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:

1. 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.
2. 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.
3. 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.
4. 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.
5. 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.
6. 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

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.