Improved regional-scale groundwater representation by the coupling of the mesoscale Hydrologic Model (mHM v5.7) to the groundwater model OpenGeoSys (OGS)

Jing, Miao; Heße, Falk; Kumar, Rohini; Wang, Wenqing; Fischer, Thomas; Walther, Marc; Zink, Matthias; Zech, Alraune; Samaniego, Luis; Kolditz, Olaf; Attinger, Sabine

doi:https://doi.org/10.5194/gmd-11-1989-2018

Articles | Volume 11, issue 5

https://doi.org/10.5194/gmd-11-1989-2018

© Author(s) 2018. This work is distributed under
the Creative Commons Attribution 4.0 License.

https://doi.org/10.5194/gmd-11-1989-2018

© Author(s) 2018. This work is distributed under
the Creative Commons Attribution 4.0 License.

Articles | Volume 11, issue 5

Model description paper

|

01 Jun 2018

Model description paper |

| 01 Jun 2018

Improved regional-scale groundwater representation by the coupling of the mesoscale Hydrologic Model (mHM v5.7) to the groundwater model OpenGeoSys (OGS)

Miao Jing, Falk Heße, Rohini Kumar, Wenqing Wang, Thomas Fischer, Marc Walther, Matthias Zink, Alraune Zech, Luis Samaniego, Olaf Kolditz, and Sabine Attinger

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Final revised paper (published on 01 Jun 2018)
Preprint (discussion started on 22 Sep 2017)

Interactive discussion

Status: closed

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

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AC1: 'A Spelling error in the title', Miao Jing, 26 Sep 2017
SC1: 'Comments to gmd-2017-231', Simon Stisen, 29 Sep 2017
- AC2: 'Responses to SC1', Miao Jing, 02 Nov 2017
SC2: 'Executive Editor comment on "Improved representation of groundwater at a regional scale – coupling of mesocale Hydrologic Model (mHM) with OpenGeoSys (OGS)"', Astrid Kerkweg, 05 Oct 2017
RC1: 'Review', Anonymous Referee #1, 27 Oct 2017
- AC3: 'Responses to Anonymous Referee #1', Miao Jing, 11 Nov 2017
RC2: 'Review of Improved representation of groundwater at a regional scale by Miao Jing et al.', Anonymous Referee #2, 13 Nov 2017
RC3: 'Review on “Improved representation of groundwater at a regional scale – coupling of mesocale Hydrologic Model (mHM) with OpenGeoSys”', Edwin Sutanudjaja, 16 Nov 2017

Peer-review completion

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

AR by Miao Jing on behalf of the Authors (26 Dec 2017) Author's response Manuscript

ED: Referee Nomination & Report Request started (04 Jan 2018) by Bethanna Jackson

RR by Olaf Arie Cirpka (15 Jan 2018)

Suggestions for revision or reasons for rejection

This manuscript describes the coupling between the spatially distributed, conceptual hydrological model mHm to the groundwater model OpenGeoSys. The overall aim is to combine the good predictive capabilities of the conceptual hydrological model with respect to river discharge and the ability of a Darvy-based groundwater model to represent groundwater tables. The topic is of high relevance to the hydrological community because the competing fully integrated, pde-based hydrological models are computationally extremely expensive, and have systematic difficulties in representing quick flow components unless the topography and subsurface properties are extremely fine resolved. Bucket-type hydrological models, by contrast, do not simulate hydraulic heads in groundwater at all and a notoriously clumsy when it comes to the calculation of transit-time distributions, which can easily be calculated in classical groundwater models.

\section{Major Remarks}

\begin{enumerate}
\item The English of the paper needs severe polishing. I have attached a corrected version, suggesting chances in almost every sentence. I highly recommend that the authors address a native speaker who surely must be available in an institution the size of UFZ before submitting a revision.

\item Unfortunately, I must say that the coupling scheme is not described very clearly. In other schemes, like the USGS-based modeling framework GSFLOW, a conceptual hydrological model is used to represent surface and soil processes, while the entire groundwater component is handled by the Darcy-based groundwater model. This is not what the authors do. The conceptual hydrological model mHm has its own groundwater storage (denote x_6) that remains untouched by the coupling. While the authors describe to some extent how mHm-based groundwater recharge is transferred to nodal loads in the groundwater model, it is not clear how groundwater leaves the OpenGeoSys domain. The authors somehow mention that OpenGeoSys and mHm have different representations of streams so that I somehow speculate that the OpenGeoSys model assumes fixed hydraulic heads along rivers, but it is not explained how river stages are calculated within or for OpenGeoSys. And it is also not clear whether any comparisons between the baseflow computed by mHm and the groundwater flux leaving the OpenGeoSys domain via river nodes are performed.

