the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 01 Jun 2026
GWSWEX v1.0: a dual-solver 1D unsaturated zone model for mass-conservative groundwater recharge and runoff computation in distributed hydrological modelling
Abstract. The faithful numerical representation of the coupled dynamics between groundwater (GW), the unsaturated zone (UZ) and surface water (SW) remains one of the more persistent challenges of regional-scale integrated hydrological modelling. Physically-based models that solve the Richards equation in three dimensions provide a rigorous description of vertical fluxes, but their computational cost, sensitivity to parameter uncertainty, and tendency to over-emphasise capillary forces at the expense of preferential and non-equilibrium flow regimes limit their suitability as self-contained UZ modules for regional integrated hydrological models, ensemble simulations, and multi-decadal climate-impact studies. Conversely, the conceptual and water-balance bucket models routinely adopted at regional scales rarely retain enough vertical resolution to track the position of a moving groundwater head, to represent capillary rise from the GW into a depleted root zone, or to impose a physically consistent evapotranspiration (ET) stress that responds to the local soil-moisture state. This paper introduces GWSWEX (Groundwater–Surface Water EXchange), a vertically resolved process-based modelling package designed to occupy the middle ground between these two extremes and to act as a UZ coupler between an external GW model and an external SW model within an integrated modelling chain. GWSWEX represents each model element as a layered 1-D soil column, tracks GW elevation, per-layer UZ storage, and SW ponding as prognostic states, and exposes two interchangeable numerical solvers behind a unified Python API: an explicit operator-split bucket-sequence solver with CFL-adaptive sub-stepping, and an implicit mixed-form Richards solver with Picard linearisation and Thomas-algorithm tridiagonal inversion. Both solvers share the same spatial discretisation, the same Mualem–van Genuchten retention and conductivity relations, the same vertically integrated drainable-volume function for the moving groundwater head, and the same atmospheric and lateral boundary conditions. The model is implemented as a modern Fortran 2008 kernel with OpenMP parallelism over elements, exposed to Python via f2py, and configured through a Pydantic-validated user-facing API. Verification against HYDRUS-1D across six soil profiles and two contrasting forcing scenarios, together with an iterated one-at-a-time sensitivity analysis of the empirical parameters of both solvers and a parallel-ensemble computational performance benchmark, demonstrates sub-centimetre to near-centimetre GWH accuracy across smooth and intensive forcing regimes, characterises the physical limits of the layered-bucket abstraction, and examines the computational cost of the model at the ensemble sizes that regional integrated modelling requires.
Competing interests: The contact author has declared that neither of the authors has any competing interests.
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The requested preprint has a corresponding peer-reviewed final revised paper. You are encouraged to refer to the final revised version.
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Preprint
(971 KB)
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The requested preprint has a corresponding peer-reviewed final revised paper. You are encouraged to refer to the final revised version.
- Preprint
(971 KB) - Metadata XML
- BibTeX
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- Final revised paper
Journal article(s) based on this preprint
Interactive discussion
Status: closed
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RC1: 'Comment on egusphere-2026-2941', Anonymous Referee #1, 05 Jun 2026
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AC1: 'Intermediate Reply on RC1', Veethahavya Kootanoor Sheshadrivasan, 11 Jun 2026
We sincerely thank the reviewer for their detailed and constructive comments.
We are currently working on incorporating this feedback to improve the manuscript, and we look forward to providing our detailed point-by-point responses and revised figures in due course.
Citation: https://doi.org/10.5194/egusphere-2026-2941-AC1 - AC3: 'Reply on RC1', Veethahavya Kootanoor Sheshadrivasan, 28 Jul 2026
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AC1: 'Intermediate Reply on RC1', Veethahavya Kootanoor Sheshadrivasan, 11 Jun 2026
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RC2: 'Comment on egusphere-2026-2941', Anonymous Referee #2, 21 Jun 2026
This paper presents a new hybrid model named GWSWEX, which is developed to address the limitations of sophisticated three-dimensional models and simple bucket-type models, enabling groundwater and runoff simulations with satisfactory accuracy and low computational cost. The authors also attempt to evaluate the accuracy and computational efficiency of this newly developed hybrid model by comparing it with a one-dimensional model under specific scenarios. The results reveal that the proposed hybrid model features remarkably less computation time while achieving accuracy comparable to the reference model. Nevertheless, the authors fail to clearly elaborate the rationale behind the comparative experiments, and their review of existing research progress is overly general and merely conceptual without detailed benchmarking against previously developed hybrid models. Even in the comparison with the selected 1D model, the discussion of results inadequately interprets the link between the model’s innovations and its numerical performance, which greatly hinders readers’ understanding of the novelty of this study. It is suggested that the authors elaborate on the innovations of the proposed hybrid model in greater detail, especially clarifying how these innovations contribute to improvements in simulation performance.
