Monitoring soil moisture from middle to high elevation in Switzerland: set-up and first results from  the SOMOMOUNT network

Pellet, Cécile; Hauck, Christian

doi:https://doi.org/10.5194/hess-21-3199-2017

Articles | Volume 21, issue 6

https://doi.org/10.5194/hess-21-3199-2017

© Author(s) 2017. This work is distributed under
the Creative Commons Attribution 3.0 License.

https://doi.org/10.5194/hess-21-3199-2017

© Author(s) 2017. This work is distributed under
the Creative Commons Attribution 3.0 License.

Articles | Volume 21, issue 6

Research article

|

29 Jun 2017

Research article |

| 29 Jun 2017

Monitoring soil moisture from middle to high elevation in Switzerland: set-up and first results from the SOMOMOUNT network

Cécile Pellet and Christian Hauck

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Final revised paper (published on 29 Jun 2017)
Preprint (discussion started on 14 Sep 2016)

Interactive discussion

Status: closed

AC: Author comment | RC: Referee comment | SC: Short comment | EC: Editor comment

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RC1: 'Review of “Monitoring soil moisture from middle to high elevation in Switzerland: Set-up and first results from the SOMOMOUNT network“ by Pellet et al.', Anonymous Referee #1, 29 Sep 2016
- AC1: 'Reply to anonymous referee #1', Cécile Pellet, 12 Dec 2016
RC2: 'Comment Referee 2', Anonymous Referee #2, 10 Nov 2016
- AC2: 'Reply to anonymous referee #2', Cécile Pellet, 12 Dec 2016

Peer-review completion

AR: Author's response | RR: Referee report | ED: Editor decision

ED: Reconsider after major revisions (further review by Editor and Referees) (13 Dec 2016) by Roberto Greco

Dear Authors,
both the reviewers have made a thorough review of your manuscript, and both raised serious doubts about the correctness of volumetric water content data indirectly retrieved through dielectric permittivity measurements when part of the water is forzen, as well as about the discussion of the obtained results, in which no clear distinction between liquid and frozen water was made.
At the same time, both the reviewers recognize some merit in the manuscript, so my decision is to reconsider it after major revisions.
I have read your point-to-point answers, and I can see that you are going to address all the issues raised by the reviewers.
I add here a few comments of my own:
1. Regarding the calibration of indirect measurements of soil water contant based upon dielectric permittivity, you should consider that also the permittivity of liquid water is sensitive to temperature variations: the value of 80 which applies at 20°C grows up to 88 at 0°C. This implies that both the specific calibration of FDR and Topp's polynomial that you use for TDR have additional uncertainty when applied at low temperatures.
2. I still find difficult to understand the supposed process of melting from below: although temperature variations in the soil are obviously more delayed at higher depths, it remains true that the amplitude of the variations decreases with depth. So, it is possible to have temperatures above melting point in depth and below freezing point in shallower layers, but only when the soil is refreezing after the summer season (the temperature is decreasing above, and is still high, or maybe even increasing, below). But the melting process in any case starts from above, and it should be clear in the discussion.
3. About your statement, criticized by both the reviewers, that under dry conditions you observed quicker infiltration, I would remind that the infiltration capacity of a soil is controlled not only by the unsaturated hydraulic conductivity (obviously smaller in drier conditions), but also on the vertical gradient of water potential, which can be very high downward when the soil is dry, so that usually the infiltration capacity is maximum when a soil is dry, and then reduces. So, before invoking preferential flows, often seen as the "miracle cure" to all problems, I would carefully check if the measurements (and the characteristics and even the visual observation of the topsoil) actually give evidence of the possible activation of by-pass flow. In other words, I would better motivate statements as "indicative for preferential flow processes", and similar.
Having said this, that I hope can help while writing the revised manuscript, I look forward to receiving it.
Best regards
Roberto Greco

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AR by Cécile Pellet on behalf of the Authors (09 Jan 2017) Author's response Manuscript

ED: Referee Nomination & Report Request started (13 Jan 2017) by Roberto Greco

RR by Anonymous Referee #1 (01 Feb 2017)

Suggestions for revision or reasons for rejection

Second review of „ Monitoring soil moisture from middle to high elevation in Switzerland: Set-up and first results from the SOMOMOUNT network“ by Pellet et al.

