The ISIMIP Groundwater Sector: A Framework for Ensemble Modeling of Global Change Impacts on Groundwater

Reinecke, Robert; Bäthge, Annemarie; Dietrich, Ricarda; Gnann, Sebastian; Gosling, Simon N.; Grogan, Danielle; Hartmann, Andreas; Kollet, Stefan; Kumar, Rohini; Lammers, Richard; Liu, Sida; Liu, Yan; Moosdorf, Nils; Naz, Bibi; Nazari, Sara; Orazulike, Chibuike; Pokhrel, Yadu; Schewe, Jacob; Smilovic, Mikhail; Strokal, Maryna; Wada, Yoshihide; Zuidema, Shan; de Graaf, Inge

doi:https://doi.org/10.5194/egusphere-2025-1181

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https://doi.org/10.5194/egusphere-2025-1181

Preprints

02 Apr 2025

| 02 Apr 2025

The ISIMIP Groundwater Sector: A Framework for Ensemble Modeling of Global Change Impacts on Groundwater

Robert Reinecke, Annemarie Bäthge, Ricarda Dietrich, Sebastian Gnann, Simon N. Gosling, Danielle Grogan, Andreas Hartmann, Stefan Kollet, Rohini Kumar, Richard Lammers, Sida Liu, Yan Liu, Nils Moosdorf, Bibi Naz, Sara Nazari, Chibuike Orazulike, Yadu Pokhrel, Jacob Schewe, Mikhail Smilovic, Maryna Strokal, Yoshihide Wada, Shan Zuidema, and Inge de Graaf

Abstract. Groundwater serves as a crucial freshwater resource for people and ecosystems, vital in adapting to climate change. Yet, its availability and dynamics are affected by climate variations, changes in land use, and excessive extraction. Despite its importance, our understanding of how global change will influence groundwater in the future remains limited. Multi-model ensembles are powerful tools for impact assessments; compared to single-model studies, they provide a more comprehensive understanding of uncertainties and enhance the robustness of projections by capturing a range of possible outcomes. However, to this point no ensemble of groundwater models was available. Here, we present the new groundwater sector within ISIMIP which combines multiple global, continental, and regional-scale groundwater models. We describe the rationale for the sector, present the sectoral output variables, show first results of a model comparison, and outline the synergies with other existing ISIMIP sectors such as the global water sector and the water quality sector. Currently, eight models are participating in this sector, ranging from gradient-based groundwater models to specialized karst recharge models, each producing up to 19 out of 23 modeling protocol-defined output variables. Utilizing available model outputs for a subset of participating models, we find that the arithmetic mean global water table depth varies substantially between models (6–127 m) and shows a shallower water table compared to other recent studies. Groundwater recharge also differs greatly in the global mean (78–228 mm/y), which is consistent with recent studies on the uncertainty of groundwater recharge but with different spatial patterns. Groundwater recharge changes between 2001 and 2006 show plausible patterns that align with droughts in Spain and Portugal during this period. The simplified comparison highlights the value of a structured model intercomparison project which will help to better understand the impacts of climate change on the world’s largest accessible freshwater store – groundwater.

How to cite. Reinecke, R., Bäthge, A., Dietrich, R., Gnann, S., Gosling, S. N., Grogan, D., Hartmann, A., Kollet, S., Kumar, R., Lammers, R., Liu, S., Liu, Y., Moosdorf, N., Naz, B., Nazari, S., Orazulike, C., Pokhrel, Y., Schewe, J., Smilovic, M., Strokal, M., Wada, Y., Zuidema, S., and de Graaf, I.: The ISIMIP Groundwater Sector: A Framework for Ensemble Modeling of Global Change Impacts on Groundwater, EGUsphere [preprint], https://doi.org/10.5194/egusphere-2025-1181, 2025.

