Hydrological Characteristics of Columnar Basalt Aquifers: Measuring and Modeling Skaftafellsheiði, Iceland

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

The manuscript analyze the characteristics of groundwater flow based on the numerical model. However, many points expecially the organization of the context should be improved. Hence I would recommend a major revision.1. The research progress should be more detailed in the Introduction.  There are various numerical models to analyze the groundwater flow. Authors should state the reason why the modflow is chosen.
2. Hydrogelogical conditions are failed to be indicated. The information of aquifer, groundwater flow and recharge-discharge is missing.
3. There are many colors in Fig.3. What are the meanings of these  colors  in Fig.3? 
4. Generally, the numerical models should be verified to prove the robustness. The monitoring data should be shown to verify the numerical results.
5. Discussion is too short.  For discussion, there should be several subsections to present the different parts of your research.  6. The conclusion is missing. In conclusion, a concise summary should be made and make a future perspective.

Author Response

Reviewer: The manuscript analyze the characteristics of groundwater flow based on the numerical model. However, many points especially the organization of the context should be improved. Hence I would recommend a major revision.

Authors respons: We agree that improvements were needed to explain the methodology better. In the original version of the manuscript we decided to leave info out to make the manuscript rather compact. But indeed that come at the cost of reproducibility of the research. Therefore we did a major revision of the manuscript and left almost no stones untouched. See the track changes in the document.

Reviewer: The research progress should be more detailed in the introduction. There are various numerical models to analyze the groundwater flow. Authors should state the reason why the modflow is chosen.

Authors: We agree that more info can be given in the introduction part of the manuscript. We added a significant portion of extra information now. Intro was 41 lines, no it is 102 lines of info. For the choice of the use of MODFLOW, see line 262 – 266 in the revised manuscript (the version without track changes) or 287 – 299 (the version including track changes). We did make the choice for a spatially distributed model like MODFLOW even though rather limited info on the physical properties of the basalt formation is available. We expect that the model uncertainty of model alternatives would be larger which would reduce the usefulness of the modeling exercise. 

Reviewer: Hydrological conditions are failed to be indicated. The information of aquifer, groundwater flow and recharge-discharge is missing.

Authors: Agree. In the revised version we added a geological map of Skaftafellsheiði, a vegetation zone map, info on the hydrogeology and info on the climate system of the region to the introduction. Now all the requested info is in the Introduction (line 33 – 135 in the version without track changes).

Reviewer: There are many colors in Figure 3. What are the meanings of these colors in Fig. 3?

Authors: These colors were used to indicate the different precipitation zones as used in the 2023 field campaign. They colors did not have any physical meaning. We expected that with colors it would be easier to see the size and shape of the precipitation zones. But we understand the confusion and included now a figure with just grey zones and white lines indication the borders of the precipitation zones. 

Reviewer: Generally, the numerical models should be verified to prove the robustness. The monitoring data should be shown to verify the numerical results.

Authors: We fully agree. We always do that in sedimentary regions where such data from piezometers is available. But in the basalt dome we do not have any (deep) drillings available. In other words: the advantage of being in a remote area with little human disturbances comes with a disadvantage of rather poor data availability. We solved this 1) by measuring at least the discharge as good a possible and 2) by looking at water levels and spring levels in the upper layers and to compare these field findings with the model outcomes. Figure 9 in the 1^st version (figure 14 in the revised version) shows the comparison of the observed and computed water levels. We did do a sensitivity analysis with the model and found the best results with the model setup and parameters as indicated in the manuscript.

Reviewer: Discussion is too short. For discussion, there should be several subsections to present the different parts of your research.

In the revised version the discussion increased from 18 lines to 57 lines. Also – as requested - we did cut the discussion into subsections (5 subsections in total)

Reviewer: The conclusion is missing. In conclusion, a concise summary should be made and make a future perspective. 

Authors: We followed the manuscript guidelines version with emphasis on the discussion, but agree that it’s better to have clear conclusions. We now have added a conclusion chapter including numbers and a future perspective regarding the research at Skaftafellsheiði and possibly in other basaltic provinces in other countries.

