|The paper presents an application of the G3M model for steady-state groundwater flow using output for the WGHM as input. The longer term objective is to fully integrate G3M into WaterGap2 for transient flow simulations, with feedback between G3M and WaterGap2. The paper heavily focuses on this long-term objective, which brings confusion because the paper does not present any results from the fully-coupled model. Any comment or observation on the fully-coupled model therefore remains purely speculative, since the work has yet to be done. I recommend to change the focus to what is actually presented in the paper, which is the steady-state uncoupled application. It would greatly clarify the paper. Section 2.2 should therefore be deleted. The long-term objective could be mentioned in the discussion, as a perspective for future work. It would then be quite acceptable for the authors to justify the choice of using G3M instead of MODFLOW (as raised by reviewers), for tighter integration with WaterGap2.|
I agree with the previous reviewers that the lessons learned from the steady-state application are not clear, compared to previous work. Since this paper does not present any results from a coupled model, arguments about coupling strategy as outcome are not valid. A future paper that actually presents results with the coupled model could address this coupling and assess challenges and issues. The other novel elements mentioned relate to scale challenges and equation solved but they are not unique to G3M. The other gradient-based GW flow models mentioned in the paper also face similar issues. The main outcomes of this steady-state application should be much more clearly highlighted and justified, in the context of previous studies.
The previous reviewers raised concerns about using the fully-saturated groundwater flow equation, as opposed to an unconfined flow equation, for the upper layer. The justification given in response to these concerns are rather confusing and difficult to understand. I think that a short description of the way WCHM treats the soil “compartment” is needed and could help justify the approach. Right now, the reader has to guess what “soil” refers to.
The paper is, in general, not clearly written. The style is often unnecessarily complicated, with very long sentences and repetition. The paper also does not focus on the essential and there is a lot of unnecessary detail. On the other hand, some important information is not presented clearly, such as a short description of how soils are treated in WCHM. Another example is Line 5 on page 8 that states : “It is assumed that there is exchange of water between GW and one river stretch in each 5' grid cell”. If I understand correctly, there is a river for every top cell in the model. If it is the case, it is an important assumption and should be stated much more clearly and earlier, and not “buried” on page 8.
The terminology used is also very confusing for a reader with a hydrogeology background. For example, groundwater is used to refer to both the contained (the “groundwater” or subsurface compartment) and the content (groundwater that flows according to the governing equation). The paper also uses the term “drainage” to represent fluid exchange between the various compartments (subsurface, rivers, soil, wetlands, etc.). The use of drainage is extremely confusing and a better terminology would greatly clarify the paper. Also, hydrogeologists do not use hydraulic head for surface water or surface water table.
The abstract provides a good example of the writing style. First, it is much too long. The abstract should be short, precise and to the point. It should not try to explain everything, such as the difference between reservoir and gradient-based GW models. It also contains sentences that are either unclear of very complicated. For example, this excerpt from lines 19-20 : “We identify challenges linked to the coarse resolution, which necessitates the deviation from established processes in regional groundwater modelling as simulation of unsaturated flow and SW body elevation”. That sentence is both complicated and unclear, and does not inform the reader.
P2. The first paragraph in the introduction (lines 1-20) presents only generalities and should be deleted. The paper should focus on the model right from the start.
P2. Line 34 : what are “macro-scale models”?
P2, Lines 37-38 : what is “the condition of SW”? Be more specific.
P2. Lines 38-39 : the excerpt “Miguez-Macho et al. (2007) linked a land surface model with a two-dimensional gradient-based GW model and computed, with a daily time step, gradient-based GW flow” is one example of unnecessary repetition that does not help the reader. There is no need to repeat that Miguez-Macho et al. used their gradient-based model to compute gradient-based GW flow. Another example of repetition is on page 3, lines 18-19 : “In this study, we present the Global Gradient-based Groundwater Model (G3M) that is to be integrated into the GHM WaterGAP 2” and just a bit further, line 29 is : “G3M is to replace this linear reservoir model in WGHM”. Actually, that last repetition is even more confusing because GHM WaterGAP 2 and WGHM are not even the same model. I have noted several such repetitions that I will not list but that the authors should identify and eliminate.
P3. Line 3 : “GW above the land surface”. Check terminology for more clarity. GW above land surface is no longer GW. Should probably write instead something like : groundwater exfiltration.