The manuscript has been extensively reworked and most of the earlier reviewer comments have been appropriately addressed. I also very much appreciate the additional chapter on the effects of soil freezing on soil moisture measurements and the restricted accuracy of liquid soil moisture measurements during frozen conditions. This limited measurement accuracy should be also mentioned later in the interpretation and discussion of the results.

However, there are still several issues that need to be resolved before publication can be recommended.

General comments

The different terms “liquid VWC”, “VWC”, “total VWC”, “total soil moisture” and “soil moisture” are used at random, which is confusing for the reader. I suggest to consistently using the term “LSM” for liquid soil moisture and “TSM” for total soil moisture (in the following I will use these abbreviations. Also the axis captions of all figures need to be adopted in this way.

All soil moisture measurements below 0 °C have now been indicated in the figures to indicate frozen soil conditions (i.e. measurements are not representing TSM anymore). However, there are cases where frozen conditions seem to occur above 0 °C. For instance in Fig. 6 (e.g. GFU) one can clearly see that LSM is dropping sharply when temperatures approaches 0 °C, indicating that a substantial part of the soil water within the measurement volume of the sensors has already started to freeze, although soil temperatures is still above 0 °C according to the temperature sensor. This indicates that the temperature measurements are not well representing the measurement volume of the sensor. This is not surprising, since the electromagnetic waves of the soil moistures sensors penetrate a certain part of the soil, whereas the temperature sensor only measures its own temperatures inducing a scale mismatch. In order to circumvent this problem, a higher threshold value for soil temperature should be to be chosen (e.g. 1 °C or higher).

There are still issues related to the special situation at DRE. Only after reading the recent HESSD paper I was able to understand the process of convective heat transport by air circulation within the talus slope. The authors suggest that is effect also explains lower LSM values. They argue that atmospheric air is transported during winter periods into the ground at the location of the monitoring station due to this process, thus when air temperature is well below 0 °C. This would induce soil freezing below the snow cover and thus explaining the observed drop in LSM. However, the soil temperature stays close to 0 °C during this (Figure 6), which is the typical soil situation below an insulating show cover. In addition the drop in LSM happens mostly in 50 cm, which is counter intuitive since freezing should be more pronounced near the soil surface.
Therefore an alternative explanation for the drop in LSM should be considered: Since the DRE soil has a very high porosity (the bulk density of 0.12 g/cm³ show in Tab. 3 means that the porosity is >90 %!), the drop in LSM could be easily explained by exfiltration of soil water into the bed rock fissures below the soil layer or by lateral water transport downhill. In addition, the drop in LSM in winter is only marginal and cannot explain the substantial lower annual mean LSM compared to MLS or FRE. It is more realistic that LSM at MLS and FRE are higher due to the influence of shallow groundwater that keeps the soil saturated for longer time periods (i.e. LSM stays constant at the maximum value), whereas DRE does not show any sign of groundwater influence (i.e. LSM shows high variability and the LSM is well below the soil porosity).

In the sensor comparison (Chapter 4.1) only the R²-values are discussed. However, the RMSE is much better indicator for the accuracy of the LSM measurements.

Chapter 3 should be restructured (only one sub-chapter is a bit awkward).

In general, the manuscript should be carefully checked for syntax and tense errors (preferably by a native speaker).

Specific comments

P1L19: “up to” instead of “until”

P1L21: “VWC” is not defined

P3L22: “Qu et al., 2013”

P3L20-21: Actually, the frequency of the SMT100 sensor is not fixed. The SMT100 sensor generates a pulse, which is inverted and then fed back to the input of the line driver resulting in an “oscillation” frequency that mainly depends on the dielectric permittivity of the surrounding medium (between 150 MHz in water and 340 MHz in air), see Bogena et al. (2017) for more details on the SMT100 technology. Bogena et al. (2017) also showed the effects of temperature on SMT100 reading and demonstrated that any temperature dependency of the measured soil moisture are related to temperature related changes in permittivity and thus are not a result of the SMT100 sensor electronics and thus can be easily corrected using temperature information.