Author Response File: Author Response.pdf

Reviewer 2 Report

Comments and Suggestions for Authors

Article

Hydrological Characteristics of Columnar Basalt Aquifers; Measuring and Modeling Skaftafellsheiði, Iceland

General Comments:

1. First sentenence in the abstract need to modify (Basalt is a good water conductor in special conditions).
2. Line 14: (he hydraulic properties of these basaltic are …) need to be reviewed.
3. The abstract need full revisions, it missed the quantitative data, and no results are presented.
4. Lines 25 -31, need to support with more references.
5. Figure 1 is hazy figure, without scale.
6. Figure 2 not supported with north arrows.
7. Figure2 not supported with color scale.
8. In Figure 2, the right part not indicated and bordered in the left part.
9. In Figure 2, the two parts must be numbered.
10. The study area needs to be identified geologically, previous geological surveys, history of the ground water, meteorological conditions and …etc.
11. Lines 56-65 not referenced.
12. Section (Materials and Methods): lines (67-96) are not referenced.
13. Equation 1 is not referenced.
14. Figure 3 is not scaled and not supported with north arrow.
15. What the definition of crop factors?
16. The results not supported with data of boreholes in the study area.

 

Comments for author File: Comments.pdf

Comments on the Quality of English Language

Article

Hydrological Characteristics of Columnar Basalt Aquifers; Measuring and Modeling Skaftafellsheiði, Iceland

General Comments:

1. First sentenence in the abstract need to modify (Basalt is a good water conductor in special conditions).
2. Line 14: (he hydraulic properties of these basaltic are …) need to be reviewed.
3. The abstract need full revisions, it missed the quantitative data, and no results are presented.
4. Lines 25 -31, need to support with more references.
5. Figure 1 is hazy figure, without scale.
6. Figure 2 not supported with north arrows.
7. Figure2 not supported with color scale.
8. In Figure 2, the right part not indicated and bordered in the left part.
9. In Figure 2, the two parts must be numbered.
10. The study area needs to be identified geologically, previous geological surveys, history of the ground water, meteorological conditions and …etc.
11. Lines 56-65 not referenced.
12. Section (Materials and Methods): lines (67-96) are not referenced.
13. Equation 1 is not referenced.
14. Figure 3 is not scaled and not supported with north arrow.
15. What the definition of crop factors?
16. The results not supported with data of boreholes in the study area.

 

Author Response

Reviewer: First sentence in the abstract need to modify (Basalt is a good water conductor in special conditions)

Authors: The original sentence was ‘Basalt can act as a good water conductor, predominantly when polygons have been formed during the cooling of the lava’. With this we thought we indicated the conditions under which basalt can act as aquifer. But we agree that this can lead to the misinterpretation that all or most basalt layers are aquifers. Therefore we changed the first sentence into ‘Basalt with columnar jointing can act as a good water conductor’. We hope that by starting with the columnar jointing the confusion is removed.

Reviewer: Line 14: the hydraulic properties of these basaltic are….) need to be reviewed.

Authors: True. Changed into ‘The hydraulic properties of these basaltic formations are always difficult to quantify’

Reviewer: The abstract need full revisions, it missed the quantitative data, and no results are presented.

Authors: Agree. We now included quantitative data and results. The old version was 10 lines, the new version is 23 lines. We hope this abstract now gives a good summary of the presented research.

reviewer: Lines 25-32, need to support with more references.

Authors: What line 25-32 was in the 1^st version is now line 34 – 57. In this section we had 3 references, we now have 7 references.

Reviewer: Figure 1 is hazy figure, without scale.

Authors: Agree, this is a figure coming from Domenico and Schwartz (D&S). For us this is a key figure in the manuscript. D&S did not use scales in their concept of REV. Most likely because the range on the scale can be large (large uncertainty). But, triggered by this feedback, we picked up the challenge to improve the D&S figure. See Figure 1 in the revised manuscript. Scales are added, less relevant info removed. We hope this new figure helps to get a better understanding of the Representative Elementary Volume concept. 

Reviewer: Figure 2 not supported with north arrows.

Authors: The north arrows in figure 2 are in the upper right corner of both maps. 

Reviewer: Figure 2 not supported with color scale.

Authors: We added numbers at the elevation lines so that the elevation range (the colors) is clearly visible. 

Reviewer: In Figure 2, the right part not indicated and bordered in the left part.

Authors: We had a border in the original manuscript, but we made it more visible in the revised version.

Reviewer: In Figure 2, the two parts must be numbered.

Authors: Done, now in the figure and subscript numbered as (a) and (b).

