Modelling river-sea continuum: the case of the Danube Delta

Ferrarin, Christian; Bellafiore, Debora; Paladio Hernandez, Alejandro; Dinu, Irina; Stanica, Adrian

doi:10.5194/egusphere-2025-606

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https://doi.org/10.5194/egusphere-2025-606

Preprints

24 Feb 2025

| 24 Feb 2025

Modelling river-sea continuum: the case of the Danube Delta

Christian Ferrarin, Debora Bellafiore, Alejandro Paladio Hernandez, Irina Dinu, and Adrian Stanica

Abstract. Understanding water transport and circulation in coastal seas and transitional environments is among the key topics of oceanographic and climate research, as well as recognizing the role of the land-sea interface. The Danube Delta represents a natural laboratory for river-sea hydrodynamic modelling due to its complex morphology and being subjected to several natural and anthropogenic stressors. In this work, we present the results of the SHYFEM finite element hydrodynamic model application to the whole river-sea continuum of the Danube Delta region. The model was run for several years to characterize: 1) the water discharge distribution among the river branches, 2) the general hydrodynamic characteristics of the coastal region of freshwater influence, 3) the transport time scale of the Razelm Sinoie Lagoon System, and 4) the processes driving the river-lagoon-sea interconnections. The unique numerical description of the transport and mixing in the different water bodies of the Delta (river branches, channels, lagoons and coastal sea) may be used to provide the scientific basis to assess the impact of human activities and to design efficient management choices. Indeed, we used the modelling framework to evaluate the effect of reconnection (restoration) measures in the Razelm Sinoie Lagoon System designed to improve hydrological connectivity.

Received: 10 Feb 2025 – Discussion started: 24 Feb 2025

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Journal article(s) based on this preprint

03 Dec 2025

Modelling river-sea continuum: the case of the Danube Delta

Christian Ferrarin, Debora Bellafiore, Alejandro Paladio Hernandez, Irina Dinu, and Adrian Stanica

Ocean Sci., 21, 3291–3310, https://doi.org/10.5194/os-21-3291-2025,https://doi.org/10.5194/os-21-3291-2025, 2025

Short summary

Christian Ferrarin, Debora Bellafiore, Alejandro Paladio Hernandez, Irina Dinu, and Adrian Stanica

Interactive discussion

Status: closed

RC1:
'Comment on egusphere-2025-606', Anonymous Referee #1, 09 Jul 2025

The manuscript “Modelling river-sea continuum: the case of the Danube Delta”, by Ferrarin et al., describes a new model configuration for the river-lagoon-sea interconnections in the Danube Delta Region. The model is first validated under current conditions and then used to explore the temporal and spatial variability of oceanographic parameters and three alternative reconnection options.