Received: 12 Mar 2025 – Discussion started: 02 Apr 2025

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Status: final response (author comments only)

RC1: 'Comment on egusphere-2025-1181', Anonymous Referee #1, 11 Apr 2025

I appreciate the efforts of Reinecke et al, and support the effort to better represent groundwater in ISIMIP - this is a much needed, and long called for effort. But overall this manuscript feels a thin, uncritical, non-exhaustive and somewhat repetitive. This may strongly worded, but it feels more like a paper written quickly after a great workshop rather than a deep effort with longer rumination and iteration.
The manuscript seems thin in that each section seems quick and brief rather than deeply insightful or critical. I think a number of the ideas could be expanded upon with more critique and reflection. For example, when I look at the models in Table 2 compared to the linkages in Figure 4, I was struck by the limited capacity of most models to simulate outputs that would be useful for other sectors. At a basic level, if water use is not even in a model, how is it useful to assess water resources? And nothing to do with groundwater quality or contamination is mentioned in Table 1 so how can this effort be useful for water quality?
Section 4 about unstructured experiments seemed repetitive to other recent articles on uncertainty in the water table depth and recharge including those of co-authors. It also felt thin and preliminary, and frankly uninspiring (in that the models seem to show little consistency) and unsurprising (due to overlap with previous articles).
Examples of it not being exhaustive is that it does not even mention the recent GroMoPo effort that a number of the authors have been involved with (Zipper et al. 2023; Zamrsky et al., 2025). This initiative has compiled hundreds of regional scale model even though line 98 claims to 'integrate currently available groundwater models that operate at regional scale'. Also missing are any mention of linking with global groundwater quality and contamination efforts such as Friends of Groundwater which seems important for the groundwater quality linkage. Finally, I was a recent reviewer of this manuscript by Huggins et al. (again with some of the same coauthors) and am struck that many of the linkages to other sectors would be much better created by taking a more holistic, social-ecological systems approach or at least bringing in insights and data from this approach than the narrow hydrologic approach outline in the manuscript. I strongly implore the authors consider and describe the synergies with these other ongoing efforts so that all these efforts are supported and elevated.
Overall, I am unsure it makes sense to consider or brand this effort as an ISIMIP ‘sector’. My understanding is that in the context of ISIMIP, a "sector" refers to a thematic area of climate impact modeling that groups together models and research focused on a particular domain of human or natural systems affected by climate change. These sectors are broad like Agriculture and Forestry and not really specific components of the water cycle like ‘groundwater’. I suggest the authors consider this framing and whether it is consistent with ISIMIP more broadly. Should groundwater really be treated as a sub-component or cross-sectoral area?
On a related note, I was also confused about what all the things around the outside of Figure 4 are… Is agro-economic modeling really a sector in ISIMIP?
I think the authors could do much more work to make Figure 4 more useful… what are the linkages that are really? how would they be developed? what models would you use? how could this be improved by better incorporating the initiatives mentioned above?
I was also surprised to see that PCR GLOB-WB was not mentioned or included eventhough it has been important to a number of global groundwater studies. I would clarify the recent for this.
Based on the review criteria of GMD….
Scientific significance: Fair (3)
Scientific quality: Poor (4)
Scientific reproducibility: N/A
Presentation quality: Fair (3)
Overall, I think I would focus the article on the idea of the ISIMIP groundwater ‘sector’ and drop section 4 since it seems scientifically inadequate as is, and significantly deepen the discussion and analysis.
References:
Huggins, X., Gleeson, T., Famiglietti, J.S. The open data landscape to study groundwater dynamics in social-ecological systems: A scoping review of global datasets and an aspirational future outlook. ERL https://eartharxiv.org/repository/view/8503/
Zamrsky, D., S.Ruzzante, K. Compare, D. Kretschmer, S. Zipper, K.M. Befus, R. Reinecke, T.Gleeson, et al. (2025) Current trends and biases in groundwater modelling using the community-driven groundwater model portal (GroMoPo). Hydrogeology Journal. doi: 10.1007/s10040-025-02882-7
Sam Zipper, Kevin M. Befus, Robert Reinecke, Daniel Zamrsky, Tom Gleeson, Sacha Ruzzante, et al. (2023) GroMoPo: A Groundwater Model Portal to promote Findable, Accessible, Interoperable, and Reusable (FAIR) practices for groundwater modeling. Groundwater. 61: 764-767 doi: 10.1111/gwat.13343

Citation: https://doi.org/10.5194/egusphere-2025-1181-RC1
RC2:
'Comment on egusphere-2025-1181', Anonymous Referee #2, 30 Apr 2025
This paper provides an overview of the recently established groundwater sector within the Inter Sectoral Impact Modeling Intercomparison Project (ISIMIP). The paper is quite succinct, and for the most part, strikes a good balance of keeping the presentation high level while still being informative. A small amount of analysis is provided in Section 4 to offer an initial glimpse at notable differences in model outputs for water table depth and recharge, but the aim of this paper is to introduce the motivation for groundwater ISIMIP sector, some background on the participating models, and the sector's short to medium term vision. Overall, I think this paper provides a very approachable and well-presented overview of the new groundwater sector. I have some minor comments that could improve the manuscript, but no major criticisms that need to be addressed.
Minor comments:
Section 6 could be improved by adding a few sentences about some potential challenges, technical, logistical, monetary (funding), for realizing the goals of new Groundwater MIP sector.