Reviewer: The study area needs to be identified geologically, previous geological surveys, history of the groundwater, meteorological conditions and …etc

Authors: We agree. We intended to create a compact manuscript, but by doing that we left info out which has to be included.  The introduction is now increased from 40 lines is the previous version to 107 lines in the revised version. We also added subchapters, a geological map and description, vegetation cover info and meteorological info.

Reviewer: Lines 56-65 not referenced

Authors: The info presented in line 56 – 65 of the old version relates to the morphology of the catchment and field observations of the top material. We did not derive any of this info from literature and therefore did not put in references for this.

Reviewer: Section (Materials and Methods): lines (67-96) not referenced.

Authors: We expanded the Materials and Methods section considerably, from line 66 – 142 in the first version to line 141 – 301 in the revised version. The first part of the methods part is describing how we did set up the fieldwork. To our opinion we do not need references in this section.

Reviewer: Equation 1 not referenced.

Authors: Agree, we did not put a reference here because we did setup this simple water balance for the area just as so many other researchers in other regions with snow.

Reviewer: Figure 3 is not scaled and not supported with a north arrow.

Authors: Done. This is figure 5 in the revised version.

Reviewer: What the definition of crop factors?

Authors: See the improved text, line 210 - 248. The crop factor is linked to the leaf area index of the vegetation. With this crop factor and the coverage of that part of the landscape you can calculate the potential evapotranspiration from the reference evapotranspiration.

Reviewer: The result not supported with data of boreholes in the study area.

Authors: In sedimentary areas we always use the info from boreholes (profile description and water levels) for the setup and calibration of the model. But in the nature reserve Skaftafellsheiði there are no boreholes. Therefore we used the info coming from the top layers, the topography and the rock outcrops at the flanks of the basalt dome to translate our conceptual model of the area into a numerical model. 

Author Response File: Author Response.pdf

Reviewer 3 Report

Comments and Suggestions for Authors

in a such citation scheme as is used in MDPI papers, it is perhaps required to start from 1 ... and continue gradually in ascending numbers...

it seems that citations were converted for the required scheme, but not subsequently ordered... As a consequence of this, References 7, 9, 17, 19, 24, 25, 29 and 32 are NOT mentioned in the paper text ...

 

 

 

 

the authors did not consider potential trans-watershed-boundary flow in the balance ... although it was neglected, this should be mentioned in the text!

 

both observation periods are, of course, too short as normally they should contain at least one complete climatic cycle (hydrological year), to minimize the influence of the water storage differences to the balance .... also this should be mentioned in the text, as the reviewer understands that such arrangements are not easy in these climate conditions and countryside...

 

...subsequent usage of the steady-state flow model is a consequence of missing data for the whole water cycle, therefore storage coefficients that can play important role in the water flow behavior is not mentioned ... it is understandable for the public, but should also explicitly appear in the text...

 

...in line 179 there is "forgotten" citation of Einarsson (1972, 1984) that should be possibly converted to the required way...while only Einarsson (1972) is present in References

 

...text in the lines 183 - 184 documents the consequence of the lack of long-term observations and water storage (groundwater levels) observations, but, of course - the presented paper represents "the art of possible". Perhaps the authors should try the usage of another evapotranspiration formula...

 

...note to Figure 9: it should be explained how and from where the observed heads were measured, as apparently there are no boreholes in the model area...

Author Response

reviewer: It seems that citations were converted for the required scheme, but not consequently ordered… As a consequence of this, references 7, 9, 17, 19, 24, 25, 29 and 32 are NOT mentioned in the paper text..

Authors: True. All references are checked now and list should be complete.

Reviewer: The authors did not consider potential trans-watershed-boundary flow in the balance… although it was neglected, this should be mentioned in the text!

Authors: Agree. Added a sentence for that. See line 155. We saw no indications on the west and east flank of water emerging from the deeper basaltic layers except from the top layer. This confirmed by the EC measurements we did on the small streams on the east and west flanks. 

Reviewer: Both observation periods are, of course, too short as normally they should contain at least one complete climatic cycle (hydrological year), to minimize the influence of the water storage differences in the balance…. Also this should be mentioned in the text, as the reviewer understands that such arrangements are not easy in the climate conditions and countryside…

Authors: True. And because of that we extended the discussion section considerably, including info on the length of the measuring campaigns.

Reviewer: …subsequent usage of the steady-state flow model is a consequence of missing data for the whole water cycle, therefore storage coefficients that can paly important role in the water flow behavior is not mentioned…. It is understandable for the public, but should also explicitly appear in the text…

Authors: True. We hope all the new text in throughout the new text is now in line with this. 