\item I see the advantages that the authors want to gain by their scheme, but I do not understand why they don’t explicitly write what they are really doing and why. My understanding is that OpenGeoSys is mainly used as a postprocessor of mHm to obtain groundwater tables, whereas the water budget is calculated by mHm alone. The advantage is pretty clear as the entire stochastic calibration procedure of mHm remains intact and the quality of discharge calculations is not hampered by using a computationally expensive groundwater model for baseflow that requires spatially distributed parameters, which are difficult to obtain. Of course, the Darcy-based groundwater model needs to be calibrated to match heads, but that does not affect the water balance. I assess that using the groundwater model as postprocessor has its intellectual elegance, but then I suggest selling the scheme this way. Otherwise the authors run into the problem why they double-account for groundwater in their two model components.

\item It is also clear that the presented coupled framework cannot overcome difficulties of mHm, or any other similar model, in representing feedbacks of shallow groundwater tables on evapotranspiration, if the shallowness of the groundwater table is caused by lateral groundwater flows. This would require two-way coupling, replacing the groundwater storage of mHm by the Darcy simulator, and joint calibration. Whether the authors will ever implement the two-way coupling, as announced in the outlook of the paper, is doubtful as the modeling and calibration philosophies of the two model components are not particularly compatible.

\item I am not quite happing with the scheme used to map mHm-based groundwater recharge to OpenGeoSys. In the example application, it seems to be no problem, as apparently the groundwater model simply uses a rectangular grid with finer resolution. (At least the authors don’t state otherwise.) In the more general case of mapping gridded fluxes to an unstructured FEM grid, checking for element centroids could be dangerous, and would definitely not ne consistent with the FEM formulation. The consistent formulation would use the weighting functions of the FEM scheme:
$$Q^{in}_i = -\int_{\partial\Omega}W_i(\mathbf{x}) q^{ex}_{mHm}(\mathbf{x}) d(\mathbf{x})$$
in which $ Q^{in}_i$ is the nodal load of node $i$, $\partial\Omega$ is the surface boundary of the FEM domain, $W_i$ is the weighting function of node $i$, $\mathbf{x}$ is the spatial coordinate on the surface, and $ q^{ex}_{mHm}$ is the exchange rate provided by mHm.

\item How does OpenGeoSys calculate river stages? This is completely missing, and knowing discharge along the streams is not sufficient for the groundwater model.

\item The example application takes place in a karstified aquifer. Muschelkalk is a limestone formation with all features of a karst system. This must not remain unmentioned, as the calibrated hydraulic-conductivity values only hold under the assumption of an equivalent porous medium. Of course, the karst features also hamper the calculation of travel times.

\item I am very astonished that the Lower Keuper (\’’ Lettenkeuper \’’ ) has such high conductivity values given the fact that this formation contains thick clay layers and acts as an aquitard.

\item Assuming no-flow boundary-conditions all around the model domain is not reasonable. There must be a lateral groundwater flux leaving the domain in the lowland-part of the aquifer in the Muschelkalk aquifer.
\end{enumerate}

\section{Major Remarks}

\begin{enumerate}
\item See my very extended list of remarks (621 comments, not all of them editorial in nature) in the attachment.

\item I would not use an incomprehensible acronym in the title. My suggestion is: Improved groundwater representation at regional scale by coupling of the mesosclale Hydrological model (mHm) to the groundwater model OpenGeoSys (OGS)

\item I would not report on version numbers in the abstract.

\item I know that OpenGeoSys can be used to compute saturate-unsaturated flow, solute transport, heat transport, and geomechanics. However, in the given context, the THMC capabilities are reduced to groundwater hydraulics alone.

\item Well identification numbers made of ten digits are totally incomprehensible.
\end{enumerate}

Referee Report: PDF

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RR by Edwin Sutanudjaja (28 Jan 2018)

Suggestions for revision or reasons for rejection

I appreciate that the authors have carefully addressed all the concerns I raised in my review. The writing has been improved. Yet, I still have some minor comments and questions that should be considered:

Page 2 Line 17 (P2L17): … Finally …

P2L24-30: This part, starting from the sentence “For example, models ...”, does not flow with the previous sentences. Please consider to rephrase it, Here you may want to consider to discuss several limitations in groundwater model packages (e.g. river, evapotranspiration packages in MODFLOW) for simulating surface water and unsaturated zones.

P3L7: What's wrong with “kinematic wave approximation” for surface flow process? Could you please elaborate? And what did you mean by “1D riil flow”?