Evaluation: overall the manuscript covers and interesting topic in a region and type of system that could be extrapolated to other similar systems. The manuscript also does a good job (through visuals) at showing the spatial and temporal variability of oceanographic parameters. However, there are questions about the datasets used, the validation of the model results and the interpretation of results could be strengthened. The manuscript would benefit from some reorganisation since the findings and the implications are difficult to follow beyond the focus on the local issue. The scientific questions and relevance are not clearly articulated in the Introduction and some of them appear further down in the Methods. New results are presented in the Discussion and on the other hand the results of the study (both those in the result section and those in the discussion section) are not put into the broader context and remain descriptive. Perhaps the validation should go in supplementary materials and the result figures currently in the discussion (11-14) could be presented in the Results section highlighting how they relate to current understanding of this type of systems. The Conclusion restates the local findings and should be strengthen by highlighting why or how this piece of work is of general interest.
Specific issues that would be good to address include:
The modelling system and Numerical experiments. Significant details on the datasets and the modelling system need to be included without these details it would be difficult to reproduce the results and to evaluate the validity of the numerical modelling: What is the bathymetric resolution of the different datasets? In coastal environments the bathymetry has a non-negligible impact on the obtained results. What is the temporal resolution of the Black Sea lateral boundary condition? Some of the datasets are only available upon request so it will be challenging to reproduce these results. What is the temporal resolution of the model outputs?
The numerical experiments: it is not clear for which period are the what-if scenarios run (L126). Please clarify whether it is 2015-2019 or 2018
L123 highlights why this study matters and yet it only appears here. This should we moved to the Introduction section
L131 you refer to four datasets not three
The model validation: my main concern is the lack of validation data in terms of the water division in the lower parts of the delta river network and salinity on coastal areas.
Fig 2. What are the red circles and the black diamonds in Fig2b and 2c? There is no legend for those. There might be a typo on the x and y axis of Fig 2b and Fig 2c, shouldn’t it be “m^3/s” instead of “m”?
The validation data for sea level and sea temperature is relatively far from the zone of interest where the manuscript focuses. Why is the validation for sea level done for mean sea level and not at the time the measurements are collected? Does the validation improve when done for mean daily values or is it that the model output are daily values? This is not clear. Validation of sea level is quite poor at times (e.g. differences of over 0.1m for 0.7m variation in sea level Fig.3). The reasons (L171) for the poor fitting should be better explained/explored, does the bathymetry play a role as well in this? Why are results not presented as scatter plots?
Similarly for the sea surface temperature, why is the validation for Constanta not shown in Fig. 4 while data is available? Temperature is usually a minor part in estuarine dynamics with most of the density gradient being driven by salinity differences. However, here the validation is presented in terms of temperature and yet you focus later on salinity results, are there no data available in terms of salinity? There are 6 satellite control SST points, however a single set of statistical parameters are given in L182. What are the implications of comparing averaged daily values with specific timings in the satellite data?
I think that if this validation section could go into supplementary materials to streamline the paper.
Table 1 could include the statistical analysis for all the validation datasets (i.e. also include sea level and satellite SST) and not just temperature for monitoring stations.
Water division: water division is only validated in the upper part of the delta, and only temperature values are shown for a point in the vicinity of the outlets. Without discharge measurements in the network or temperature or salinity in the outlets it is difficult to assess how well or bad the model is performing. The water division is likely to influence the coastal dynamics and the plumes observed. These results also don’t seem to be further explored in the paper.
Spatial and temporal variability of coastal dynamics
L216 please define the scales of variability you refer to in the text. Here it is implicit you are considering seasonal scales as per your figure 6.
L234 there is no correlation calculated here, suggest change “are highly correlated” to “can be explained by”
River lagoon connectivity:
Over which period are calculated the averages and estimates in L241 to L249?
what causes the difference in WRT in the different years in L253 to L256? Are the weather regimes very different? Is it the different river discharge? It would be good to further explore the reason for these differences.
L259 The sense of the gradient and the number of days seem reversed.
Assessment of lagoon-sea reconnection solutions
L270 please indicate for which period where the what if scenarios run.
You could combine Fig. 8 and Fig. 9 and present current Fig. 9 as differences with respect to the reference run instead of absolute values. That way the differences would be more easily perceived.
L277-282: please point to table 2
Fig 10, please change the colours they’re not very colour blind friendly particularly sol.A and sol. B
Discussion The discussion further presents new results (there are 4 results figures in this section) and while they are interesting, they’re not put in context or related to similar systems. I believe the discussion would be strengthened if these new 4 figures were moved into the results section and the results were further discussed and contextualised in the discussion. It is difficult to see what is of interest beyond the regional area and what learnings could be taken to other regions.
L305-309: I believed this is the first time that stratification is mentioned in the manuscript. So far, the analysis has been limited to the surface. Beside there is no reference to the different wind regimes, could you please elaborate on its influence on the circulation and vertical mixing patterns? Fig 6b and 6c correspond to surface salinity and currents.
L306 This is the first reference to the stratification of the water column and the stratification has not been shown anywhere in the paper, please point the reader to the figures as needed or rephrase.
L309 Fig6b and 6c only show surface, not vertical processes as indicated, please rephrase. The wind regimes are not shown anywhere in the paper so it is difficult to follow the reasoning.
L315 Upwelling is wind driven although it may interact with river plumes. You may wish to clarify the reasoning in this paragraph.
L325 is the first reference to salt intrusion, there is no previous information or profile to assess the type of salt intrusion. You could include a profile along the main branches in Fig., 12.
Fig 13, please change the colours. Sol. A and Sol. B are difficult to distinguish. It is not clear how or where the differences are calculated. Is this over the whole lagoon? Is it the differences between two points one in the lagoon and one at sea?
L337 please clarify what you mean by hydraulically limited here. You again refer to the wind here, but you do not present any information about the wind. You could include wind information in the figure in the same way that you include the river discharge.
L340. Fig14 would benefit from including wind regime
L345-347 please clarify what you mean by with hydraulically controlled.
Conclusions The conclusion could be strengthened by highlighting why or how this work is relevant beyond the study area at present it just restates what the paper does, and it is difficult to see why it would be of interest of the broader community beyond the study area. Could this be further explored?
L382 “four lagoon-sea reconnections” I believe you explore 3 different options.