Reviewer: …in line 179 the is ‘forgotten’ citation of Einarsson (1972, 1984) that should be possibly converted to the required way.. while only Einarsson (1972) is present in References.

Authors: True. Is now complete.

Reviewer: …text in the lines 183-184 documents the consequence of the lack of long-term observations and water storage (groundwater levels) observations, but, of course – the presented paper represents ‘the art of possible’. Perhaps the authors should try the usage of another evapotranspiration formula..

Authors: We added give now more info on the evapotranspiration formula used. We hope this covers this comment.

Reviewer: …note to Figure 9: it should be explained how and from where the observed heads were measured, as apparently there are no boreholes in the area. 

Authors: True. We had to rely on different types of field observation to get to a good representation.

Author Response File: Author Response.pdf

Reviewer 4 Report

Comments and Suggestions for Authors

The topic of the manuscript is of general interest in regions where groundwater is abstracted from basalt formations for drinking water supply and irrigation. However, the presented modeling approach is lacking scientific quality. Therefore, I cannot recommend the manuscript for publication.

Mainly, the description of the water budget and the related discussion of the simulation results does not fulfil scientific standards. In the context of equation 1 used to calculate the storage it is not described how the budget of the snowpack is considered (line 77). For the description of the calculated evapotranspiration only some parameters are presented in table 1 but no formula is given for the calculation (line 107). The description of the model setup is incomplete as table 2 is missing (line 115). The simulation results regarding the water budget is also not described in all detail. The evapotranspiration was determined with temporal variation but from the presented numbers and units of the discharge (line 165) and the storage (line 182) it seems that the results were evaluated cumulatively.

Furthermore, there are several technical shortcomings:

Fig. 2:  a reference should be given as the figure is probably taken from an external source

line 148, 149:  'H' is not defined, the presented term is not complete

references 7, 9, 17, 19, 25, 29, 32 are not mentioned in the text

Comments on the Quality of English Language

several misspellings in particular in section 4

Author Response

Reviewer: The topic of the manuscript is of general interest in regions where groundwater is abstracted from basalt formations for drinking water supply and irrigation. However, I cannot recommend the manuscript for publication. 

Authors: Hopefully the major revision of the manuscript makes it of interest for publication

Reviewer: Mainly, the description of the water budget and the relation discussed of the simulation results does not fulfill scientific standards. In the context of equation 1 used to calculate the storage it is not described how the budget of the snowpack is considered (line 177). For the description of the calculated evapotranspiration only some parameters are presented in table 1 but no formula is given for the calculation (line 107). The description of the model setup is incomplete as table 2 is missing (line 115). The simulation results regarding the water budget is also not described in all detail. The evapotranspiration was determined with temporal variation but from the presented numbers and units of the discharge (line 165) and the storage (line 182) is seems that the results were evaluated cumulatively. 

Authors: We did add quite some detail on the methodology, the results and the conclusions. The backbone of the manuscript is still intact, but we believe that with the help of the reviewers comments we managed to give a concise description on what we did, how we did it and why. For the more specific comments:

• We did do measurements on the snowpack in combination with pictures. We now added info in the revised manuscript on our findings regarding snow during both field campaigns (line 193 – 196 and line 331 – 344).
• Formula ET: done (see line 209 – 248)
• Model setup. We expanded the model setup description extensively (line 251 – 301, including table 2
• Water budget. We expanded the discussion on the model results.

Reviewer: Fig 2: a reference should be given as the figure is probably taken from and external source.

Authors: Done. Figure was taken from kort.gis.is and adapted from there. Refence is added.

Reviewer: Line 148, 149: ‘H’ is not defined, the presented term is not complete.

Authors: Indeed. Given the expansion of the manuscript and the fact that we have figure 10 (the correlation figure) we removed this part regarding H.

Reviewer: References 7, 9, 17, 19, 25, 29, 32 are not mentioned in the text. 

Authors: Done.

Author Response File: Author Response.pdf

Round 2

Reviewer 1 Report

Comments and Suggestions for Authors

This version of the manuscript has been improved significantly. I still have some issues for revisions.

1. A geological section with hydrogeological information should be added in Fig.3. 

2. Monthly variation of temperature and precipitation should be indicated by figure.

3. Some words are difficult to read in Fig.5.

4. Model validation is absent.

5. Limitations should be indicated for further research.

Author Response

Reviewer comment 1: A geological section with hydrogeological information should be added in Fig.3. 

