Characterization of stony soils' hydraulic conductivity using laboratory and numerical experiments

Beckers, Eléonore; Pichault, Mathieu; Pansak, Wanwisa; Degré, Aurore; Garré, Sarah

doi:https://doi.org/10.5194/soil-2-421-2016

Articles | Volume 2, issue 3

https://doi.org/10.5194/soil-2-421-2016

© Author(s) 2016. This work is distributed under
the Creative Commons Attribution 3.0 License.

https://doi.org/10.5194/soil-2-421-2016

© Author(s) 2016. This work is distributed under
the Creative Commons Attribution 3.0 License.

Articles | Volume 2, issue 3

Original research article

|

12 Aug 2016

Original research article |

| 12 Aug 2016

Characterization of stony soils' hydraulic conductivity using laboratory and numerical experiments

Eléonore Beckers, Mathieu Pichault, Wanwisa Pansak, Aurore Degré, and Sarah Garré

Download

Final revised paper (published on 12 Aug 2016)
Preprint (discussion started on 29 Oct 2015)

Interactive discussion

Status: closed

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

- Printer-friendly version

- Supplement

RC C588: 'Referee comments', Anonymous Referee #1, 17 Nov 2015
- AC C802: 'Reply to anonymous referee #1', Sarah Garré, 29 Apr 2016
RC C718: 'comments of a second reviewer', Anonymous Referee #2, 31 Jan 2016
- AC C803: 'Reply to anonymous referee #2', Sarah Garré, 29 Apr 2016
EC C726: 'Overview of reviewer comments', Jan Vanderborght, 04 Feb 2016
- AC C805: 'Reply to Editor's comments', Sarah Garré, 29 Apr 2016
RC C728: 'Review', Wolfgang Durner, 04 Feb 2016
- AC C804: 'Reply to W. Durner', Sarah Garré, 29 Apr 2016

Peer-review completion

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

ED: Revision (08 May 2016) by Jan Vanderborght

AR by Sarah Garré on behalf of the Authors (09 May 2016) Author's response Manuscript

ED: Referee Nomination & Report Request started (17 May 2016) by Jan Vanderborght

RR by Anonymous Referee #1 (31 May 2016)

Suggestions for revision or reasons for rejection

General comments
After revisions, I think the manuscript has been greatly improved but still leave one point unresolved or questionable:
In page 13-14, the authors give few explanations or discussions on the effects of stone shape on Kr, and attribute the decrease of Kr with stone size to two reasons: one is the stronger overlapping effect of the influence zone of smaller inclusions for soils with the same stone content; the other is the bigger contact area between stones and fine earth for soils with smaller inclusions. The assumption underlying both of them is that the increased pore tortuosity should be mainly responsible for the decrease of Kr with the decreased stone size.
I think the authors may neglect the other side behind the phenomenon, that is, the reduction of the minimum cross-section area for water flow. Even with the same content of impermeable rock fragments, the minimum cross-section area for water flow in a soil column can increase with decreased stone size, change with stone shape according to the arrangement of stones shown in Figure 1 or used in the numerical simulations. I strongly suggest the authors calculate the minimum cross-section area for water flows for all treatments, which may better explain the effects of stone size and shape on Kr than that mentioned in the manuscript. If the phenomenon still cannot be well explained after considering this reason, we can then attribute unexplained part to the change of soil pore tortuosity.

Specific comments
On line 8 in page 7, “adapted” should be “adopted”.
On line 22 in page 10, “concomitant” should be “consistent”.
On line 10 in page 11, “depend of” should be “depend on”.
On line 22 in page 17, “Zhang, J.H.” should be “Zhang, J.B.”.

Hide

RR by Anonymous Referee #2 (19 Jun 2016)

Suggestions for revision or reasons for rejection

In their revised version of the manuscript, the authors satisfactorily responded to the comments of the reviewers by including extra measurements to confirm their findings. I think this paper is of importance for the soil science community since it addresses an important topic, namely the hydraulic conductivity of stony soils. The authors present interesting data that are in contradiction with currently available models that are used to relate the hydraulic properties of stony soils to the properties of the fine earth in these soils.
However, there are still a few main points that need to be clarified.
First, the author speak of replicate samples when they discuss the effect of stoniness on the saturated hydraulic conductivity. But, it is not clear what these replications in fact represent. Neither is it clear what the common factor is between these so-called replication.
Second, there are still some problems with the numerical simulations of the evaporation experiments. In fact, the authors should be able to evaluate the spatial variation of locally measured hydraulic heads due to the presence of stones and the effect that this may have on the estimation of effective or ‘average’ unsaturated hydraulic properties. But, the authors are referring to numerical instabilities. In case these numerical instabilities are important, the quality and relevance of the numerical simulations should be questioned.
Third, concerning the experimental procedure, the authors write that all samples were packed to the same bulk density. But, doesn’t this lead to lower bulk densities of the fine earth in the samples with high stone content and couldn’t this be the reason for the increase in saturated conductivity in samples with higher stone content?
Fourth, the presentation of the experiments and simulations that were carried out is not yet very well streamlined and should be better structured.