Citation: https://doi.org/10.5194/egusphere-2025-606-RC1
- AC1: 'Reply on RC1', Christian Ferrarin, 21 Aug 2025
  
  The comment was uploaded in the form of a supplement: https://egusphere.copernicus.org/preprints/2025/egusphere-2025-606/egusphere-2025-606-AC1-supplement.pdf
  
  Citation: https://doi.org/10.5194/egusphere-2025-606-AC1
RC2:
'Comment on egusphere-2025-606', Anonymous Referee #2, 25 Jul 2025

Please find my review in the attached document. I have recommended the manuscript for major revision because I think it needss to be revised and better structured with the focus on a clear resear question and story line. Right now it presents a lot of model results without to much structured analysis. I assume the main focus of the authors is to demonstrate the general quality of the model. But this should be more clear from the structure of the paper. However, I think the authors have developed a very interesting model configuration and have carried out a thourough model study. I have no daubts that the manuscript can develop into a very interesting paper.

Citation: https://doi.org/10.5194/egusphere-2025-606-RC2
- AC2: 'Reply on RC2', Christian Ferrarin, 21 Aug 2025
  
  The comment was uploaded in the form of a supplement: https://egusphere.copernicus.org/preprints/2025/egusphere-2025-606/egusphere-2025-606-AC2-supplement.pdf
  
  Citation: https://doi.org/10.5194/egusphere-2025-606-AC2

Interactive discussion

Status: closed

RC1:
'Comment on egusphere-2025-606', Anonymous Referee #1, 09 Jul 2025

The manuscript “Modelling river-sea continuum: the case of the Danube Delta”, by Ferrarin et al., describes a new model configuration for the river-lagoon-sea interconnections in the Danube Delta Region. The model is first validated under current conditions and then used to explore the temporal and spatial variability of oceanographic parameters and three alternative reconnection options.