Specific comments:
The abstract is overly lengthy with excessive elaboration on research motivation, while lacking descriptions of the innovations of GWSWEX and its specific accuracy metrics. The current version reads more like a project summary rather than a standard academic abstract.
Section 2: As a research article, the model overview section should explicitly specify the advanced techniques or design philosophies adopted in model development, as well as the underlying reasons why the proposed model outperforms existing 3D models and bucket-type models. However, this section mainly describes each component of the model, which fails to meet the academic communication requirements of a research paper.
Line 125: “Two forcing setups probe complementary regimes.” Although the authors state that two complementary experimental schemes are designed to evaluate model performance, the general applicability of these experimental configurations needs further clarification, along with the real-world environmental conditions they represent. If these are idealized numerical experiments, the authors should also specify the general evaluation objectives for adopting these two experimental designs.
Lines 221–223: “In the intensive scenario, both GWSWEX solvers report higher peak ponding and longer ponded duration than HYDRUS-1D in the four simulations” What physical implications do these results carry? Does this observation indicate that GWSWEX yields reasonable or biased outputs? What potential mechanisms could account for such discrepancies?
Figures 3–7: Descriptions of these figures only identify the variables illustrated, without interpreting the physical characteristics shown in the figures or linking these features to the physical mechanisms embedded within the GWSWEX model. The authors are advised to either incorporate observational ground truth data for comparison or thoroughly discuss the physical meaning of the discrepancies between different models reflected in these figures.
Line 482: “Several GW–UZ–SW couplers in the same architectural family warrant specific mention.” It is recommended to relocate this part to the introduction section. Additionally, the authors need to provide a clear justification for selecting HYDRUS-1D as the benchmark instead of conducting direct comparisons with these existing hybrid coupling models mentioned herein.
Citation: https://doi.org/10.5194/egusphere-2026-2941-RC2 -
AC2: 'Intermediate Reply on RC2', Veethahavya Kootanoor Sheshadrivasan, 25 Jun 2026
We sincerely thank the reviewer for their thorough reading and thoughtful constructive comments.
We are currently working on incorporating this feedback to improve the manuscript, and we look forward to providing our detailed point-by-point responses in due course.
Citation: https://doi.org/10.5194/egusphere-2026-2941-AC2 - AC4: 'Reply on RC2', Veethahavya Kootanoor Sheshadrivasan, 28 Jul 2026
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AC2: 'Intermediate Reply on RC2', Veethahavya Kootanoor Sheshadrivasan, 25 Jun 2026
Peer review completion
Interactive discussion
Status: closed
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RC1: 'Comment on egusphere-2026-2941', Anonymous Referee #1, 05 Jun 2026
Review of "GWSWEX v1.0: a dual-solver 1D unsaturated zone model for mass-conservative groundwater recharge and runoff computation in distributed hydrological modelling" 2026.
Summary: This manuscript introduces GWSWEX v1.0, a vertically resolved, process-based one-dimensional unsaturated-zone modelling package designed to improve the representation of coupled groundwater, unsaturated-zone, and surface-water dynamics in regional-scale integrated hydrological modelling. The model aims to bridge the gap between computationally expensive 3-D Richards-equation-based models and simplified conceptual bucket models. GWSWEX represents each model element as a layered 1-D soil column and tracks groundwater elevation, per-layer unsaturated-zone storage, and surface-water ponding as prognostic states. The model includes two interchangeable numerical solvers: an explicit operator-split bucket-sequence solver with CFL-adaptive sub-stepping and an implicit mixed-form Richards solver with Picard linearisation. Verification against HYDRUS-1D, sensitivity analysis, and computational performance benchmarking demonstrate the model’s potential for mass-conservative groundwater recharge and runoff computation in distributed hydrological applications. Overall, the topic is relevant and has potential significance for regional hydrological modelling and groundwater–surface-water interaction studies.