I am confident that the author can address these remaining issues relatively easily.

Detailed comments:
P1 Ln 22: Change to: ‘We pointed out several factors that determine saturated hydraulic conductivity of stony soil but that are not considered by these models.’ To my understanding, a driver is something that will lead to a dynamic change.
P2 ln 24: …other factors: include here which factors.
P2 ln 26- 27: ‘… the reduced volume available for flow might compensated by other factors.’ If you think of increased tortuousity, the effect of reduced volume is in fact amplified and not compensated. I think you must explicitly mention here the different factors you have in mind.
P 2 ln 27: ‘contradictory effect’ Change to ‘compensation factor’
P3 ln 2: add saturated and unsaturated hydraulic conductivity
P3 ln 3: aforementioned models: Change to: predictive models that have been proposed in the literature.
P4 ln 14-15: change non-porous to non-conductive since heat flow also takes place in non-porous media.
P5 Eq 7. The second equation for h>0 is not needed since K is given as a function of Se and Se cannot be larger than 1.
P5 Ln 17: add: ‘ for soil with rock fragments’ I suppose you do not mean the water content and water potential in the rock fragments.
P5 ln 21: ‘coarse inclusions’ add > ?? mm. You could also think of inclusions of pockets of sand in the clay.
P5 ln 27: Was the bulk density of the overall sample 1.51 g/cm³ or was the bulk density of the fine earth in the sample 1.51 g/cm³? In case there are stones in the sample (which have a larger density than the fine earth), packing the stony soil to the same density of the fine earth will lead to a lower density of the fine earth in the stony soil. Hence the porosity of the fine earth will be larger, which of course influences the hydraulic properties.
P 6 ln 12: add unsaturated hydraulic conductivity.
P 7 ln 17: Since you use K already for the hydraulic conductivity, I propose to change K to H here (to be consistent with Eq. 12).
P8 ln 18: change this sentence to: HYDRUS 2D solves the two dimensional Richards equation using the Galerkin finite element method.
P8 ln 21: ‘…rock fragments were supposed to be circular.’ This is confusing since you also did simulations for non-circular rock fragments.
P8 ln 24: ‘fitting on’ --> fitting of.
P9 paragraph 2.3.1. This paragraph should be rewritten.
P9: ln 12: what do you mean with setup extension?
P9 ln 14: What do you mean by numerical instability? Numerical instability refers to a numerical error that is made when the flow equation is solved numerically. But, in the stony sample, you may as well have spatial variations in the hydraulic head at one depth because of the influence that the stones have on the flow field. It is not clear from your line of argumentation whether you are talking about spatial variations of hydraulic heads or numerical errors. In case of the latter, you cannot argue that you use simulated pressure heads only at the top and bottom of the sample since the pressure heads inside the sample are prone to numerical errors. If there are important numerical errors at some locations, then you cannot use the results at other locations either because these will be impacted by the errors that you make. If the variations in pressure heads inside the soil columns are due to the impact of the stones on the pressure heads, then they represent a true effect that can be expected in such soils and that will have an impact on the measured pressure heads. In that case, you can use these simulations to evaluate the effect of these variations on the estimated hydraulic properties. You write that you use only the simulated pressure heads at one observation node. I think this is reasonable if you want to evaluate the effect of the tensiometer location on the estimated hydraulic functions. But then I think you should use data from different observation nodes and compare the obtained results if you consider these different data.
P10 ln 20: write: ‘and four volumetric stone fractions’
P 10 ln 19-27: This paragraph is not clear. First, it is not clear from the start that this is about true experiments. Second, it is not clear which stone content was considered in the samples with glass beads. Third, it is not clear how many replicates were actually considered. I am not saying that after reading, it could not be figured out: 5 replicates for the samples with rock fragments and 1 for the glass beads and the volumetric stone content of the glass bead samples is 20%. But, the way it is formulated is very indirect and not straightforward.
P10 ln 23-25. This I cannot follow. What do you mean by: The four replications were processed all together? What do you mean by: between replications, the soil was oven dried for 24 hours and passed through a 2 mm sieve? Do you mean that for one stone content you packed 4 times a sample using the same soil material?
P11 ln 5: change ‘developed’ into ‘presented’.
P11 ln 18: What does the 95% refers to? This always requires some statistical inference. But, you only have a few replicates (and for the predictions by the different models, you can hardly assume that the replicates are truly random repetitions). Furthermore, for the experiments, can you assume that the Kse values are normally distributed? Mostly, Kse values are lognormally distributed. To avoid all these problems and discussions, I propose to plot just the range between the minimal and maximal value and the median.
Ln 23 and figure 3: I do not understand why you can treat different samples with different stone contents as replicates of each other. In other words, what is the relation between the samples denoted by ‘gravels 1’? Why are these replicates? Replicate may be the wrong wording here. It seems as if you assume that there are two factors: one is the stone content and one is another factor but I could not figure out what it should represent. What is common among measurements that belong to the set or factor level ‘gravels 1’? First, you need to explain what these other factor levels represent. However, at this moment, I cannot find a reason why there should be other factor levels.
Ln 24-28: Since I do not understand what the replications represent and whether a common factor can be assumed between these samples, it does not seem to make sense to discuss how different individual samples compare with each other.
P12 ln 11: I would propose to replace ‘sampling’ by ‘packing’. Sampling rather refers to taking an soil sample whereas here, you ‘make’ a sample.
P12 ln 25: change ‘depending on’ to ‘with’.
P12 ln 17: again, I don’t understand the mean of ‘replicate’.
P13 ln 3: correct ‘permeability’
P13 ln 15: Change ‘drivers’. Table 3 illustrates the effect of different factors on the simulated Kse.
P15 ln 7: Which effect? Do you mean that with higher stone contents, the unsaturated conductivity will increase with increasing stone content as the Kse did?