Evaluation: overall the manuscript covers and interesting topic in a region and type of system that could be extrapolated to other similar systems. The manuscript also does a good job (through visuals) at showing the spatial and temporal variability of oceanographic parameters. However, there are questions about the datasets used, the validation of the model results and the interpretation of results could be strengthened. The manuscript would benefit from some reorganisation since the findings and the implications are difficult to follow beyond the focus on the local issue. The scientific questions and relevance are not clearly articulated in the Introduction and some of them appear further down in the Methods. New results are presented in the Discussion and on the other hand the results of the study (both those in the result section and those in the discussion section) are not put into the broader context and remain descriptive. Perhaps the validation should go in supplementary materials and the result figures currently in the discussion (11-14) could be presented in the Results section highlighting how they relate to current understanding of this type of systems. The Conclusion restates the local findings and should be strengthen by highlighting why or how this piece of work is of general interest.
Specific issues that would be good to address include:
The modelling system and Numerical experiments. Significant details on the datasets and the modelling system need to be included without these details it would be difficult to reproduce the results and to evaluate the validity of the numerical modelling: What is the bathymetric resolution of the different datasets? In coastal environments the bathymetry has a non-negligible impact on the obtained results. What is the temporal resolution of the Black Sea lateral boundary condition? Some of the datasets are only available upon request so it will be challenging to reproduce these results. What is the temporal resolution of the model outputs?
The numerical experiments: it is not clear for which period are the what-if scenarios run (L126). Please clarify whether it is 2015-2019 or 2018
L123 highlights why this study matters and yet it only appears here. This should we moved to the Introduction section
L131 you refer to four datasets not three
The model validation: my main concern is the lack of validation data in terms of the water division in the lower parts of the delta river network and salinity on coastal areas.
Fig 2. What are the red circles and the black diamonds in Fig2b and 2c? There is no legend for those. There might be a typo on the x and y axis of Fig 2b and Fig 2c, shouldn’t it be “m^3/s” instead of “m”?
The validation data for sea level and sea temperature is relatively far from the zone of interest where the manuscript focuses. Why is the validation for sea level done for mean sea level and not at the time the measurements are collected? Does the validation improve when done for mean daily values or is it that the model output are daily values? This is not clear. Validation of sea level is quite poor at times (e.g. differences of over 0.1m for 0.7m variation in sea level Fig.3). The reasons (L171) for the poor fitting should be better explained/explored, does the bathymetry play a role as well in this? Why are results not presented as scatter plots?
Similarly for the sea surface temperature, why is the validation for Constanta not shown in Fig. 4 while data is available? Temperature is usually a minor part in estuarine dynamics with most of the density gradient being driven by salinity differences. However, here the validation is presented in terms of temperature and yet you focus later on salinity results, are there no data available in terms of salinity? There are 6 satellite control SST points, however a single set of statistical parameters are given in L182. What are the implications of comparing averaged daily values with specific timings in the satellite data?
I think that if this validation section could go into supplementary materials to streamline the paper.
Table 1 could include the statistical analysis for all the validation datasets (i.e. also include sea level and satellite SST) and not just temperature for monitoring stations.
Water division: water division is only validated in the upper part of the delta, and only temperature values are shown for a point in the vicinity of the outlets. Without discharge measurements in the network or temperature or salinity in the outlets it is difficult to assess how well or bad the model is performing. The water division is likely to influence the coastal dynamics and the plumes observed. These results also don’t seem to be further explored in the paper.
Spatial and temporal variability of coastal dynamics
L216 please define the scales of variability you refer to in the text. Here it is implicit you are considering seasonal scales as per your figure 6.
L234 there is no correlation calculated here, suggest change “are highly correlated” to “can be explained by”
River lagoon connectivity:
Over which period are calculated the averages and estimates in L241 to L249?
what causes the difference in WRT in the different years in L253 to L256? Are the weather regimes very different? Is it the different river discharge? It would be good to further explore the reason for these differences.
L259 The sense of the gradient and the number of days seem reversed.
Assessment of lagoon-sea reconnection solutions
L270 please indicate for which period where the what if scenarios run.
You could combine Fig. 8 and Fig. 9 and present current Fig. 9 as differences with respect to the reference run instead of absolute values. That way the differences would be more easily perceived.
L277-282: please point to table 2
Fig 10, please change the colours they’re not very colour blind friendly particularly sol.A and sol. B
Discussion The discussion further presents new results (there are 4 results figures in this section) and while they are interesting, they’re not put in context or related to similar systems. I believe the discussion would be strengthened if these new 4 figures were moved into the results section and the results were further discussed and contextualised in the discussion. It is difficult to see what is of interest beyond the regional area and what learnings could be taken to other regions.
L305-309: I believed this is the first time that stratification is mentioned in the manuscript. So far, the analysis has been limited to the surface. Beside there is no reference to the different wind regimes, could you please elaborate on its influence on the circulation and vertical mixing patterns? Fig 6b and 6c correspond to surface salinity and currents.
L306 This is the first reference to the stratification of the water column and the stratification has not been shown anywhere in the paper, please point the reader to the figures as needed or rephrase.
L309 Fig6b and 6c only show surface, not vertical processes as indicated, please rephrase. The wind regimes are not shown anywhere in the paper so it is difficult to follow the reasoning.
L315 Upwelling is wind driven although it may interact with river plumes. You may wish to clarify the reasoning in this paragraph.
L325 is the first reference to salt intrusion, there is no previous information or profile to assess the type of salt intrusion. You could include a profile along the main branches in Fig., 12.
Fig 13, please change the colours. Sol. A and Sol. B are difficult to distinguish. It is not clear how or where the differences are calculated. Is this over the whole lagoon? Is it the differences between two points one in the lagoon and one at sea?
L337 please clarify what you mean by hydraulically limited here. You again refer to the wind here, but you do not present any information about the wind. You could include wind information in the figure in the same way that you include the river discharge.
L340. Fig14 would benefit from including wind regime
L345-347 please clarify what you mean by with hydraulically controlled.
Conclusions The conclusion could be strengthened by highlighting why or how this work is relevant beyond the study area at present it just restates what the paper does, and it is difficult to see why it would be of interest of the broader community beyond the study area. Could this be further explored?
L382 “four lagoon-sea reconnections” I believe you explore 3 different options.

