Herzog et al. Study the longitudinal, vertical, and lateral connectivity of GW and SW in the Dreisam valley. The study is nicely done, generally well described and scientifically interesting. Mainly, I suggest adding some more explanation and discussion to better guide the reader and thus better help her to understand the storyline presented. I generally like the study and have no major comments beyond the constructive suggestions made below.

[1] There are a few editorial changes needed regarding the English. An example from the abstract: “This raises the question on” should be “question of”. Or “By reason of the physically-based” would better read as “Due to the phy…”. Please have another read through the document for these instances.

[2] In the abstract, “model reality” should probably be “model realism” to be more in line with the language used in other papers (e.g. Gharari et al. 2014 HESS; Hrachowitz et al. 2014 WRR; Wagener 2003 HP).

[3] I like the abstract, but could you quantify terms a bit more. E.g. what is a short time scale in the context of this study? How many measurement locations did you consider? Etc. This is all discussed in the manuscript but might make the contributions of the study clearer right away if mentioned in the abstract.

[4] It might be interesting to connect the metrics discussed, developed, and estimated in this study to the metrics (often called signatures) used in other studies. E.g. the recent study on reservoir impact in the UK by Salwey et al. (2023, WRR).

[5] The authors state in lines 69ff: “While such parameter uncertainties are relevant when it comes to obtaining the best model results, they are less relevant if the focus is on process understanding.” I do not agree with this statement. Understanding which parameter dominate system responses, and what preferred values they take when they do so, has long been part of assessing models regarding their physical realism (e.g. Reusser et al., 2009, HESS). So dismissing parameter uncertainty as a simple problem of model performance is really understating the problem. I therefore would expect a discussion of the potential influence of parameter uncertainty on the study outcomes in the conclusions or discussions sections. Even if a more detailed analysis is not feasible in this study.

Late in the paper, the authors stated “The third research question addressed the sensitivities and changes of modelled connectivities in response to the applied stress test scenarios.” Is this question really completely unrelated to parameter uncertainty? I can accept if the authors cannot add this element to this study, but a basic discussion of the potential influence would be good.

[6] I am afraid that I am a bit lost when looking at Figure 5. The super short caption is hardly helping me to understand what I am looking at here. The text discusses gaining and loosing conditions. Maybe making those explicit in the figure would be a start? More info please.

[7] A general comment after looking at the next figure. Can you please make the captions more extensive. It is a bit annoying to have to go through much of the paper to look for abbreviations, variable names, location details etc. to understand figures. Please make the captions much more detailed so that the reader does not have to go through the text to understand the figure content. Or at least tell the reader exactly where to find the info needed to interpret the figure.

[8] Regarding the conclusions. I understand that the authors discuss what they specifically learn about their study region. However, it might be nice to add a short paragraph on what innovations, understanding or questions might be transferrable to other studies. What outcomes are general?

[9] As future work, the authors might want to consider a broader sensitivity analysis which could include both the stress test to the system as well as uncertainty in parameters or other model inputs. That would create a generic framework for analyses of the type presented here.

Review Herzog et al (HESS-D).

I have read this discussion paper with great interest. I do not doubt the science that went into this study. However, the way it is presented is rather unsatisfying. This feeling covers all aspects of the paper, specifically how the methodology and results are presented as well as the interpretation of results and conclusion.

Overall I am a bit puzzled about how I should interpret this study. It starts off with a larger context. But, much more than a ‘very general, first attempt to model SW-GW interactions in the Dreisam valley (L422-424)’ it is not. I am questioning how much we learned ‘to help gain insights into factors to consider in future modeling studies’ (L424). What are these insights specifically and to what extent are the findings from this study transferable to other studies? (and is that not already done, for example, in the larger scale groundwater-surface water models available?).