Assessment: The manuscript addresses an interesting and potentially valuable topic by proposing GWSWEX, a vertically resolved 1-D unsaturated-zone model for coupled GW-UZ-SW dynamics. The dual-solver framework and mass-conservative design provide a useful attempt to balance physical realism and computational efficiency. However, several important issues remain in the current version. Therefore, I recommend major revision before the manuscript can be considered for publication. The detailed comments are provided below.
Major Comments:
- The Introduction should be condensed and reorganized to clearly highlight the importance of GW-UZ-SW coupling, the limitations of existing 3-D Richards-equation-based models and bucket-type models, and the specific novelty of GWSWEX.When presenting the results, the authors should improve the readability of figures and tables.
- Some figures contain multiple soil profiles, forcing scenarios, and solver comparisons, which makes them difficult to interpret. The layout, font size, legends, and color scheme should be improved to better highlight the key findings.When emphasizing the novelty of GWSWEX, the authors should provide a clearer comparison with existing models.
- A comparative table is suggested to compare GWSWEX with HYDRUS-1D, MODFLOW-UZF, MIKE SHE, ParFlow, and conventional bucket models in terms of vertical resolution, groundwater-head tracking, capillary rise, mass conservation, computational efficiency, and coupling capability.When validating the proposed model, the authors should consider using field observations.
- The current verification mainly relies on comparisons with HYDRUS-1D under idealized numerical scenarios. Field data, such as observed groundwater levels, soil moisture, runoff, or evapotranspiration, would help demonstrate the practical applicability of GWSWEX.
- The manuscript should clarify which parameters are physically based, empirical, or numerical. Recommended parameter ranges and calibration guidance for different soil types and hydrological conditions would improve the practical value of the model.
- When discussing the nonlinear responses of soil hydraulic parameters, ET-stress thresholds, infiltration parameters, and inter-layer connectivity parameters, the authors should consider relevant recent studies on nonlinear correlations and predictive modelling, including:
https://doi.org/10.1029/2024JB029018.
https://doi.org/10.1029/2023WR036494.
https://doi.org/10.1029/2023WR036203
Citation: https://doi.org/10.5194/egusphere-2026-2941-RC1 -
AC1: 'Intermediate Reply on RC1', Veethahavya Kootanoor Sheshadrivasan, 11 Jun 2026
We sincerely thank the reviewer for their detailed and constructive comments.
We are currently working on incorporating this feedback to improve the manuscript, and we look forward to providing our detailed point-by-point responses and revised figures in due course.
Citation: https://doi.org/10.5194/egusphere-2026-2941-AC1 - AC3: 'Reply on RC1', Veethahavya Kootanoor Sheshadrivasan, 28 Jul 2026
-
RC2: 'Comment on egusphere-2026-2941', Anonymous Referee #2, 21 Jun 2026
This paper presents a new hybrid model named GWSWEX, which is developed to address the limitations of sophisticated three-dimensional models and simple bucket-type models, enabling groundwater and runoff simulations with satisfactory accuracy and low computational cost. The authors also attempt to evaluate the accuracy and computational efficiency of this newly developed hybrid model by comparing it with a one-dimensional model under specific scenarios. The results reveal that the proposed hybrid model features remarkably less computation time while achieving accuracy comparable to the reference model. Nevertheless, the authors fail to clearly elaborate the rationale behind the comparative experiments, and their review of existing research progress is overly general and merely conceptual without detailed benchmarking against previously developed hybrid models. Even in the comparison with the selected 1D model, the discussion of results inadequately interprets the link between the model’s innovations and its numerical performance, which greatly hinders readers’ understanding of the novelty of this study. It is suggested that the authors elaborate on the innovations of the proposed hybrid model in greater detail, especially clarifying how these innovations contribute to improvements in simulation performance.
Specific comments:
The abstract is overly lengthy with excessive elaboration on research motivation, while lacking descriptions of the innovations of GWSWEX and its specific accuracy metrics. The current version reads more like a project summary rather than a standard academic abstract.
Section 2: As a research article, the model overview section should explicitly specify the advanced techniques or design philosophies adopted in model development, as well as the underlying reasons why the proposed model outperforms existing 3D models and bucket-type models. However, this section mainly describes each component of the model, which fails to meet the academic communication requirements of a research paper.