Citation: https://doi.org/10.5194/egusphere-2025-606-RC1
- AC1: 'Reply on RC1', Christian Ferrarin, 21 Aug 2025
  
  The comment was uploaded in the form of a supplement: https://egusphere.copernicus.org/preprints/2025/egusphere-2025-606/egusphere-2025-606-AC1-supplement.pdf
  
  Citation: https://doi.org/10.5194/egusphere-2025-606-AC1
RC2:
'Comment on egusphere-2025-606', Anonymous Referee #2, 25 Jul 2025

Please find my review in the attached document. I have recommended the manuscript for major revision because I think it needss to be revised and better structured with the focus on a clear resear question and story line. Right now it presents a lot of model results without to much structured analysis. I assume the main focus of the authors is to demonstrate the general quality of the model. But this should be more clear from the structure of the paper. However, I think the authors have developed a very interesting model configuration and have carried out a thourough model study. I have no daubts that the manuscript can develop into a very interesting paper.

Citation: https://doi.org/10.5194/egusphere-2025-606-RC2
- AC2: 'Reply on RC2', Christian Ferrarin, 21 Aug 2025
  
  The comment was uploaded in the form of a supplement: https://egusphere.copernicus.org/preprints/2025/egusphere-2025-606/egusphere-2025-606-AC2-supplement.pdf
  
  Citation: https://doi.org/10.5194/egusphere-2025-606-AC2

Peer review completion

AR – Author's response | RR – Referee report | ED – Editor decision | EF – Editorial file upload

AR by Christian Ferrarin on behalf of the Authors (17 Sep 2025) Author's response Author's tracked changes Manuscript

ED: Reconsider after major revisions (22 Sep 2025) by John M. Huthnance

ED: Referee Nomination & Report Request started (23 Sep 2025) by John M. Huthnance

RR by Anonymous Referee #2 (16 Oct 2025)

RR by Anonymous Referee #1 (20 Oct 2025)