I regret there is no proper sensitivity analysis done of at least the most important parameter settings that impact the groundwater-surface water interactions. However, I do understand that this is probably not something the authors want to add to their analysis at this stage. I found all conclusions on uncertainties of the simulated leakage, because of the very limited model evaluation, somewhat hard to interpret.  I have several points of concern:

[1] Model concept. There are several aspects that I cannot fully understand. (L118) ‘We used a combination of..’; I am not sure what this means. Are the models coupled in a way, or does one model use the outputs of the other, or did you use a combination of model results of the analysis? Not clear. (L123-124) ‘Surface water fluxes, well extractions, and recharge are added in the form of boundary conditions’. How does this work in Modflow6? As far as I know, you have to translate surface water fluxes to heads and use it as a general head boundary in Modflow. To be able to calculate heads from fluxes there needs to be some calculations using assumptions on river depth and width. What was used here? For wells in Modflow, you can define this as a specific pumping rate or specific head. Recharge is probably used as an upper boundary and as an input flux. Is capillary rise considered as well? Not enough information is provided to understand how the Modflow model is built and how boundary conditions are implemented. (L129-136): ‘the model’ is this referring to the smaller Dreisam model or the bigger model? ‘requires spatially distributed parameters determining’ what kind of parameters? Determining surface and subsurface flows so I am assuming soil parameterization, elevation etc but this is not mentioned anywhere. (L134): as the aquifers are unconfined (at least I did not read otherwise) does it make sense to use specific storage (neglectable of unconfined conditions) compared to specific yields? (L156-157) ‘based on Manning’: I think this needs a bit more explanation (or reference to the original code and/or coupling between surface and groundwater).

(L142-158) A general remark for this whole section is that there is not a clear structure, especially not in this last part. It might help to show a small model conceptualization or flow chart: what goes in (parameterization, forcing) and what comes out (discharge, which is then used to calculate river heads), etc. I truly recommend going through this section again and making sure everything is very clear: which input data and where did you get it from, what is calculated in which model component, and how does it feed into the next model component? A proper understanding of the model setup is crucial to understanding and interpreting results as a reader (also, specifically as no sensitivity analysis is performed on any parameter settings. Then at least be clear and open on how you construct and parameterize your models).

Some additional questions: the RoGeR model was at daily resolution, how about the groundwater model and the river routing? Same for spatial resolution? And vertical discretization of both the hydrological and groundwater models is also not that clear to me. Human interactions are implemented by groundwater extraction only. And is there a return flow to surface water and/or groundwater I cannot follow that either.

(L142-150) More generally, I have some trouble understanding the term ‘leakage’ in this study. As I understand it now, it is used here to describe groundwater drainage and river infiltration. For me, leakage would mean more the unintended movement or loss of groundwater from its natural subsurface flow and not the dynamic interaction between groundwater and surface water that changes due to groundwater pumping. Maybe the terminology is something to verify with a groundwater expert as well.

[2] Evaluation of model results. Similar to the previous I have many questions. More in general I find it hard to understand what ZWL represents. A conceptual figure just showing a connected groundwater-surface water system and a disconnected system might already help.

Another more general comment, in the writing you write ‘validation’ while the heading reads ‘evaluation’. Reading this section I am wondering if validation should not be replaced by evaluation in this section.  L166: ‘We preselect a set of stations’ How did this pre-selection go, or is that what is described next? (if so then use something like ‘to this end’ instead of ‘hereby’). (L167-170)  I cannot follow this. L172: ‘calibrated’: there was nothing on a calibration before.

[3] Stresstest scenario definition It is not completely clear what goes into the first stress ‘changes in groundwater recharge’. Is this driven by climate input only or also by varying soil parameterization? I found the use of scenario somewhat misleading as ‘scenario’ refers to projections and potential future and the analysis is on the recent past (or current climate). So analysis or assessment might be a better term.

Check throughout the manuscript how you refer to these tests. I also read the stress test (without scenario) this heading can then also read the stress test definition.

[4]  validation of zero water levels. The first finding is that simulated groundwater levels (heads or depths?) are underestimated (too deep or too shallow?) but that this does not impact simulated discharge. The underestimation has to do with the parameterization of the river bed. It is not clear which part of the parameterization is impacting the results. Probably the drainage level (aka the boundary condition) is meant here as the river bed resistance (for example) does not impact groundwater heads that much. Results are not well explained here. More in general in the result section (here and other pasts) there are several parts that are redundant and describe methods instead of results, for example :L216, L223-226. L279-271.