Something not touched on is that certain models could perform better in specific regions on for specific output variables. There could be value in more directly stating that the Groundwater ISIMIP sector could inform region-specific model recommendations for specific outputs.

L 135: could the authors expand slightly on “functional relationships”?

L 136-138 Groundwater is new to ISIMIP, but are there any notable previous groundwater model intercomparison efforts worth mentioning, even if not global in scale?

L179-189: In addition to using the same forcing data, are there plans to run these models at the same resolution or are outputs going to be scaled to the same resolution as a post-processing step?

Figure 1 & Lines 195-196: “This difference in ensemble WTD points to conceptual differences between the models, which should be investigated further.” Given the authors on this paper have expertise with these models, could there be a couple sentences offered positing as to why there could be such stark differences in the water depth for G3M & CLM compared to WBM and VIC-wur?

Figures 2 and 3: It is my understanding that for this initial comparison the groundwater recharge model results have different forcings (Table A1). I think it would be good to remind the audience of this because the rest of the paper is focused on the forthcoming efforts using the same forcings for the MIP.

Minor spelling comments:
Spelling L150: “and surface water exchange fluxes as upper boundary conditionals without later fluxes”: Lateral missing “al”

Line 275: ISMIP missing I, ISIMIP
Citation: https://doi.org/10.5194/egusphere-2025-1181-RC2
AC1:
'Comment on egusphere-2025-1181', Robert Reinecke, 23 Jun 2025

Please see the attached PDF for a full response to both reviewers, including a track-changed document with proposed changes.

Citation: https://doi.org/10.5194/egusphere-2025-1181-AC1
- EC1:
  'Reply on AC1', Thomas B. Wild, 28 Jul 2025
  Dear Authors,
  Thanks for preparing your thorough revisions in response to the first round of reviewer comments. I am going to pass your manuscript and responses back to reviewers now, but I have a few of my own comments that I would like you to consider addressing at this stage. I have a set of high-level comments, but also some line comments as well.
  High-level comments:
  I suggest further distinguishing the paper’s contribution in the text. I think I am still struggling to discern this paper’s core contribution. I don’t mean to suggest it’s not a contribution, only that I think further clarifying it could help readers. As far as I can tell, the paper does not deliver any protocol, and it also does not really deliver any results. So, is its purpose ultimately to motivate the value of having the groundwater sector in isimip, and describe some of the models and variables that will be looked at, but without detailed protocols or results given? If so, I would suggest stating this much more explicitly. Also, regarding results, I do like have your section is titled to clarify its purpose “4. Unstructured experiments point out model differences that should be explored further”. But earlier in the abstract you refer to ‘initial results’, which I suggest modifying because I think it could lead to misinterpretation about what this paper actually provides. I do actually agree with reviewer one’s original comment that the paper might be better off spending less time showing results from a range of papers/experiments whose points of commonality are hard to discern (section 4), and instead spending more time on the paper’s core contribution to introduce the new isimip-groundwater sector, but I respect that you wish to keep this section, and you already provided rationale in your response to reviewers about why you feel it is important to keep.
  
  Consistency with ‘global change’ terminology and literature. The paper’s title and exposition invoke ‘global change’, but I find that this paper is more aligned with climate change and global hydrology, rather than global change. It is true that some of the key models referenced in your study represent dynamics around land use change, climate change scenarios, and human groundwater abstraction. So, in this sense your study is theoretically aligned with global change, assuming you ultimately conduct experiments that explore these dimensions. But, what I think has the potential to create confusion is that there are many researchers who focus centrally on ‘global change’, and many of them study or consider water resources in various ways, usually focused on human dimensions, economics, etc. I think it will be unclear to those communities how your contribution aligns (or does not) with their efforts, and it could be helpful clarify this in the paper. For example, as I mention in my line comments below, there are researchers who work on groundwater and global change through a hydro-economics lens, focused foremost on the economics of groundwater extraction, but also including some physics-based representations of groundwater extraction. Many of those teams are also directly linked (and internally consistent) with the IAMs that produce the emissions and land use change trajectories (e.g., via ScenarioMIP) that are responsible for feeding climate impacts information into the hydrology models that you are focused on. How will the groundwater sector in ISIMIP relate to those communities and any global change scenarios they produce related to groundwater?
  