Line 125: “Two forcing setups probe complementary regimes.” Although the authors state that two complementary experimental schemes are designed to evaluate model performance, the general applicability of these experimental configurations needs further clarification, along with the real-world environmental conditions they represent. If these are idealized numerical experiments, the authors should also specify the general evaluation objectives for adopting these two experimental designs.
Lines 221–223: “In the intensive scenario, both GWSWEX solvers report higher peak ponding and longer ponded duration than HYDRUS-1D in the four simulations” What physical implications do these results carry? Does this observation indicate that GWSWEX yields reasonable or biased outputs? What potential mechanisms could account for such discrepancies?
Figures 3–7: Descriptions of these figures only identify the variables illustrated, without interpreting the physical characteristics shown in the figures or linking these features to the physical mechanisms embedded within the GWSWEX model. The authors are advised to either incorporate observational ground truth data for comparison or thoroughly discuss the physical meaning of the discrepancies between different models reflected in these figures.
Line 482: “Several GW–UZ–SW couplers in the same architectural family warrant specific mention.” It is recommended to relocate this part to the introduction section. Additionally, the authors need to provide a clear justification for selecting HYDRUS-1D as the benchmark instead of conducting direct comparisons with these existing hybrid coupling models mentioned herein.
Citation: https://doi.org/10.5194/egusphere-2026-2941-RC2 -
AC2: 'Intermediate Reply on RC2', Veethahavya Kootanoor Sheshadrivasan, 25 Jun 2026
We sincerely thank the reviewer for their thorough reading and thoughtful constructive comments.
We are currently working on incorporating this feedback to improve the manuscript, and we look forward to providing our detailed point-by-point responses in due course.
Citation: https://doi.org/10.5194/egusphere-2026-2941-AC2 - AC4: 'Reply on RC2', Veethahavya Kootanoor Sheshadrivasan, 28 Jul 2026
-
AC2: 'Intermediate Reply on RC2', Veethahavya Kootanoor Sheshadrivasan, 25 Jun 2026
Peer review completion
Journal article(s) based on this preprint
Model code and software
GWSWEX v1.0: Source and Verification Data Veethahavya Kootanoor Sheshadrivasan and Jakub Langhammer https://doi.org/10.5281/zenodo.20268583
Interactive computing environment
GWSWEX v1.0: Source and Verification Data Veethahavya Kootanoor Sheshadrivasan and Jakub Langhammer https://doi.org/10.5281/zenodo.20268583
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Veethahavya Kootanoor Sheshadrivasan
Jakub Langhammer
The requested preprint has a corresponding peer-reviewed final revised paper. You are encouraged to refer to the final revised version.
- Preprint
(971 KB) - Metadata XML
Review of "GWSWEX v1.0: a dual-solver 1D unsaturated zone model for mass-conservative groundwater recharge and runoff computation in distributed hydrological modelling" 2026.
Summary: This manuscript introduces GWSWEX v1.0, a vertically resolved, process-based one-dimensional unsaturated-zone modelling package designed to improve the representation of coupled groundwater, unsaturated-zone, and surface-water dynamics in regional-scale integrated hydrological modelling. The model aims to bridge the gap between computationally expensive 3-D Richards-equation-based models and simplified conceptual bucket models. GWSWEX represents each model element as a layered 1-D soil column and tracks groundwater elevation, per-layer unsaturated-zone storage, and surface-water ponding as prognostic states. The model includes two interchangeable numerical solvers: an explicit operator-split bucket-sequence solver with CFL-adaptive sub-stepping and an implicit mixed-form Richards solver with Picard linearisation. Verification against HYDRUS-1D, sensitivity analysis, and computational performance benchmarking demonstrate the model’s potential for mass-conservative groundwater recharge and runoff computation in distributed hydrological applications. Overall, the topic is relevant and has potential significance for regional hydrological modelling and groundwater–surface-water interaction studies.
Assessment: The manuscript addresses an interesting and potentially valuable topic by proposing GWSWEX, a vertically resolved 1-D unsaturated-zone model for coupled GW-UZ-SW dynamics. The dual-solver framework and mass-conservative design provide a useful attempt to balance physical realism and computational efficiency. However, several important issues remain in the current version. Therefore, I recommend major revision before the manuscript can be considered for publication. The detailed comments are provided below.
Major Comments:
https://doi.org/10.1029/2024JB029018.
https://doi.org/10.1029/2023WR036494.
https://doi.org/10.1029/2023WR036203