Reply authors: We agree. in line 73 - 85 (version with track changes) already some info on the formations was given, but a translation to the hydrogeology makes it more clear. therefore a new section was added: line 89 - 101 in the version with track changes. We hope this makes it more clear.

Reviewer comment 2: Monthly variation of temperature and precipitation should be indicated by figure.

Reply authors: In the section on climate (line 148 - 154 in the version with track changes) the climate was already described in general terms (precipitation evenly spread year round), but indeed that is too vague. We now added the monthly and annual averaged T and P for a nearby weather station. Given the number of figures we already have in the manuscript we decided to put this info in a table.

Reviewer comment 3: Some words are difficult to read in Fig.5.

Reply authors: agree. We improved the figure and now all axis and rain gauge codes are with a larger font. It was a bit difficult to enlarge the font for the rain gauge codes without creating a bit of a messy figure. Hopefully the adaptations are indeed improving the readability.

Reviewer comment 4: Model validation is absent.

Reply authors: In the section 433 - 449 in the version with track changes we explain that we checked the modeling results by comparing the leakage (discharge) with the measured discharge and also compared the modeled water levels with field observations. We now added a section (line 449 - 453) where we give a bit more detail on how we did look at the stationary approach. We also added info on the calibration we did by using PEST (line 314 - 320 in the version with track changes).

Reviewer comment 5: Limitations should be indicated for further research.

Reply authors: We already gave some limitations regarding this research in the discussion section (line 458 - 518). But indeed that can be more clear. Therefore we added an extra section on this (line 519 - 527 in the version with track changes).

Hopefully these adaptations in the manuscript help to make the manuscript easier to read and can trigger further research on this topic.

Reviewer 2 Report

Comments and Suggestions for Authors

Article

Hydrological Characteristics of Columnar Basalt Aquifers; Measuring and Modeling Skaftafellsheiði, Iceland

General Comments:

Most of the comments were implemented, and the following notes must be considered:

1. Figure 4, caption must be detailed.
2. Figure 14, caption must be detailed.

Comments for author File: Comments.pdf

Comments on the Quality of English Language

Article

Hydrological Characteristics of Columnar Basalt Aquifers; Measuring and Modeling Skaftafellsheiði, Iceland

General Comments:

Most of the comments were implemented, and the following notes must be considered:

1. Figure 4, caption must be detailed.
2. Figure 14, caption must be detailed.

Author Response

Reviewer comment 1: Figure 4, caption must be detailed.

Reply authors: Agree. We added more info in the caption now.

 

Reviewer comment 2: Figure 14, caption must be detailed.

Replay authors: Agree. We now added more info in the caption.

Reviewer 4 Report

Comments and Suggestions for Authors

The authors addressed all my comments and revised the manuscript substantially.  The reader can now follow the descriptions quite well. 

However, the descriptioon of the water balance of the groundwater model might be described in more detail.  This is recommended to prvide smoe more in formation to support the statement in the conclusions 'The output of the modelled water balance in MODFLOW showed that the stream 575 leakage in the model is in the same order as the measured discharge' (l. 575-576).

Further, I recommend to carefully check the type setting and spelling finally to avoid mistakes caused by the use of the revision mode.

Author Response

Comment reviewer 1: However, the descriptioon of the water balance of the groundwater model might be described in more detail.  This is recommended to prvide smoe more in formation to support the statement in the conclusions 'The output of the modelled water balance in MODFLOW showed that the stream 575 leakage in the model is in the same order as the measured discharge' (l. 575-576).

Reply authors: Agree. We already described the model performance in line 427 - 448 in the revised version with track changes, but added some extra info on how we calibrated and validated the model (line 449 - 453 and line 314 - 321. Hopefully it is now more clear how we came to the conclusion that the model does mimic the field observations and therefore that such a generalized model from the REV concept can describe water flow in columnar basaltic formations.

Reviewer comment 2: Further, I recommend to carefully check the type setting and spelling finally to avoid mistakes caused by the use of the revision mode.

Reply authors. Fully agree. The major revisions were so major that type setting and spelling error were looming around the corner. We did find quite a few while checking the manuscript. Hopefully it is now clean and free of typos and/or double spaces.

General comment: the reviewer feedback did really help to improve the manuscript to the quality as can be expected for Geosciences. Thanks for that.