P3L27-28: “The process uncertainty decreases as one goes depper into the subsurface storage. In the subsurface storage, hydrological process are relatively well understood …” I am not sure that I can agree with this statement. Please include some references to support this statement.

P4L2: “OGS has been demonstrated its ability of dealing with data uncertainty in groundwater aquifers.” Please give references.

Figure 1: What is K that leaves donward from the groundwater part? How do you include it in your coupling strategy?

P6L22, Equation 22: For clarity, could you please expand the source/sink term qs? I believe that this should consist at least, the recharge C and the baseflow q4.

Section 2.3: While explaining the coupling mechanism, I suggest to include the variable symbols as given in Sections 2.1 and 2.2. An example is given as follows:

P7L4: The basic idea is to feed fluxes generated by mHM, e.g. distributed groundwater recharge C and baseflow q4 ...

P8L14-21, step 3: Please clarify that your OGS mesh actually has a regular shape: square 250 m x 250 m (not an irregular ones as suggested in Fig. 2c).

P9L1: Did you consider groundwater abstraction in your modelling experiment? Please clarify.

P9L7: So, your OGS mesh is rectangular grid: 250 m x 250 m? (not an irregular ones as suggested in Fig. 2c).

P10, Section 3.2: Please include the number of aquifer layers in the OGS model. 11?

P11, Section 3.3: Please clarify to which OGS aquifer layer you force the recharge and baseflow. To the uppermost one only?

P11L21: 1500 mm/month is a very high threshold. Please check.

P12L23: What are K values? Hydraulic conductivities? Or K values in Fig. 1? Please clarify.

P13L26: What is K? The calibrated K values?

P14L8, Figure 8: Please use the same maximum and minimum values on the color scales for all sub-figures in Fig. 8 (page 15) so that they can easily be compared among each other.

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ED: Reconsider after major revisions (29 Jan 2018) by Bethanna Jackson

AR by Miao Jing on behalf of the Authors (02 Mar 2018) Author's response Manuscript

ED: Referee Nomination & Report Request started (11 Mar 2018) by Bethanna Jackson

RR by Olaf Arie Cirpka (29 Mar 2018)

Suggestions for revision or reasons for rejection

This is the second time that I review this paper (which seems to have a somewhat longer history). In general, the paper has improved significantly, and the major coupling approach has become clearer.

1. While the general approach is explained clearly in section 2.3, the normal naïve reader would not think that the approach works the way as described after reading the abstract and even the introduction. From the abstract, it should be clear that the groundwater storage is NOT taken out of mHm; instead fluxes into and out of the groundwater body are evaluated by mHm and used as Neumann boundary condition for the groundwater model. That is not stated explicitly.

2. Likewise, the introduction reads as if the approach was towards two-way coupling, which is not done in the presented method. While I agree that shallow groundwater tables influence actual ET in reality (pae 2, lines 8-12), this is not the case in the chosen approach where ET belongs to mHm which does not know the groundwater table. You really need to explain the problem that you DO solve by the approach, not the problems that you do not address. The motivation should be:

a. Conceptual hydrological models are good to get water balances right (essentially meeting hydrographs), they are quick to run and can be calibrated in stochastic frameworks. But these models cannot handle groundwater tables as that is not a state variable of the models.

b. Pde-based subsurface models are good to get hydraulic heads and travel-time distributions. In the unsaturated zone, they often fail getting quick flow components right because the relevant features are not resolved. Adding pde-based surface models is possible in general, but requires the resolution of fine-scaled features (which are sometimes not known), a tremendous number of uncertain parameters, and takes ages to run. Thus calibrating these models is cumbersome, and doing this in a stochastic framework is computationally unfeasible. Without pde-based surface models, the subsurface models still need boundary conditions at the surface.

c. The general idea of the paper is to perform the hydrological calculations with mHm, including the simplified linear groundwater storage, extract fluxes into- and out of groundwater from mHm, and use this as Neumann BC for a pde-based groundwater model using OGS. By this, mHm is “pimped up” to predict hydraulic heads in groundwater.

d. The model can thus NOT consider feedbacks between land-surface processes and the groundwater table. It is also clear that an aquifer system that inherently acts completely different than a linear storage cannot be simulated right. The approach builds on the assumption that the groundwater storage calibrated within mHm really represents groundwater and is not an alias of other storage mechanisms.

I highly recommend having a discussion on thinkable coupling strategies upfront and justifying the general choice already in the introduction. Do definitely not argue with features that you deem necessary but that you can’t deliver.

