Review comment: DECIPHeR-GW v1: A coupled hydrological model with improved representation of surface-groundwater interactions by Yanchen Zheng et al.

This manuscript by Yanchen Zheng et al. presents a new coupled hydrological model called DECIPHeR-GW v1, which has a specific focus on enhanced representation of surface-groundwater interactions. The model couples two previously published models: an HRU-based hydrological model (DECIPHeR, Coxon et al., 2019) and a 2D groundwater model (Rahman et al. 2023). The coupling results in feedbacks between receiving recharge, simulating surface-groundwater interactions and returning groundwater levels and – discharges, of which the latter is then again incorporated in the river routing of the HRU base model. These interactions are all based on three interaction scenarios: groundwater head below bottom of the root zone, groundwater head is within the root zone, and  groundwater head is higher than the topography. The aim of this study was to develop a coupled version, that is computational efficient even at large scales and able to represent the surface-groundwater interactions with high skill.

The calibration and validation was done on 669 catchments and 1804 groundwater wells. While the calibration was solely focused on streamflow data as the objective, the groundwater observations were used to evaluate the internal dynamics of the coupled model. The coupled model improved the simulation results in groundwater-dominated catchments, however strongly human influenced catchments remain challenging. Overall, the coupled model seems to produce robust streamflow simulations thanks also due to the incorporation of the temporal dynamics of groundwater levels and outperforms the original DECIPHeR model in catchments with minor human influence.

The manuscript is well written and easy to follow, the additional extensive supplement provides the reader with even more information, where of interest.

The following points, remarks and questions are mostly raised for further clarifications, no major comments:

Line 165 capital S for section 2.2 (or check to keep consistency)

Line 185 what buffer zone was defined for the demonstrated model?

Figure 3, description, capital S for section 4.2  (or check to keep consistency)

Line 236 50m gridded elevation map mentioned to define HRUs, does this differ from the original DECIPHeR model? Or where there in general specific changes (besides the parameters listed in 3.4) done for this version of DECIPHeR(-GW) presented here compared to the original DECIPHeR model?

Line 243 Citation does not need to be in brackets I believe

Line 244 capital S for section 3.3 (or check to keep consistency)

Line 300-302 could there be a potential pitfall doing the calibration like that?

Line 325 what is the benchmark model?

Line 324 Citation does not need to be in brackets I believe

Line 325 was the national calibration done on top of the catchment calibration, or both separate and the parameter values saved for the specific use of the model (e.g. national vs catchment runs)?

Line 357 any educated guess what are the driving factors are in the model for the overestimated streamflow locations? Or how they could be changed to include for example the waste water discharges mentioned (or other human influences)?

Line 365 Would there also be an option to not use equal weights? E.g. including a sort of ratio weight for different catchment sizes included in the national calibration?

Line 405 is the KGE of 0.85 referring to the model that after the national calibration or the catchment only? And how would they differ (also in relation to the benchmark model)?

Line 410 could there be structural components that could be added that represent the human influences? (maybe more for a future study)

Title: DECIPHeR-GW v1: A coupled hydrological model with improved representation of surface-groundwater interactions

Summary

This study presents a coupled hydrological modeling framework integrating the DECIPHeR land surface hydrological model with a 2D groundwater model. Applied to 669 watersheds across England and Wales, the coupled model (DECIPHeR-GW) demonstrates improved streamflow simulations, particularly in areas with strong groundwater-surface water interactions.

DECIPHeR-GW features an HRU-based structure that feeds a gridded groundwater model, allowing for dynamic water exchange based on water table and root zone elevations. The model incorporates six key parameters for stochastic calibration (including soil and aquifer properties) and enables simulations across large domains.

Evaluation against observations shows enhanced temporal variability and streamflow magnitude compared to the uncoupled model. While generally successful, the study acknowledges challenges in watersheds significantly impacted by human activities. 

Positive Aspects

• The manuscript shows great clarity and organization, making it highly readable and accessible. The manuscript effectively guide the reader through the study's objectives, methodology, results, and discussion.
• The presentation of results is good, with a clear and concise narrative that effectively conveys the key findings. The authors interpret the results, providing insightful discussions on their implications and limitations.
• The figures and tables are well-designed and informative, effectively illustrating the key findings and supporting the conclusions. 
• The supplementary information is also valuable and well-presented, providing important details and supporting data that helps in the overall understanding of the study.