P3. Line 12 : The difference with the de Graaf et al paper is that they added “an additional drainage flus to GW drainage”. That explanation is given several times in the paper but it is never clearly described and it is difficult to understand what de Graaf et al. did exactly.
P3, Line 30 : The paper mentions the G3M model and now the G3M-f framework. First, what is the difference between G3M and G3M-f? Second, is it relevant to mention anything else that the model, G3M, since the application here is totally decoupled from a global hydrologic model?
P3, Lines 33-34 : The formulation “We want to find out whether we can use gradient-based groundwater modelling at the global scale, …, to improve estimation of flows between SW and GW … and capillary rise” is rather surprising. Isn’t the working hypothesis that gradient-based models will improve simulations?
P3, lines 36-40 : This is an example of a very complicated and unclear sentence : “Steady-state simulations are a well-established first step in groundwater model development to understand the basic model behaviour limiting model complexity and degrees of freedom, thus providing insights into dominant processes and uncovering possible model-inherent characteristics impossible to observe in a fully coupled transient model.” For example, what is “the basic model behaviour limiting model complexity and degrees of freedom”? What are the “model-inherent characteristics” that can’t be observed in a fully coupled transient model?
P3, lines 40-41 : I don’t know what is meant by “A transient model might obfuscate model inherent trends due to the slow changing nature of groundwater processes.”
P3, line 41 : The following statement is quite bold and I am not sure that I agree : “A fully coupled model furthermore adds complexity and uncertainty to the model outcome”. If it is the case, what do the authors want to develop a fully-coupled model if its outcome will be more uncertain?
P4. Section 2 on Model description. I suggest to reorganize that section because it is not clear. I suggest to start by presenting the governing equations (equations 2 to 7) and then present the global-scale components. All simplifying hypothesis should be clearly stated. The exact input data originating from the global hydrologic model should also be clearly presented.
P4, lines 10-11 : “G³M differs from traditional local and regional GW models”. Is it really the case? I think that the main difference is the scale of application and the use of WCHM output as input.
P4, line 17 : “At this scale, information listed above is poor or non-existing”. It should be reworded. The information contained for smaller-scale (as mentioned just above) is still available at the large (global) scale. You probably mean something else.
P4, line 23 : Not clear what is meant by : Due to the lack of the distribution of hydrogeological properties.
P4, line 33 : I assume that “groundwater boxes” are actually “groundwater cells”. If it’s the case, then “cells” should be only consistently.
P5. Figure 1 could be improved because it is not clear what is shown exactly. Also, what are “virtual layers”?
P5. Equation 2 is not the correct partial differential equation (PDE). The cell volumes (delta_x, delta_y, delta_z) only appear when the PDE is integrated over a 3D cell. Also, writing that the partial differential equation is “a function of hydraulic head gradients” is not rigorously correct. The PDE is derived from applying mass conservation to a representative elementary volume, where groundwater flow is described by its mass flux. The hydraulic head gradients appear because the mass flux is expressed with Darcy’s Law.
P5, lines 20-21 : It is confusing to write that “Inflows in the groundwater are accounted for as…” because the equation is for both inflow and outflow.
P7, line 35 : in the exponential, what is m? what is the value of f?
P8, line 4 : the value of c_ocean is set to 100 m2/day. It appears to be several orders of magnitude greater than other conductances. Based on equation (4), I suspect that this large value is similar to specifying a first-type boundary condition for all cells located on the ocean boundaries. Is it the case?
P9, lines 11-15. The paragraph is not clear.
P12, line 15 : “High conductance values are reached in the tropical zone due to a higher GW recharge”. Is it really the case and not the opposite, i.e. because the conductance is large, groundwater recharge is larger?
P14, line 11 : what is meant by : comparison to local studies suggested a unit conversion error?
P15, lines 18-19 : I don’t understand the sentence : Plotting hydraulic head instead of depth to GW has the disadvantage that the goodness of fit is dominated by the topography as the observed heads are calculated based on the surface elevation of the model.
P17, lines 29-30. I don’t understand the reference to “model decision”. What does it mean?
Table 2 : It is incorrect to write that the first 3 models solve the 3D Darcy equation. They solve a 3D mass conservation equation where the fluid flux is expressed with Darcy’s law. It is also the case for ParFlow, which uses Darcy’s Law to represent fluid fluxes in Richards’ equation.