Representing lateral groundwater flow from  land to river in Earth system models

Liao, Chang; Leung, L. Ruby; Fang, Yilin; Tesfa, Teklu; Negron-Juarez, Robinson

doi:https://doi.org/10.5194/gmd-18-4601-2025

Articles | Volume 18, issue 14

https://doi.org/10.5194/gmd-18-4601-2025

© Author(s) 2025. This work is distributed under
the Creative Commons Attribution 4.0 License.

https://doi.org/10.5194/gmd-18-4601-2025

© Author(s) 2025. This work is distributed under
the Creative Commons Attribution 4.0 License.

Articles | Volume 18, issue 14

Model description paper

|

29 Jul 2025

Model description paper |

| 29 Jul 2025

Representing lateral groundwater flow from land to river in Earth system models

Chang Liao, L. Ruby Leung, Yilin Fang, Teklu Tesfa, and Robinson Negron-Juarez

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Final revised paper (published on 29 Jul 2025)
Preprint (discussion started on 30 Oct 2024)

Interactive discussion

Status: closed

RC1: 'Comment on gmd-2024-178', Anonymous Referee #1, 05 Dec 2024

The paper details a study of preliminary implementation of lateral groundwater flow in the land component of the E3SM model. The goal of the model developments is to allow for subsurface flow along hillslopes to rivers. The formulation of the model allows for water table depth variation such that the slope of the groundwater table may vary in time in response to water infiltration across the land model grid cell and the elevation of water in the connecting river channel segment. Figure 9 of the manuscript highlights the large discrepancy between the default (current) model treatment of groundwater elevations and the observed spatial variability of water table along a hillslope. The ability to more realistically simulation water table depths and seasonally varying lateral groundwater fluxes has the potential to significantly improve the representation of the terrestrial hydrological dynamics in the E3SM model.
The manuscript is well written and clear. I do not have major revisions to request, however, I believe the paper’s value could be enhanced with additional details regarding the observed differences in LGFs between some of the cases. For example, case 9 has a high LGF raising the elevation of the downslope boundary would intuitively seem to reduce the gradient of the WTP. However, Figures 9 and 10 show that the water rose significantly and apparently steepened. Is the higher LGF only in response to the increase in water table slope or also due to the thickness of the zone discharge at the river or due to more flow in the shallower soil layers which may have higher Ksat? Is there a strong transient response in this case for the WT to rise?
The plotted slope for case 6 on Figures 9 and 10 is confusing. The plotted slope looks much steeper than all other slopes, but the case is supposed to be the surface slope. On Table 4 the slope is listed as 0.07 which is identical for the other cases.
Is it possible from the analysis to quantify how including LGF would impact river discharges across a basin seasonally? Or is the most significant potential change related to the discussion of soil moisture and hydrological feedbacks on plants.
Is there seasonal observational data that can be shown as well? Or is the actual groundwater table at the modeled site truly time invariant. If so, why is the model seasonally responsive but the actual system is not? Can the authors provide any discussion on why all of the modeled cases have WTD much greater than the observed? Is this related to the selection of Ksat and/or the vertical infiltration values?
Minor comments:
Line 91: WTD not defined yet.
Line 291: “Case 6 produced the largest WTD ( 19.9m) due to its large LGFs (caused by high WTG, 0.07)I” Can more detail be provided? The WTG for case 6 is the same for 3, 7, 8, 9, and 10.
Figure 10: the title at the top says February but the results are for August.

Citation: https://doi.org/10.5194/gmd-2024-178-RC1
CEC1:
'Comment on gmd-2024-178 - No compliance with the policy of the journal', Juan Antonio Añel, 07 Dec 2024

Dear authors,

Unfortunately, after checking your manuscript, it has come to our attention that it does not comply with our "Code and Data Policy".

https://www.geoscientific-model-development.net/policies/code_and_data_policy.html
First, after checking the Zenodo repository we have seen that it does not contain an E3SM version. Moreover, you have archived part of your code on GitHub. However, GitHub is not a suitable repository for scientific publication. GitHub itself instructs authors to use other long-term archival and publishing alternatives, such as Zenodo. Therefore, the current situation with your manuscript is irregular. Please, publish your code in one of the appropriate repositories and reply to this comment with the relevant information (link and a permanent identifier for it (e.g. DOI)) as soon as possible, as we can not accept manuscripts in Discussions that do not comply with our policy. Also, please include the relevant primary input/output data. Also, you must include a modified 'Code and Data Availability' section in a potentially reviewed manuscript, containing the DOIs of the new repositories.
I note that if you do not fix this problem as soon as possible, we will have to reject your manuscript for publication in our journal.
Juan A. Añel

Geosci. Model Dev. Executive Editor

Citation: https://doi.org/10.5194/gmd-2024-178-CEC1
- AC1: 'Reply on CEC1', Chang Liao, 09 Dec 2024
  
  Hi, editor,
  Thank you for bringing our attention to the issues with the data and code section. After reviewing, we have revised the manuscript and updated the data and code-sharing practice. We will replace the existing Open research with the following content in the revision.
  The data and code used in this paper are available from Zenodo https://doi.org/10.5281/zenodo.14003482.
  The HexWatershed model used for the hillslope definition can be installed as a Python package (https://doi.org/10.5281/zenodo.6425880) Liao (2022a).
  The E3SM model with the hillslope-based subsurface lateral flow capability is available at: https://doi.org/10.5281/zenodo.14338209.
  Thank you.
  
