Gravitational and capillary soil moisture dynamics for distributed hydrologic models

Castillo, A.; Castelli, F.; Entekhabi, D.

doi:https://doi.org/10.5194/hess-19-1857-2015

Articles | Volume 19, issue 4

https://doi.org/10.5194/hess-19-1857-2015

© Author(s) 2015. This work is distributed under
the Creative Commons Attribution 3.0 License.

https://doi.org/10.5194/hess-19-1857-2015

© Author(s) 2015. This work is distributed under
the Creative Commons Attribution 3.0 License.

Articles | Volume 19, issue 4

Research article

|

21 Apr 2015

Research article |

| 21 Apr 2015

Gravitational and capillary soil moisture dynamics for distributed hydrologic models

A. Castillo, F. Castelli, and D. Entekhabi

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Final revised paper (published on 21 Apr 2015)
Preprint (discussion started on 30 Jun 2014)

Interactive discussion

Status: closed

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

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- Supplement

RC C2784: 'Review of hessd-11-7133', Andrew J. Guswa, 28 Jul 2014
RC C2846: 'Interactive comment on "Gravitational and capillary soil moisture dynamics for hillslope-resolving models"', Stephanie Kampf, 31 Jul 2014
EC C2915: 'Start interacting', Nunzio Romano, 04 Aug 2014
AC C3171: 'Response to Comments of A. J. Guswa', Aldrich Castillo, 18 Aug 2014
RC C3558: 'Review of HESS-2014-271"Gravitational and capillary soil moisture dynamics for hillslope-resolving models" by Castillo et al.', Anonymous Referee #3, 02 Sep 2014

Peer-review completion

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

ED: Reconsider after major revisions (13 Sep 2014) by Nunzio Romano

AR by Aldrich Castillo on behalf of the Authors (24 Oct 2014)

ED: Referee Nomination & Report Request started (12 Nov 2014) by Nunzio Romano

RR by Andrew Guswa (13 Nov 2014)

Suggestions for revision or reasons for rejection

Review of
A. Castillo, F. Castelli, and D. Entekhabi, Gravitational and capillary soil moisture dynamics for distributed hydrologic models

Review by Andrew J. Guswa, aguswa@smith.edu

In this paper, the authors present a proof-of-concept for using a zero-D dual-porosity soil-moisture model in lieu of the 1D Richards equation to represent hydrologic fluxes in the vadose zone. The new representation is implemented in the MOBIDIC model. The authors compare predictions of soil-moisture (integrated over the top 50-cm) from a calibrated version of the MOBIDIC model to predictions from a calibrated SHAW model for two sites: one semi-arid (Arizona) and one sub-humid (Mississippi). The paper shows that predictions of the temporal dynamics of average soil-moisture can be adequately captured by either model.

In my original review of this manuscript, I found that the two case studies provided a reasonable demonstration of the ability of the simplified model to represent the soil-moisture dynamics as predicted by the SHAW model. I had two primary comments:
1. that the authors also include a performance comparison with a bucket-type model of the vadose zone to demonstrate the conditions under which the dual-porosity model is warranted (also suggested by reviewer 3)
2. that the authors clarify the description of the equations that represent the soil-moisture dynamics in the MOBIDIC model

The authors have elected not to pursue my first suggestion. I find this puzzling as the effort required would not be much, and the results have the potential to greatly increase the scientific impact of the work. Instead, the authors articulate that the comparison is not made “because of the advantages and merits of the dual-pore structure as discussed,” such as that hydrologic processes can act “separately on the dual reservoirs.” This may indeed be an advantage, but my point is that it has not been demonstrated as such – only claimed.

Thus, I still think the paper would be strengthened with a comparison against a bucket-type model. Even if the bucket and the dual-porosity models gave the same results for soil-moisture dynamics, I think that would be valuable information. In such as case, the argument for the dual-porosity model could be made on the basis of other characteristics, such as modeling for transport and mixing – for example for stable isotopes.

I also found the last sentence of the added paragraph on p. 7, which claims that bucket models do not capture hysteresis, a bit odd. I do not see bucket models as fundamentally different from the dual-porosity model in this regard. The inclusion of hysteretic or non-unique behavior simply requires an inclusion of history-dependence, which could be incorporated in either type of model.

With respect to my second suggestion, I found the paper to be improved with respect to the articulation of the equations, but I still think the clarity could be improved. In particular, my recommendation is to present the balance equations for the water stores, Wg and Wc in particular, followed by the expressions for the flux terms. E.g.,

dWg/dt = I1 + I2 – Rr + QL,up – QL,down – Qper – Qas
I =
Qper =

By including only the expressions for the flux terms, I think clarity suffers. The authors have added an expression for the updated storage in the gravity reservoir (Wgu), which helps with respect to Qper. The expression for QL, however, needs to be revisited – should that depend on Wg or Wgu? Also, one of the Qas terms in eq 7 should be Qper.

Lastly, a minor point, the authors refer to the dual-reservoir model in MOBIDIC as being 1D. In fact, it is a dual-reservoir model (zero-D). There is only one soil layer – there is no spatial discretization in any dimension.