P3L26: According to Bogena et al. (2017) the accuracy of the SMT100 for ideal conditions/media is about 1 vol.% (factory calibration) and even better in case of sensor specific calibration.

P4L4-10: Remove redundancies (e.g. penetration depth)

P10L2-4: This statement is not fully correct. Watanabe and Wake (2009) showed that the relationship of liquid water fraction measured with NMR and the permittivity measured with TDR can be approximated with Topp’s equation for sand (except −0.1 < T > 0 °C), but not for other soil textures like loam. Thus, for most of your sites this means that the LSM measurements have less accuracy during frozen conditions.

P11L3: Check grammar

P11L8: Here you also should mention the very high RMSE at MLS.

P11L21: In fact, the temperatures are typically staying close to 0 °C.

P11L29: A high retention capacity should lead to less variability in temporal soil moisture dynamics.

P11L32: Is the high evaporation rate really only due higher temperature? What about other meteorological parameters, especially low precipitation rates?

P12L18: This is an indication for preferential flow.

P12L29: Why “also”?

P13L13: “event” instead of “daily”

P14L9: Check grammar.

P14L12: Check grammar.

P14L13: Which depth?

P14L27: Please indicate the soil type in terms of FAO classification.

P15L4: Check grammar and repetition.

P15L12: “large elements” is not an appropriate term in this respect.

P16L8: “soil material”, “near the soil surface”

P16L11: The slope is higher than 45°. Better present the slope of a regression.

P16L12: The slope was less steep.

P16L18: Please add the LSM values.

P17L23-24: The statement that temperature is a result of the radiation balance is not correct. Air temperature in mountainous regions typically decreases with elevation according to the moist adiabatic lapse rate due the decrease in atmospheric pressure with elevation and latent heat exchange processes.

P18L2: Explain “surface offset”

P18L18: The term “evolution” is not appropriate here.

P19L10: Fig. 13 is not a conceptual model. You could call it a generalised schematic or similar.

Fig. 6 is overcrowded with time series making it very difficult to read, especially since the colours are also quite similar. Since you are later using only the SMT100 data, I suggest removing all other L. SM data. The 0°C-threshold is not working always (see general comment).

Fig. 7: Precipitation should be shown for the whole period. Use different axis for snow and LSM (the LSM range is too wide).

Fig. 8: You should use the 10 cm temperature data to indicate freezing conditions.

Fig. 10: Yellow is hardly visible.

Additional literature

Bogena, H., J.A. Huisman, B. Schilling, A. Weuthen and H. Vereecken (2017): Effective calibration of low-cost soil water content sensors. Sensors 17(1), 208, doi:10.3390/s17010208.

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RR by Anonymous Referee #2 (21 Mar 2017)

ED: Publish subject to revisions (further review by Editor and Referees) (21 Mar 2017) by Roberto Greco

AR by Cécile Pellet on behalf of the Authors (05 May 2017) Author's response Manuscript

ED: Referee Nomination & Report Request started (08 May 2017) by Roberto Greco

RR by Anonymous Referee #1 (08 May 2017)

ED: Publish as is (23 May 2017) by Roberto Greco

AR by Cécile Pellet on behalf of the Authors (24 May 2017)

Short summary

This paper presents a detailed description of the new Swiss soil moisture monitoring network SOMOMOUNT, which comprises six stations distributed along an elevation gradient ranging from 1205 to 3410 m. The liquid soil moisture (LSM) data collected during the first 3 years are discussed with regard to their soil type and climate dependency as well as their altitudinal distribution. The elevation dependency of the LSM was found to be non-linear with distinct dynamics at high and low elevation.