3. Running a groundwater model that has only Neumann boundary conditions (recharge, no flow, exchange with streams) can cause non-uniqueness problems. In the confined case, it is obvious that this is an ill-posed forward problem. In the phreatic case it’s somewhat better, but definitely not optimal (specifically not at steady state). I teach my students that they need at least one Dirichlet boundary to fix the head. This should be discussed somewhere. (In the previous version I thought that the rivers either form a fixed-head or leaky boundary conditions).

4. The Pearson correlation coefficient is NOT a metric for the goodness of a fit, as it tests linearity between two data sets rather than identity. The correct metric would be the root mean square error or the mean absolute error. Please exchange all correlation coefficients by RMSE or MAE.

Detailed Comments:

1. Page 1, lines 5-6: “by which grid-based fluxes of groundwater recharge and river-groundwater exchange generated by mHm are converted to fixed-flux boundary conditions of the groundwater model OGS.”
2. Page 1, line 8: “Specifically, the grid-based vertical reservoirs in mHM, including a linear groundwater storage, are completely preserved”
3. Page 1, line 11: “using discharge for mHm and long-term mean of groundwater head measurements for OGS, respectively.”
4. Page 1, line 12 and several times thereafter: Eliminate the correlation coefficient as metric for the goodness of a model fit.
5. Page 1, line 14: “so that the one-way coupled model”
6. Page 1, line 16: “provide an adequate representation”
7. Page 2, lines 8ff.: While this is true, it is not resolved at all by the presented approach. The arguments given here speak against using the method presented.
8. Page 2, line 24: I have no clue what you mean by “parametrization of topographicsl features” in the context of a groundwater model.
9. Page 3, lines 3-5: Confusing arguments. What you really need to say here is that these models use pde's for both subsurface and surface flow, requiring a fine resolution and many parameters. There is also no “unsaturated groundwater”.
10. Page 3, line 6: replace “field-scale” by “hillslope-scale”
11. Page 3, line 7: Warning! Both Reed Maxwell and Ed Sudicky sell their pde-based models with applications on the continental scale. Whether the parameters in these models have any physical meaning is a different story, but you can’t say that the pde-based models are restricted to small-scale applications. Sabine Attinger is co-author on a paper submitted to GMD using TerrSysMP on the scale of the state of Baden-Württemberg.
12. Page 3, line 9: Sorry, but it’s conceptual models that neglect the momentum balance altogether. Darcy’s law is derived from a momentum balance, and the shallow-water equations even more so.
13. Page 3, lines 23-24: Yes, but you can of course use more complex formulations for land-surface processes in pde-based models (see TerrSysMP coupling CLM to PARFLOW). The true difficulty is that, unless you resolve all relevant fine-scale features, you don't get quick flow components right with pde-based approaches. This is an inherent scaling problem. The conceptual hydrological models overcome this difficulty by introducing quick flow out of the blue, which is conceptually impossible in the pde-based approaches.
14. Page 3, line 30: Remove “therefore”
15. Page 3, lines 31-32: If the overall aim was to model regional-scale groundwater flow dynamics, you would choose a stand-alone groundwater model. So that cannot be the overall goal. What you really want is to extend a regional-scale conceptual hydrological model to also predict hydraulic heads (and eventually groundwater transport and a proceeding step).
16. Page 4, line 25: eliminate “by applying the MPR methodology”.
17. Page 4, line 31: “we only listed the equations needed for the coupling”
18. All explanations related to equations: In the mHm part, the variable symbols are followed by untis (e.g., “[mm]”), in the OGS part they are followed by dimensions (e.g., “[L]”). Either way is OK, but it should be consistent. (I’d prefer the dimensions over the units)
19. Page 5, line 11: beta_5 is not a rate, it’s a rate coefficient. And it has a dimension (of an inverse time, which explains why it is NOT a rate but a rate coefficient).
20. Page 5, line 14: same arguments for beta_6
21. Page 6, line 25: In which direction is z oriented? According to equation (10), where z is subtratct from the pressure head, it must be downwards. Please specify.
22. Page 6, lines 26 & 27: “the OGS source code” and “the OGS mesh”
23. Pahe 8, line 1: “OGS is used to simulate groundwater flow for prescribed fluxes at all boundaries” rather than “hydrological processes”
24. Page 8, line 14: “mHM is run independently of OGS to calculate land surface fluxes including exchange fluxes of the groundwater storage”
25. Page 8, line 21: “is transformed to distributed river discharges along the streams and spatially distributed exchange rates between streams and groundwater needed in OGS”
26. Equation 11: What happens if the stream nodes are irregularly spaced?
27. Page 9, line 15: “For each node” rather than “each of the nodes”
28. Equations (12) and (13). This is still not clear. I understand equation (13), provided that W_i(x) is the FEM weighting function. But what exactly is done in equation (12)? W_j(x) seems to be a shape function of the mHM grid, but that is not stated. Actually, my understanding is that the index j relates to an mHM grid cell and that W_j(x) is unity within the mHM cell and zero elsewhere. But that is not written in the text accompanying the equation.
29. Page 9, line 30: “the groundwater model simulates groundwater flow to obtain hydraulic heads. The groundwater model may also be used to compute travel times and solute transport within the groundwater domain, requiring additional boundary conditions; but this is not described in the present paper.”
30. Page 11, line 7: “3.2 Meteorological and surface properties”
31. Page 11, line 12: “kriged onto”
32. Page 11, lines 12ff.: “Potential ET was estimated by the method of Hargreaves and Samani (1985). Other datasets used in mHM are the digital elevantion data ... soil and geological maps, and derived properties such as sand and clay contents, bulk density, CORINE land-cover information…” (geology appeared twice, I have no clue what meta-data you meant)
33. Page 11, section 3.3: It’s not “stratified aquifer model”, it’s “stratigraphic” (as you relate aquifer properties to stratigraphic units, which need not be stratified).
34. Section 3.6 and also 4.1, 4.3: See my remarks related to the Pearson correlation coefficient. Calculate and report RMSE or MAE instead.
35. Page 14, line 25: “the wells” rather than “those wells”
36. Page 15, line 1: “which is due to the unknown local or eve sub-grid-scale properties”.
37. Page 16, line 3: “where the Muschelkalk crops out”
38. Page 16, line 9: Which figure is “the figure”
39. Page 20, line 16: The model was not calibrated in the transient mode. Thus, you did not calibrate the specific yield and the specific storativity, which largely determine the dynamic behavior of hydraulic head.
40. Page 20, lines 30-31: This again is the trap of arguing with something that you do not resolve at all. Your hydrological model does not consider feedbacks of groundwater to surface hydrology at all.
41. Page 22, line 16: Double “derived recharge rates”
42. Page 22, line 25: “capability” rather than “ability”
43. Page 22, line 29: “computational effort” rather than “consumption”
44. Page 22, line 34: “can explicitly be modeled” (word order)