General Comments:

• The manuscript presents a coupled land-surface and groundwater model. While the importance of incorporating groundwater is recognized, a more focused research question is needed. The authors should clearly articulate how their approach differs from existing coupling methods, highlighting the novelty of their model. Additionally, a discussion on the positioning of their model within the spectrum of simplified to physics-based groundwater representations is needed.
• ​​The manuscript emphasizes the model's scalability. However, a more detailed discussion on the potential scale mismatch between the regional land surface model and the large-scale groundwater model is required. Specifically, the authors should clarify what type of  groundwater flow represented at the 1 km grid scale, considering the local-scale flows discharging into streams. The manuscript should address how these different scales are reconciled within the model.
• The manuscript highlights computational efficiency as a key advantage of DECIPHeR-GW. A more thorough discussion on the model assumptions that contribute to this efficiency is needed. The authors should explicitly state which processes are simplified or neglected in both the land surface and groundwater components. A comparison of the model's assumptions to those of other computationally expensive models, particularly those that incorporate fine-resolution environmental data and capture land surface heterogeneity, would provide valuable context.

Specific Comments

• Abstract:

[19] A more specific description of the catchment characteristics would help the understanding of the study area.

[20] Please specify the variable being analyzed in this context.

[23-24] The abstract currently does not explicitly establish a strong connection between computational efficiency and large-domain simulations as a significant challenge in traditional land surface modeling. Consider incorporating some of the performance measurements described in the Discussion.

- Consider including a brief statement in the abstract regarding the spatial resolution of both the land surface and groundwater components of the coupled modeling framework to provide further context for the reader.

• Introduction:

[46-49] It would be beneficial for the authors to explicitly articulate the specific aspects of groundwater representation in existing models that are challenging or form the basis of their research hypothesis.

[80-86] A dedicated section discussing the novelty of the proposed approach would add more to the relevance of the study. This section should clearly differentiate the current methodology from previously mentioned modeling approaches, highlighting the unique contributions and advancements of the presented work.

- The inclusion of a dedicated paragraph discussing the scales of the modeling framework is recommended. This paragraph should address potential scale mismatches between the land surface and groundwater components, and how these differences are addressed within the model.

• The DECIPHeR-GW model

[93-101]

To enhance clarity, it would be beneficial to include a more detailed description of the HRU construction process. This would clarify how the domain is discretized and how this discretization may influence the representation of key hydrological processes.

When referring to "previous studies" in line 97, please specify whether this refers specifically to the DECIPHeR model or to land surface models in general.

In line 101, it would be helpful to elaborate on the specific requirements for large-scale simulations, as defined by the authors.

[106-113]

While other large-scale coupled models can be computationally expensive due to the inclusion of detailed processes (such as vertical water movement), it is unclear how DECIPHeR-GW balances computational efficiency with process representation.

If computational demands and input data requirements are reduced, it is essential to clearly describe which hydrological processes are simplified or omitted, and how these simplifications are compensated for through the calibration process.

[120] Given that the variable Qex in Figure 1 may represent recharge rates exceeding infiltration capacity, it is important to discuss whether the model considers the potential for saturated overland flow.

[147] Does the hydrological model allow for two-way interactions between river routing and the HRUs, enabling water from the river to contribute to aquifer recharge?

[154] Please provide further details on how the parameterization of the groundwater grid is connected to the characteristics of the overlying HRUs. Does each HRU has a set of soil parameters and those are weighted average to parametrize the groundwater grid?

[187] How are the HRUs over the buffering zone updated within the model if they belong to a different catchment?

[191]

How are the systems of watersheds connected, considering that each watershed has its own groundwater model?

Is the connection solely through river routing, or do the buffering zones of adjacent watersheds also interact or overlap?

• Model implementation and evaluation across England and Wales

[202-205] To enhance clarity and reproducibility, a detailed description of the meteorological, soil properties and elevation data used in the study would be beneficial. This should include information on the temporal and spatial resolution of the data, as well as a list of the variables used to parameterize the model.

[244] The manuscript would benefit from a more detailed description of the spatial resolution and configuration of the HRUs. The inclusion of a figure illustrating the HRU distribution within the study area would significantly help.

• Results

[358] To further strengthen the analysis, it would be valuable to quantify the correlation between human activity and the model's performance. If these specific catchments are not being monitored, it is important to discuss the potential implications for the calibration process, particularly with regard to the representation of human-induced water abstractions.

Figure S8 of the supplementary information appears to be missing units for surface water abstractions, groundwater abstractions, and wastewater discharges. 

[370] The inclusion of a figure showing the temporal mean water table elevation for the study area would provide insights into the spatial and temporal variability. This would allow for a visual assessment of the water table's consistency with the expected topographic trends.