Suggestions for revision or reasons for rejection

The authors have made clear efforts to address the comments; the revised manuscript shows significant improvement in structure and clarity. The messages are more explicit and effectively conveyed and the discussion is substantially strengthened. The responses are detailed and most of the requested information has been incorporated. I am generally satisfied but I have some minor remaining concerns that would be good to address before acceptance
·L105 and R1.2 is the reanalysis only available at daily frequency or do you chose to select daily input for the boundary. If that is the case, you’re reasoning is circular. If you chose to use save daily outputs and higher resolution data is available for open boundary conditions, your choice is because smaller scales are not that relevant (i.e. no tide and meteorological events larger scale). If the dataset is only available at daily scales is also likely that it is because shorter scales are not relevant. You explain this better in the reply to reviewer. Could you add this in the manuscript L105?
·In R1.8 the authors state that SHYFEM performs slightly better than BLKSEA, however in Table 1 RMSE is larger and CC is lower for BLKSEA than it is for SHYFEM which contradicts their statement. Looking at Fig 1 it would seem that BLKSEA presents slightly elevated values than SHYFEM across the time seriessith BLKSEA overestimaring peaks and SHYFEM reproducing better the low values. Please better indicate the reason for discrepancies between BLKSEA and observations
·R1.9 and Fig3 I am curious as to why include station 15360 instead of Constanta when you already have sea levels for Constanta. I understand that you the figure may look unbalanced with 3 sea levels and 2 salinities but I would like to see Constanta time series in the figure. Please do include the time series temperature for Constanta
·R1.31 please add the example at the beginning of March (L287)
·L121 I think it should be “leads”
·L178 please avoid constructions such as “underestimate (overestimate the peak discharge values in the Chilia (Tulcea)” Could you reformulate? This was highlighted as well by R2
·L301 you could use WFT instead of flushing time since already defined and you use the acronym afterwards. You have also described what it is, so no need to define it again here.
·L319 I think you can remove “where”
·L334 I think it should be “does not”
·L367 I don’t think Zhang and Yu 2025 is relevant here, their model is 2D barotropic so it does not resolve the type of processes you are looking at in your paper. Please remove it. If you wanted more Pearl River Delta examples, the following would be more relevant since they do resolve baroclinic processes, you could cite https://doi.org/10.1007/s44218-022-00008-0, or more recent https://doi.org/10.1029/2021JC017523 or https://doi.org/10.1016/j.ocemod.2025.102634
·L412 It is the first time you talk about the standard deviation of the relative discharge among branches and temporal variability, in Section 3.1 you’re giving averages for the whole period. How accurately these averages are is also relevant for the plumes you obtain later. Perhaps you could develop more the discussion on that aspect.
·L419 “configuration” is confusing here.
·L426 presumably the temperature of the smaller scale patterns between river mouths either warmer or colder has to do with the background temperature of the ocean and that of the river discharge. Is you river temperature constant? Or does it change throughout the year?
·L432 the salt intrusion length values are introduced here for the first time and it is difficult to see where you show those values, perhaps point to the figure or introduce those values in the result section so the discussion here makes more sense.

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ED: Publish subject to minor revisions (review by editor) (22 Oct 2025) by John M. Huthnance

Dear Authors
I now have the referees’ comments on your revised manuscript; I assume you have seen them (please tell me if you do not have them). Below I have some italicised comments (see the attached .pdf file) on how you might respond to them. This looks like a lot but I should tell you that both referees rated this manuscript as “good” and asked for (only) “minor revision”; please do what is practicable to address these comments.
Yours sincerely
John Huthnance (editor)

Referee 1
First paragraph of the review “The manuscript would still benefit from the development of a more consistent story line and the formulation of a research question.” In your line 30 “investigates” is rather vague and would be clarified by immediate connection to lines 34-38 where – implicitly – you state the aims by stating what you quantified. “But the limitations of the paper should be stated clearly, preferably in the introduction or method section, rather than, as the authors have chosen, in the result-discussion section (4.1).” I think probably in the method section, but only as limitations known before doing the work. It is quite conventional to discuss limitations as part of a Discussion section, perhaps especially the limitations that appear by doing the work.
“Chapter 1: Line 13-38: Introduction . . .” I mostly agree. Your present introduction is quite short and could include more about “the state of the art” and how you intend to advance it. However, in the introduction you should avoid referring to what the manuscript achieves or to the results obtained.
“Chapter 2.1: Line 70-115: . . .” The specific points on lines, 74, . . 90 are all improvements. “Line 99-104: . . .” I am not sure about this. Implementation values are most naturally kept with the formulation they relate to. But perhaps aspects of formulation (wind drag, bottom friction, vertical diffusivity – and boundary conditions?) should come before time step and output frequency. More references would help.
“Line 103-104: . . .” Maybe the point is that (for example) stress resulting from a daily-average flow is not equal to (usually greater than) daily-average stress. So it is necessary to use variables at the frequency of the time step for all (especially non-linear) calculations before “final” output.
“Line 108-109: please correct: …. and is computed by dividing …. with …. .” I think “…. and is computed by dividing …. by …. .”
“Chapter 2.2: line 117-144: Numerical experiments . . .” I generally agree with these comments.
“Chapter 2.3: Line 146-198: Model validation . . .” I generally agree with these comments so please consider them. Of course for validation you can only use data that you can find. You should put enough of your previous responses in the manuscript for other readers not to ask the same questions that you responded to.
“Chapter 3.1: line 204-228: Water division . . .” This is probably fair comment but the last paragraph of this section 3.1 is important regarding some form of validation and its first and last sentences. Respond as best you can but do keep this section in some form.
“Chapter 3.2: line 230-276: Spatial and . . .” I agree with a majority of the comments. In lines 232, 265 maybe “peculiar” –> “particular”. However, I think you should choose your wording for lines 240, 254, 255, not necessarily follow the referee. In line 260, best to avoid “surficial” but you might simply want “2 m surface layer”.
“Chapter 3.3: line 278-320: Lagoon . . .” Line 283 I agree with the referee but Line 284 either “induced” or “driven” is OK.
“Chapter 3.3.1: line 322-359: Assessment . .” Line 321 - I agree. Figure 11 – I am not sure; there is a strong point that “Sol. A” and “Sol. B” are very close and “Sol. C” and “REF” are very close. Difference plots (A-B or C-REF) would need a different scale. Line 322-359 – is there an “expected outcome of the reconnection”?
“Recommendation . .” I think this is a suggestion that you might move the discussion relating to (1), (2), (3) to the respective sub-sections of section 3. I do not insist on this but you should think about it.
“Chapter 4: line 360-464, River-sea modelling . .” What you do about “There should be a chapter 4: Discussions, before sub-chapter 4.1” depends on what you do about “Recommendation . .”.
“Line 361-404:” See above about “First paragraph of the review”
“Line 399” and “Line 463-464”. Please respond to these comments in the revised manuscript.
“Chapter 5: Line 465-499: Conclusions . .” I agree
“Reply to the authors response:” Not all of this asks for changes, but do consider what requested changes would be easy to do.