  Citation: https://doi.org/10.5194/gmd-2024-178-AC1
RC2: 'Comment on gmd-2024-178', Alexandre Gauvain, 31 Mar 2025

The manuscript by Liao et al. presents the implementation of a lateral groundwater flow model coupled with a component of the E3SM model. The objective of the model developments is to account for groundwater flow along hillslopes down to rivers. The hillslope definition is based on watershed delineation at subgrid spatial scales, and a physics-based lateral groundwater flow model is implemented using Darcy's Equation. The model is coupled with the land component (ELM) and river component (MOSART) in the Energy Exascale Earth System Model (E3SM). Tests are conducted on the hillslope definition, lateral groundwater flow, and sensitivity using different configurations, with the model's application demonstrated in the Amazon basin.

The main results show that the subsurface processes are significantly improved using this hillslope approach. Figures 9 and 10 clearly demonstrate the benefits of the implementation compared to Case 1 (without the hillslope method).

The manuscript is well-written and easy to read. No major revisions are required for this manuscript. However, I believe the manuscript could be further improved. First, some figures could be enhanced for better visibility (see minor comments below). It could also be useful to clearly identify the domain of applicability of this model (e.g., shallow groundwater systems, etc.).

Moreover, the model introduces a subgrid heterogeneity definition to resolve lateral groundwater flow (LGF), and the analysis of results has primarily been conducted on an averaged state of the system. It is unfortunate to present a model with hillslope-based resolution without offering an analysis of the results at the same resolution. For example, adding a spatial comparison of simulated river discharges (and/or the contribution of LGF) across the domain (Figure 5) would have provided a more in-depth analysis of the results. Is there observed discharge data available for this domain to compare with the model results?

Also, it would be interesting to understand how the model handles a hillslope domain spanning multiple grid cells of the land surface model (LSM).

Minor Comments

Introduction

- Lines 35–38: What about the following reference : J. Marçais, J.-R. de Dreuzy, J. Erhel. (2017) Dynamic coupling of subsurface and seepage flows solved within a regularized partition formulation. https://doi.org/10.1016/j.advwatres.2017.09.008. Upon reading your manuscript, the description of your model reminded me of this reference, which is not cited in your work.

- Figure 1: The color of the left hillslope appears more yellow than orange, as mentioned in the caption. This could be confused with other orange polygons in the figure.

Method Description

- Line 91: What does WTD stand for? I understand that it means "Water Table Depth," but it is not clearly stated. Please define it earlier in the text.

- Line 150: This sentence might be better placed in the legend of Table 1, or the sentence could be adapted to explicitly reference Table 1 (Table 1 is not referenced in the text).

- Line 168: Could you provide more details to define K0, K1, and K2? Additionally, using the symbol "K" for shape parameters may be confusing, as it is commonly associated with hydraulic conductivity. Consider changing the notation for the shape parameters.

Model Application and Evaluation

- Figure 4: Is it possible to add the watershed delineation in this figure? This would help to better situate the results shown in Figure 5.

- Figure 8: The placement of the zoomed-in plot makes the figure appear cluttered. Moving the zoomed-in plot outside the main figure could make the visualization clearer. Same comment for Figures 9, 10, and 11.

- Line 270-308: Clearly mention in the text, at the start of paragraphs 3.2.3 and 3.2.4, that the modeled data (LGF, WTD and WTG) presented are averages.

- Figure 10: The title of the figure mentions data presented for the month of February, whereas the legend indicates it is for the month of August.

Citation: https://doi.org/10.5194/gmd-2024-178-RC2

Peer review completion

AR: Author's response | RR: Referee report | ED: Editor decision | EF: Editorial file upload

AR by Chang Liao on behalf of the Authors (22 Apr 2025) Author's response Author's tracked changes Manuscript

ED: Publish as is (26 Apr 2025) by Lele Shu

AR by Chang Liao on behalf of the Authors (28 Apr 2025) Manuscript

Short summary

Understanding horizontal groundwater flow is important for understanding how water moves through the ground. Current climate models often simplify this process because they do not have information about the land surface that is detailed enough. Our study developed a new model that divides the land surface into hillslopes to better represent how groundwater flows. This model can help improve predictions of water availability and how it affects ecosystems.