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RR by Edwin Sutanudjaja (06 Apr 2018)

Suggestions for revision or reasons for rejection

I have few minor points/suggestions for the latest version of the paper:

Page 3 lines 3-5 (P3L3-5): I suggest to remove the sentence “Although the methods for subsurface flow …, the approaches for surface flow are inevitably based on some approximations and conceptualizations (e.g., diffusive-wave approximation, 1-D rill flow).”, which is not relevant for the manuscript. I am not sure that the approach used by the authors for modeling surface water flow (e.g. the “Muskingum algorithm”, Eqs. 5-8) and their simplified conceptualization for surface water-groundwater interaction (see my comment for Equation 11) are better than the ones used in some well-known ISSHMs.

Figure 1: Based on your response to my previous review, the term K is always zero. Could you please expand the caption of Figure 1 to explain it? Alternatively, you may want to remove K from Figure 1 such that it avoids the confusion with the symbol Ks used for hydraulic conductivities.

Figure 5: What does the color represent? Please provide the legend. In the caption, please provide your threshold value (used for removing the original stream network).

Equation 11: I think that the fluxes Q4 are always positive. Therefore, in terms of surface water and groundwater exchange, you conceptualize that water can only flow from groundwater to surface water, not vice versa. Please clarify this assumption/weakness in the manuscript.

P14L14-16: Could you please clarify and explain with what you meant by “... a Dirichlet boundary condition is assumed at the northwestern and northeastern edges”? Is it a fixed head boundary condition? It seems that this is inconsistent with your previous sentence stating that “... no flow boundaries are set at the outer perimeters …”.

Figure 7b: Please provide the y-axis label, including its unit (m?)

Figure 8: As suggested in my previous review, please use the same maximum and minimum values (and the same color scales) for all sub-figures so that they can easily be compared among each other.

Figures 12a-d: Please use the same maximum and minimum values/colors for all sub-figures so that they can easily be compared among each other.

Figures 12e-f: Please use the same maximum and minimum values/colors for all sub-figures so that they can easily be compared among each other.

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ED: Publish subject to minor revisions (review by editor) (10 Apr 2018) by Bethanna Jackson

AR by Miao Jing on behalf of the Authors (19 Apr 2018) Author's response Manuscript

ED: Publish subject to technical corrections (10 May 2018) by Bethanna Jackson