication. I have a detailed, point-by-point list of specific comments below, but these comments generally fall under a few broader thematic points. These points are:
- The manuscript would benefit from a more precise description of what the Zwater parameter physically corresponds to. From my reading of prior work, the Zwater parameter is the equivalent water depth of all pore-bound water undergoing seasonal freeze/thaw within the active layer. However, the Zwater parameter is variously referred to in slightly different ways throughout the manuscript, which could raise questions from readers as to what exactly Zwater physically corresponds (i.e., is this the equivalent water depth, or the thickness of the active layer that is fully saturated?) I have a few specific suggestions to address this.
- First, I would recommend including a physical cartoon or schematic that unambiguously illustrates the Zwater parameter. For example, Figure 2 could be modified by including two additional cartoon cross-sections next to the original soil profiles, with the total porebound water separated from the soil matrix into a single homogenous column of pure water, with the depth of this water column labeled Zwater.
- I recommend choosing a standard way to describe Zwater and being consistent throughout the manuscript. At various times Zwater is referred to as ‘saturated active layer soil water storage’, ‘saturated soil water thickness’, ‘soil water equivalent depth’, ‘soil water depth in the saturated active layer’, ‘soil water column that experiences the ice-to-water phase change in the saturated active layer (denoted as zwater)’, ‘active layer water storage’ and ‘saturated active layer soil water column’.
- As currently written, the section of the manuscript on Zwater estimation and the section on DEM errors feel rather disjoint from each other. I think that the manuscript would benefit from a tighter coupling of these two elements, as well as a more thorough and explicit discussion of the novel advancements made in this manuscript on top of the Chen et al. 2020 proof of concept manuscript.
- The manuscript would benefit from a more explicit framing of the novel contributions that this manuscript introduces to the field over the prior Chen et al. 2020 study. The discrete dem error due to pixel misregistration analysis is one such novel contribution. However, I would also highlight any advancements made in the Zwater estimation process introduced.
- Currently, the Zwater and DEM error sections read as pretty disjoint from each other; these sections could potentially be submitted separately as two stand-alone papers; or, perhaps more discussion linking them could be included in the revised manuscript. For example, to what degree to observed DEM errors propagate into an effective uncertainty for Zwater estimates? Bridging these two sections together through an uncertainty propagation analysis would be one such way to strengthen the cohesiveness of the manuscript.
Individual comments are organized by section below:
Introduction:
Page 1, line 22: “Whether the carbon held by the active layer soils will be transformed to carbon dioxide or methane (a more powerful greenhouse gas), or whether it will flow towards rivers and lakes as dissolved carbon in groundwater, depends largely
on the wetness or dryness of the active layer (i.e., how much water is stored).” I would recommend including a citation or reference to a few relevant papers that support this statement, as it is a central point that underlines much of the scientific justification for this manuscript.
Page 2 line 37: “Because ice density is less than water density (and thus ice volume is greater than water volume), the land surface subsides as the active layer thaws from winter to summer (Liu et al., 2010)” I would recommend being explicit here and stating that the amount of surface subsidence depends upon the overall volumetric water content of the thawing permafrost, as this further motivates the proposed methodology. One second thought, this may not be necessary to state here, as you later state it on line 44.
Methods:
An important point to raise is that, in addition to assuming stationary thaw conditions from year to year, the interannual stacking method also implicitly assumes no variations in excess ground ice content from year to year. While I think that this is a justifiable assumption, it might be worth explicitly mentioning this, and discussing recent work that has demonstrated that InSAR is sensitive to interannual variations in excess ground ice formation and melting: https://doi.org/10.5194/tc-15-2041-2021 ,
https://doi.org/10.1029/2023WR035331
Section 2.3:
-What are typical pixel misregistration values for ALOS? They are surely processor dependent, and the InSAR processor used is not explicitly mentioned. However, I imagine they are still relatively small, no more than 1 or 2 pixels in any direction, and usually sub-pixel.
Page 10 line 252: This is a minor point, but the comparison between the InSAR pixel (10-100 m) and field measurement (30x30 cm^2 area plot) is a ‘linear to area’ comparison.
Page 11 line 260: I suggest removing, or restating ‘waviness of the ice-table’, as it is not precise.
Page 11 line 260: “we found that the fitted PDFs stayed mostly the same” This statement is also imprecise. Can you quantify what ‘mostly the same’ means?
Results and Discussion:
Section 3.3:
Page 18 line 387: Why do these three scenes likely exhibit more severe pixel misregistration errors compared to the other scenes?
Can an alternative (or compounding) interpretation for the observed phase difference between east and west facing slopes be due to viewing geometry rather than DEM misregistration? Comparing to a descending path frame track over the same area would shed light on this, and allow the authors to rule out a difference in the projection of downslope deformation (e.g., solifluction) onto the LOS vector vs. a pixel misregistration issue.
Conclusions:
Page 22, line 431: “InSAR-estimated seasonal surface thaw subsidence measures the amount of water stored in the saturated soil active layer above
permafrost, which can be used to constrain hydrologic models and water mass budgets.” Rather than saying InSAR thaw subsidence measures water storage, I might suggest instead something like ‘is sensitive to’ or ‘is related to’, as this is not a direct measurement of soil water storage, but rather a model-based estimation.
Citation: https://doi.org/10.5194/hess-2024-362-RC1 - The manuscript would benefit from a more precise description of what the Zwater parameter physically corresponds to. From my reading of prior work, the Zwater parameter is the equivalent water depth of all pore-bound water undergoing seasonal freeze/thaw within the active layer. However, the Zwater parameter is variously referred to in slightly different ways throughout the manuscript, which could raise questions from readers as to what exactly Zwater physically corresponds (i.e., is this the equivalent water depth, or the thickness of the active layer that is fully saturated?) I have a few specific suggestions to address this.
Viewed
| HTML | XML | Total | BibTeX | EndNote | |
|---|---|---|---|---|---|
| 91 | 25 | 5 | 121 | 1 | 0 |
- HTML: 91
- PDF: 25
- XML: 5
- Total: 121
- BibTeX: 1
- EndNote: 0
Viewed (geographical distribution)
| Country | # | Views | % |
|---|
| Total: | 0 |
| HTML: | 0 |
| PDF: | 0 |
| XML: | 0 |
- 1