Enhancing global water cycle representation through two-way coupling of VIC-WUR and MODFLOW 6

Liu, Sida; Nauta, Lisanne; Marinelli, Bryan; Datadien, Karun; Karandish, Fatemeh; Becker, Eileen S.; Yuan, Haochen; de Graaf, Inge

doi:10.5194/egusphere-2026-3448

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https://doi.org/10.5194/egusphere-2026-3448

Preprints

13 Jul 2026

| 13 Jul 2026

Enhancing global water cycle representation through two-way coupling of VIC-WUR and MODFLOW 6

Sida Liu, Lisanne Nauta, Bryan Marinelli, Karun Datadien, Fatemeh Karandish, Eileen S. Becker, Haochen Yuan, and Inge de Graaf

Abstract. Groundwater is a critical component of the global water cycle, sustaining streamflow during dry periods and supporting evapotranspiration where water tables are shallow. However, in many global hydrological models, groundwater processes are represented in a simplified way, which limits the simulation of water tables, lateral flow, groundwater–river exchange, and capillary rise. Here, we present VIC-WUR v3.0, a new global hydrological modelling framework in which the land-surface model VIC-WUR is coupled with MODFLOW 6, a physically based groundwater flow model. Compared with many existing global hydrological models, the new framework explicitly simulates transient groundwater flow and introduces a groundwater-depth-dependent capillary rise parameterisation constrained by soil hydraulic properties.

The coupled model is applied globally at 5 arcmin resolution under naturalised conditions for 1980–2009. The coupled model results are broadly consistent with the large-scale behaviour reported by other global hydrological and groundwater modelling studies. Groundwater recharge falls within the range of existing global model estimates, while VIC-WUR v3.0 tends to simulate comparatively higher recharge. Simulated capillary rise is spatially concentrated rather than spatially widespread, emerging mainly in regions with shallow groundwater where it can support soil moisture and evapotranspiration. Relative to the uncoupled VIC-WUR configuration, the coupled simulation also improves the representation of seasonal river discharge dynamics, particularly under low-flow conditions. These results show that VIC-WUR v3.0 provides a more physically based representation of groundwater–surface water interactions at the global scale and highlights the importance of capillary rise in shallow-groundwater environments. The model offers a new platform for investigating water availability under future human impacts, climate change, and land-use change.

Received: 12 Jun 2026 – Discussion started: 13 Jul 2026

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Sida Liu, Lisanne Nauta, Bryan Marinelli, Karun Datadien, Fatemeh Karandish, Eileen S. Becker, Haochen Yuan, and Inge de Graaf

Status: final response (author comments only)

RC1:
'Comment on egusphere-2026-3448', Anonymous Referee #1, 06 Aug 2026
The article "Enhancing global water cycle representation through two-way coupling of VIC-WUR and MODFLOW6" addresses an important gap in global hydrological modeling. The article requires minor revisions. Please address the comments below, as it would strengthen the article and improve the overall clarity. The following are my recommendations:
The abstract should be formatted as a single paragraph. Multi-paragraph abstracts are not typical for research articles and should be revised accordingly.

Please add keywords after the abstract as it shows the extent and domain of the research article.

The objective, significance, and novelty of the research should be explained in detail in the last paragraph of the introduction. Although the authors have tried to explain it, more details need to be added to make it clearer for the audience.

It is mentioned in the manuscript that the “coupled configuration outperforms the uncoupled simulation at 70.5% of stations and often produces higher discharge, reflecting the role of groundwater in sustaining river flow under water-limited conditions. Improvements are also evident during the wet season, when the coupled configuration outperforms the uncoupled simulation at 81.6% of stations”. Please explain how many GRDC stations were used and what criteria were used for selecting the stations. Were any performance metrics used for the selection of the stations?

It is mentioned in the manuscript that “The observed water tables, averaged over the model period, are largely limited to northern Europe and North America”. Please explain if the coefficient of determination performance is stronger in regions with observed water table data or globally. Conduct an analysis for bias and error distribution, as the coefficient of determination is not enough by itself. Analysis for shallow, intermediate, and deep water tables separately will also add value to the study.

Please add a table that summarizes the differences between VIC-WUR and VIC-WURv3.0.

It would be very beneficial to add a separate section after the conclusion for “Limitations and Future Research”.

Line 141: Change “Eagleson’s capillary flux equation Eagleson (1978)” to “Eagleson’s capillary flux equation (Eagleson 1978).

Please consider increasing the font size for labels in Figure 1, as it is very hard to read. The same applies to the arrows in Figure 2.

Please define all abbreviations when they are first used in the manuscript. Some examples include RIV, DRN, WTD, Q10, etc.

Figure B1 is very helpful, but it is very hard to read due to its size and color. Either split the figure into multiple panels or add a summary of the seasonal behavior.
Citation: https://doi.org/10.5194/egusphere-2026-3448-RC1
RC2: 'Comment on egusphere-2026-3448', Anonymous Referee #2, 28 Aug 2026

The authors present a new global hydrological modeling framework, VIC-WUR v3.0, which couples VIC-WUR with MODFLOW 6 to improve the representation of surface water–groundwater interactions. The manuscript is well written overall, and I have only a few comments, as detailed below.
Section 2.3: It is not entirely clear to me how the modeling framework operates, particularly given the different temporal resolutions of VIC-WUR and MODFLOW 6. VIC-WUR operates at a daily time step, whereas MODFLOW 6 operates at a monthly time step. Regarding the statement, “Within the next daily VIC-WUR run, the MODFLOW-derived baseflow is added to each river reach prior to routing, thereby augmenting surface discharge with groundwater contributions,” does this mean that VIC-WUR-derived groundwater recharge and total surface streamflow are provided as inputs to MODFLOW 6 at the end of each calendar month, and that the resulting MODFLOW-derived baseflow is then provided to VIC-WUR at the beginning of the following calendar month? Please clarify the temporal coupling and data exchange between the two models.
Section 3.1: VIC-WUR and MODFLOW 6 use different grid systems. Could the authors clarify the rationale for using different grids rather than adopting a consistent spatial grid for both models? It would also be helpful to briefly discuss whether the differences in spatial discretization affect the exchange of information between the two models.
Section 3.2: The computational times for VIC-WUR and MODFLOW 6 are reported separately. Since the two models are coupled within the proposed framework, could the authors clarify whether they are run sequentially or concurrently? It would also be helpful to report the total computational time required for a complete coupled VIC-WUR–MODFLOW 6 simulation.
Section 4.2: Line 194 mentions that available large-scale observation-based groundwater datasets (Fan et al., 2013; Jasechko et al., 2024) were used for model evaluation. Could the authors specify which datasets were used and explain how they were incorporated into the evaluation?
Specific comments:
Line 178: “A steady-state solution of MODFLOW provides initial water tables”. Does it refer to a previous work? If so, add reference.
Figure 6: Too small to see the details.
Figure 7: High groundwater support ratios also appear in cold regions, such as Canada and Russia. Please explain.

Citation: https://doi.org/10.5194/egusphere-2026-3448-RC2

Sida Liu, Lisanne Nauta, Bryan Marinelli, Karun Datadien, Fatemeh Karandish, Eileen S. Becker, Haochen Yuan, and Inge de Graaf

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Short summary

We developed a global modelling framework that couples land-surface processes with physically based groundwater flow. A key feature is a groundwater-depth-dependent capillary rise scheme, where upward water movement from groundwater to soil is constrained by water table depth and soil properties. The model simulates recharge, water tables, river exchange, and capillary rise together, improves seasonal river flow, and shows where shallow groundwater may support crop water availability.


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