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RR by Stephanie Kampf (11 Dec 2014)

ED: Reconsider after major revisions (04 Jan 2015) by Nunzio Romano

AR by Aldrich Castillo on behalf of the Authors (15 Feb 2015) Author's response Manuscript

ED: Referee Nomination & Report Request started (04 Mar 2015) by Nunzio Romano

RR by Anonymous Referee #4 (14 Mar 2015)

RR by Andrew Guswa (17 Mar 2015)

Suggestions for revision or reasons for rejection

Review of
A. Castillo, F. Castelli, and D. Entekhabi, Gravitational and capillary soil moisture dynamics for distributed hydrologic models

Review by Andrew J. Guswa, aguswa@smith.edu

In this paper, the authors present a proof-of-concept for using a zero-D dual-porosity soil-moisture model in lieu of the 1D Richards equation to represent hydrologic fluxes in the vadose zone. The new representation is implemented in the MOBIDIC model. The authors compare predictions of soil-moisture (integrated over the top 50-cm) from a calibrated version of the MOBIDIC model to predictions from a calibrated SHAW model for two sites: one semi-arid (Arizona) and one sub-humid (Mississippi). The paper shows that predictions of the temporal dynamics of average soil-moisture can be adequately captured by either model.

In my original reviews of this manuscript, I found that the two case studies provided a reasonable demonstration of the ability of the simplified model to represent the soil-moisture dynamics as predicted by the SHAW model. I had two primary comments:
1. that the authors also include a performance comparison with a bucket-type model of the vadose zone to demonstrate the conditions under which the dual-porosity model is warranted
2. that the authors clarify the description of the equations that represent the soil-moisture dynamics in the MOBIDIC model

In response to point 1, the authors have focused on the characteristics of the MOBIDIC model that cannot be captured with a bucket model, such as the isotopic differentiation of streamwater and vegetation water. This seems like a reasonable approach to distinguish the dual-porosity model from simpler bucket-type models.

In response to the second point, the authors have included more detail on the stocks and flows for the MOBIDIC model, which improves the communication of the model structure. I have a few remaining specific questions regarding the model formulation:

Equation 4, which is the balance equation for Wp, appears to be missing a sink term – throughfall or some equivalent. As written, Wp can only be depleted by canopy evaporation.

As described, infiltration moves water into the gravity storage, and, from there, it is moved into capillary storage (lines 20-26 on p. 11 is a nice description of this). The authors mention that the rate of infiltration to the gravity storage is limited by Ks, which raises two questions:
- the first is about the rate of infiltration prior to saturation, i.e., when rain first arrives at the surface of a dry soil. In such a case, one would expect infiltration to exceed gravity drainage, but equations 9 and 10 indicate that the infiltration rate will not exceed Ks.
- second is that total infiltration appears to be the sum of I1 and I2. Since each of these rates appears to be independently limited to Ks, then the limit on the total rate of infiltration would be 2*Ks.

I raise these two questions not because I think they limit the value of the paper but rather because I think the issues have the potential to confuse the reader and warrant some discussion.

Lastly, a minor comment: the discussion on p. 18, lines 3-9 is somewhat confusing as to whether it refers to both sites or to just site 1. The comment about the gravelly soil and low water content at saturation is for site 1, but the paragraph and larger section are written for both sites.

In summary, the authors have clarified both the intent and the formulation of the simpler dual-porosity model.

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ED: Publish subject to minor revisions (Editor review) (22 Mar 2015) by Nunzio Romano

Dear Authors,
Just a final editorial comment and then my decision after this additional round of referee appraisals on a contribution that definitely raised some, even contrasting evaluations.

Firstly, Dr. Guswa’s major complaints (which I also supported) seem to be resolved especially by the comparisons against the SHAW model.

I do think that the validity of the “gravity/capillary partitioning” should be further evaluated. This need for improvement cannot be a reason for rejection at this stage, but rather it is a way to open a wider discussion among scientists.
However, the connection between the “gravity/capillary partitioning” and Dr. Beven’s macroporosity debate, albeit intriguing, is definitely questionable, and I would like if this part might be left out or a bit reduced in the final version of your work.

Even the part in the introductory section about the “scale mismatch” (between vertical and horizontal spatial discretizations) should be a bit lowered, at least allowing for various successful applications of RE in many different situations. Solving RE is always demanding and this comes from both physical and numerical problems. Uniform soil profiles are an exception in real life or only computation abstractions. Layered soil profiles are instead the rule and they (with the complex soil layer sequences) do regulate the evolution of vadose zone processes. These facts should perhaps be discussed a bit in the introduction, instead of invoking scale issues.

This stated, the paper can certainly be accepted as it is, but I would like to see the final manuscript before my final decision, allowing also for Dr. Guswa's and my last comments.

Nunzio Romano

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AR by Aldrich Castillo on behalf of the Authors (28 Mar 2015) Manuscript

ED: Publish as is (31 Mar 2015) by Nunzio Romano

AR by Aldrich Castillo on behalf of the Authors (07 Apr 2015)

Short summary

Using two sites with different climates, we show that a 1-pixel version of a distributed hydrologic model that uses a single soil layer with a novel dual-pore structure and employs linear parameterization for infiltration and other fluxes and has comparable performance to a benchmark detailed soil water physics solver in capturing the essential local-scale soil moisture dynamics.