[5] L250:  ‘leakage’ is the same as ‘leakage flow’ that was used before in this Alinea. And can you explain why simulated leakage flow is highly uncertain for areas with increasing flows? How did you evaluate the level of uncertainty? L252 “Which conditions they may change’ only considering limited model choices of groundwater pumping and recharge right?

[6] Figure 4: I do not fully understand this figure. What do the violins represent? Green is the reference (and not a green dot) and orange is the well scenario (and not an orange box) (see legend). What are the percentiles representing? What is the threshold? Where do ‘losing’ and ‘gaining’ come from? I can understand this but throughout the whole methodology and result this terminology was not used (but positive and negative leakage).

[7] L335-344: I disagree with the argument for not doing a sensitivity analysis. Also for a general interpretation of the result a sensitivity analysis, and varying parameter settings of a few key parameters, would have been useful to better understand what we learn from this modeling experiment (for example look at what is done in large-scale modeling studies). Also, I am a bit skeptical about the ‘reduce calculation times’ argument. How long does your model run the calculation times do not increase in when you re-run your model for different parameter settings, it is just more work. I would strongly recommend to rewrite your argumentation for not doing a sensitivity analysis or simply don’t bother to explain.

Minor comments

[8] L105-107. In this sentence, it is not clear if ‘baseflow’ is the groundwater that is released to the stream or the constant flow in a stream that comes from the gradual release of groundwater. Also, what follows is confusing ‘the degree of connectivity’; connectivity of what? Also, this suggests you discuss the groundwater discharge to the stream that contributes to rivers’ baseflow. I recommend being as clear as possible on the terminology, and baseflow is a difficult term.

[9] writing in general. I recommend an English language check to correct grammar and common writing mistakes. For example, linking sentences together with ‘and’ where the start of a new sentence would be preferred; referring to previously mentioned aspects with ‘this’ or ‘these’ where it is not always clear where it refers to; misuse of commas and often no use of comma’s where comma’s are needed for the readability. Sometimes the meaning of the sentence also completely changes when a comma is not placed (I had the read some sentences several times to understand a comma was missing). The level of the manuscript will increase significantly if the writing is improved.

L128: To represent both the surface and surface systems

L123: well extractions à groundwater extractions

L127: “The extension of the model domain”à the first part is confusing: the model domain covers.

L129 ‘< comma> as well as’

L131-133: ‘The percolation … ‘: a clear example where commas should be placed or new sentence should start as now it reads as if you sum up recharge and the sum of interflow and overland runoff.

L133: ‘time invariant’: constant or static is a more common way of writing.

L160-161: leave this out.

L162: A direct validation is not possible

L164: If I understand it correctly you compared observed water table heads and streamflow to simulated values. Thus, ‘negative outliers of stream stages …. For gw heads far below the surface only concerns the simulated values? Not clear from the writing.

L245 is slightly more positive (meaning leakage in the natural system is lightly higher)

L255-256: to stay consistent with the unit provided for the area. Also, a unit for leakage flow and specific leakage should be given.

L257 (setting the min…. ); I do not understand this. Is L271 similar (I can understand the latter).

L274 modelled à simulated.

L318: is it, not the other way around? Because of the resolution of your modflow model, you are not able to represent your drainage at a level of detail needed to accurately estimate the infiltration of losing streams. Which results in an underestimation of groundwater heads (or overestimation of depths).

L321-327: this section hints at a hillslope effect but does not explain anything. (it turns in circles).

L328: are weirs and bridges impacting the river bed to such an extent that it will impact your modeling more than, for example, the parameterization you use for riverbed conductance and the uncertainty related to that?

L332: not ‘would be’: a sensitivity analysis will be meaningful (and will be needed to properly interpret the modeling exercise.

L427 “modeled leakage’

L430: I have read this before, but here it says ‘Due to the uncertainties we are not able to investigate the change in the magnitude’. Of course, you can study the changes in magnitude. You are not able to fully interpret the results, not because of the uncertainties but because of the lack of sensitivity analysis and/or observed or more reliable modeled data.