Table 2: Use footnotes to explain Rv and n and the meaning of the circles, triangles and bars. Add also the number of replicates used in the experiments.
Figure 2: Check whether the graph becomes clearer when the Ks axis is logarithmically scaled. The 95% interval in the caption is not well defined. I would propose to change the 95% interval simply by the range between the maximal and minimal Kse that is measured or predicted by the different models.
Figure 5: explain in the figure caption what the dotted line refers to and explain that the triangles are saturated conductivities (closed is measured with black for the stony and grey for the fine earth, and open is predicted by the model (indicate which model exactly). Make also a distinction between the data points that are obtained from the two replicates.

Hide

ED: Reconsider after minor revisions (review by Editor) (19 Jun 2016) by Jan Vanderborght

AR by Sarah Garré on behalf of the Authors (06 Jul 2016) Author's response Manuscript

ED: Publish subject to technical corrections (09 Jul 2016) by Jan Vanderborght

I think that the paper can now be accepted for publication. The reviewers' comments have been properly addressed. There are however still a few small technical corrections that should be done:

1) Figure 2. The caption says: 'The bars show the maximum and minimum around the median predicted by these models.' But, there are also bars around the measurements. I propose to make these bars also represent the maximum and minimum of the measurements.

2) Figure 3 and the discussion on the replicates. It is still not clear what the replicates actually are. Did you use the same soil and stone material which was five times dried, sieved and repacked or did you make 5 columns at the same time (and consequently used different soil and stone material in each replicate)? Secondly, why should the measurements on for instance replicate 5 for the different stone contents be connected by a line? This suggests that replicate 5 at for instance a stone content of 20% has something in common with replicate 5 at a stone content of 40 %. What is it that these replicates have in common? If there is no reason why there should be a common factor and explanotary factor between replicates at different stone contents, then there is no reason to connect them with a line. It would even not be relevant to know then the replicate number of a certain data point.

3) 'One could think that this observation is directly related to change in the minimal cross section for water flow.' Add how the minimal cross section was defined. Did you calculate the minimal cross section from calculating the flow cross sections at different hights in the sample?

4) 'the relationship is not perfect as we could expect with numerical simulations, and so forth could support the hypothesis' Reformulate

5) 'on the one hand, numerical instabilities are more plausible at the limits of the sample and on the one hand, the use of bigger samples than conventionally used (6.5 cm height) might reduce the accuracy of the evaporation method' You use two times 'on the hand' here.

Hide

ED: Publish subject to technical corrections (19 Jul 2016) by Kristof Van Oost (Executive editor)

AR by Sarah Garré on behalf of the Authors (26 Jul 2016) Author's response Manuscript

Short summary

Determining the behaviour of stony soils with respect to infiltration and storage of water is of major importance, since stony soils are widespread across the globe. The most common procedure to overcome this difficulty is to describe the hydraulic characteristics of a stony soils in terms of the fine fraction of soil corrected for the volume of stones present. Our study suggests that considering this hypothesis might be ill-founded, especially for saturated soils.