I read this manuscript with great interest, knowing the complexity of modeling karst systems and the unique and important role they play in hydrology. 

However, in the current reading of the discussion paper, the novelty of the work is not clear beyond the study area. The introduction states that the study demonstrated that “good results can still be obtained by using few experimental data and time series analysis” (L66) but the research experiment itself does not follow a systematic approach that shows how lesser and lesser data still results in similar results. Is the goal of the study to show that less data can still result in good modeling of a karst system or that it is not necessary to fully couple the surface-water and groundwater systems in a karst system (L65)? The study design does not seem to follow a systematic testing for either approach; rather it applies the GEOframe-Newage tools to the study area. It would be more compelling to compare these results to how the system could be modeling with other couplings or with more (or less) data. 

The contribution is made more difficult to understand because the experiment organization in L70-79 appears to read more as results than hypotheses about what the study will test. The lack of clear hypotheses makes it difficult to understand the broader contribution of this work. 

There are also quite a few qualitative statements that are not for the authors to decide about the quality of the modeling results. For example, on L234, the text states “these values are more than acceptable.” It is not possible for the authors to make this assessment because they do not know what applications the readers may deem are “acceptable” - this is a qualitative statement based only on the authors’ subjective assessment. The results should simply be reported and allow the reader to decide if these results are acceptable for their application or need. Another example is in L254, where the sentence reads, “It is apparent that the model is very good at reproducing the lowest discharges…” This should be changed to read something like, “The model is able to reproduce flows at the lowest discharges…” and then report or reference the accuracy at which the flows can be reproduced.

The results, interpretations, and discussion all relate specifically to the study area and there appears to be no further attempt to generalize or broaden the findings to the wider audience of HESS. There are also few stations used in the analysis, further limiting the interpretation of the results more widely. It would be helpful to frame these sections with a broader audience in mind beyond the study area. 

The conclusions make some interesting points, which actually do emphasize some of the potential novel aspects of the work but they are not emphasized in the manuscript elsewhere. For example, Conclusion #1 and the sentence on L356-357 discuss the insight that the classical approach for delineating basins is not appropriate and a preliminary check on the water balance is needed for karst system, especially if runoff coefficients are high. I am not sure of the novelty of this finding but this is a point that is noted in the title but then not mentioned again until the conclusions. The paper should be reframed with these contributions in mind. I will note again that I am not sure this will improve the novelty of the work but the conclusions are much more clearly stated as to the contribution of the work and it was unfortunate to wait until the end of the paper to understand the potential contributions of this work. 

In summary, there would need to be a reframing and substantial improvement to the paper in the next revision for this to be considered in HESS. For these reasons, I am recommending Major Revision. 

Minor comments. 

Statements made in the introduction without support:

L27: "Karst landscapes cover 15% of the Earth surface"

L34-35: "Both observations (using tracer’s theory) and time series analysis are useful tools..." Examples are given later in the paragraph but this could be an opportunity to cite a more broad set of papers that have dealt with karst systems. 

L238: The table number is missing. 

The manuscript from Shima et al. is dealing with a relevant topic, having potential interest for the readers of EGUsphere. The manuscript is well organized and containing useful information, sufficient data, good modeling efforts. Nevertheless, the goal of their research seems not well focused on novelties. In addition, the english language tremendously suffers for an misleading use of italian sentences which have been translated in english maintaining a classical italian structure.

The starting point is the non-correspondance of hydrographic basins and hydrogeological basins. This finding is a very basic one, everyone knows the difference and it is absolutely non a novelty for the scientific community. In their text, the Authors are presenting this issue as a novelty, instead of presenting the problem in the introduction chapter, using the relevand and abundant literature on this topic. So, the novelty of their manuscript has to be searched in the methods they applied to solve the problem. This is in my opinion the logical approach and I suggest them to completely rewrite the introduction focusing on the problem they want to analyse (how to take into account the overflow in river discharge due to external groundwater flow feeding your basin). By this way, they can easily highlight their findings, mainly related to the useful modeling and methodology they performed during the study.

The second concern is related to the english language. Too many sentences are too long, with secondary sentences included. The uses of commas is limited and this approach cannot be approved by international readers. Please rewrite the entire document using shorter and clear sentences: one concept, one phrase. I strongly suggest the support of a mothertongue for providing a successful review.

Below I provide my specific comments:

- line 1: chains or chain?

- line 12: hydrogeological is one word

- line 16: use by instead of against

- line 18: candel "in this study" (here and all along the text): it is obvious, you are presenting your study...

- line 22: I suggest to avoid "geomorphological", you are not analysing geomorphology here, but simply the hydrographic boundaries of your basin

- line 23: what are the "groundwater recharge effects"? and how they can be "gradually attenuated". The abstract must be explanatory, and this sentence is not

- line 35: use flow, not flux (in english their meanings are different)

- line 81: what do you intend with "features"? Perhaps you intend "characteristics"? Also in this case use of english seems not appropriate

- line 86: Nera is a River, so write Nera River, as Tiber too. Here and through the entire document, please

- line 86: is is. Please check carefully your text, you are 9 authors and we do not expect such mistakes in a submitted manuscript

- line 87: aquifers normally "are drained by" and not "feeding"

- line 90: use % instead of per cent

- line 91: I suggest to use concentrated instead of occurring

- line 92: what do you intend with "linear" springs? Perhaps "streambed" springs?

- line 114: case study? Better "study area"

- line 120: My?

- line 121: a classical mistake in english: "starting from" has to be changed in "since"

- caption of Figure 1: define HRUs in the caption too

- line 140: you are using "runoff" for the "river discharge"; this is confusing, because runoff has a different meaning in hydrology/hydrogeology; I suggest to change with "river discharge", here and all along the text

- line 140: higher is better than larger

- line 145: higher is better than larger

- line 160: you are in a karst domain, so a response in 3 days would be due to karst circuits. Please evaluate this possibility and if you exclude this possibility please explain why

- line 164: the sentence is not coplete or there is some mistakes, please check it

- caption of figure 3: " is still high" is qualitative evaluation, please specify the number (it seems that in this case is lower than 1, so why you think is high?)

- line 194: fate, not destiny

- line 205: using the period 2017-2018, do you not have problems with the reaction to the earthquake? I read some papers indicating a long reaction in discharge in this zone

- line 234: "more than acceptable"? I suggest to limit your evaluation with "acceptable"

- line 250: while at CSA was the contrary happens.... This sentence has no meaning in english, please revise

- line 297: this is the real core of your manuscript and this has to be highlighted both in the discussion and in the conclusion!

- line 316: Visso River? Visso is a gauging station, not a River

- line 317; m3/2 ???

- line 321: where is Pescara spring? Out of your study area? So why you includes this spring in your comments here? I suggest to cancel this reference

- line 324: I did not find a "lack of clear recharge signal" in the reference you cited here. I suggest to cancel this part, not necessary and not included in your study area

- line 326: the sentence is not clear, please rephrase the concept. I know that aquifer recharge is EVER going to springs/river, producing discharge (not runoff)

- line 328: you have not discussed the role of Karst, so I suggest to not include karst in the conclusion

- line 329: hydrogeological, not geohydrological

- line 332: if you have karst, please discuss in the text, not in the conclusion

- line 346: average

- line 351: please add the value demonstrting the good agreement

- line 352: from from

- line 354: hydrographic, not geomorphological

- line 357: your findings are not based on isotopes neither in tracer tests, so why you added in the conclusion?

- line 359: underground basin? Please use hydrogeological basin!