  I don’t find that it’s possible for me to reproduce the figures/results you present in the paper. Please provide a code repository that complies with GMD standards.
  
  Line comments (line numbers, when provided, pertain to the tracked changes PDF)
  Abstract
  Line 35. I suggest avoiding normative language like “excessive”, and instead using more specific language (i.e., excessive in what way), if possible, to avoid misinterpretation.
  
  Line 38. “no ensemble of groundwater models has been available”. I think it would be helpful to articulate how you define a groundwater model for purposes of this paper. I assume you are referring to models focused largely on the physical system. But, for example, there are global hydro-economic models that deal explicitly with the economics of groundwater extraction along with some groundwater physics. Are those models excluded? I assume so, because they are not cited.
  
  The sudden discussion of Spain and Portugal in the abstract felt a bit surprising and arbitrary. Maybe rearranging your current text could soften some of the surprise.
  
  It is only at the very end of the abstract that we learn that this entire exercise is really focused on climate impacts. I personally think it could lead to misinterpretations (from those not very familiar with ISIMIP) to feature that only at the end of the abstract, particularly given the title's use of "global change".
  
  Introduction
  “The pressure on groundwater systems intensifies due to the combined effects of population growth, socioeconomic development, agricultural intensification, and climate change, e.g., through a change in groundwater recharge (Taylor et al., 2013; Reinecke et al., 2021).” I find the cited literature to be insufficient. These papers may have referenced these issues or addressed a subset of these issues, but as far as I can tell, the list of papers does not cover the full space. It could be helpful to cite the papers that have addressed these issues holistically, including those that have looked at combined effects of multiple global change factors on groundwater.
  
  Line 71: (1) what is a “large-scale perspective”? (2) You are citing economics as a key issue, which again makes me concerned that your paper actually omits a lot of humans- and economics-focused global groundwater literature. (3) The cited literature is also non-exhaustive.
  
  Line 79: This statement seems like it could benefit from further literature support: “While large-scale climate-groundwater interactions are starting to become understood (Cuthbert et al., 79 2019), current global water and climate models may not always capture these feedbacks as most either do not consider groundwater at all or only include a simplified storage bucket, limiting our understanding of how climate change will affect the water cycle as a whole.”
  
  I suggest better explaining what the role of the global water sector in isimip is, so readers can better understand the full picture and the implications of a groundwater sector.
  
  Sections 2/3: ISIMIP framework/current generation of models
  Here could be a place to consider describing what kinds of models are included versus excluded—like hydroeconomic models and global change models that are more focused on humans and less focused on groundwater physics.
  
  Results
  In Figure 4, I found it confusing that you say you’ll focus on green and orange—and then orange is called ‘focus’.
  
  Vision
  You note that “In summary, the ISIMIP Groundwater sector aims to enhance our understanding of the impacts of climate change and direct human impacts on groundwater and a range of related sectors.” It’s certainly possible I missed it, but I don’t see that this is actually supported/made clear in the paper. What set of experiments will be run that will isolate the impacts of humans on groundwater? What dimensions of human impacts will be looked at, and how? I think this probably just relates to my earlier comment that I’m struggling with wanting more details about what experiments will be done, whereas doing so isn’t necessarily the focus of this paper.
  
  A lot of what you describe is indeed what other communities (including, but not limited, to other isimip sectors) have found about the value of producing ensembles that enable intercomparison. I suggest citing those other communities’ work.
  
  Citation: https://doi.org/10.5194/egusphere-2025-1181-EC1

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Short summary

Here we describe a collaborative effort to improve predictions of how climate change will affect groundwater. The ISIMIP groundwater sector combines multiple global groundwater models to capture a range of possible outcomes and reduce uncertainty. Initial comparisons reveal significant differences between models in key metrics like water table depth and recharge rates, highlighting the need for structured model intercomparisons.


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