Referee 2. Please take account of these comments as best you can. The references to L105 and L121 might be looking at a different version of the manuscript

Editor comments previously sent to you.
Line 276. This part of the sentence does not relate to the previous part. Maybe “. . and of the winds . .” but the sentence is too long.
Line 318. “which resulted mostly influenced” needs correction.

Comments to the Author: PDF

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AR by Christian Ferrarin on behalf of the Authors (26 Oct 2025) Author's response Author's tracked changes Manuscript

ED: Publish subject to technical corrections (30 Oct 2025) by John M. Huthnance

Dear Authors
Thank-you for your revised manuscript. I am now asking for “Technical Corrections” meaning that you should please consider the “Detailed Comments” below and then upload your final manuscript directly to the Copernicus/OS editorial system. I do not need to see it again but please note that all comments become available to readers when the manuscript is published. It will be copy-edited; please check that this retains your intended meaning. Thank-you for publishing in Ocean Science.
Yours sincerely
John Huthnance (editor)

Detailed comments
Line 43. Better “higher” –> “greater” (twice)?
Line 78. “by the beginning of the years 2000” is unclear; “. . year 2000” or “beginning of the 2000s”?
Line 122. “the difference” –> “different” or “the various”?
Line 125. Better “. . water, assuming a hypothetical fully mixed basin, and is . .”?
Line 126: “diving” –> “dividing”
Table 1 and Figure 2 legends: River discharge. Ref. 1 suggested that the correlation coefficient should go to more decimal places to show how close the value is to one.
Line 209. “mayor” –> “major”
Line 220. Better “lower” –> “less”
Line 280. Better “low” –> “ weak”
Line 284. Better “a high” –> “strong”
Lines 301-302 do not quite explain how you obtain the seasonal standard deviations shown in figure 7. (i) For each month (line 201) do you use all the daily values in that month in 2015-2019 or just the standard deviation of the mean values for (e.g.) January 2015, January 2016, . . January 2019. (ii) How do you go from the three monthly standard deviations to the seasonal value?
Figure 7. Change “fall” to “autumn” in the figure panels if possible.
Line 333. Better “overpasses” –> “exceeds”
Line 347. Better to omit “,” after “which”
Line 371. Better “lower” –> “shorter”
Line 382. “. . details of the . .”
Line 430. Better “. . as in many . .”
Line 456. Omit or move “coastal”?
Line 486. “slow” –> “little” (actually faster renewal!)

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AR by Christian Ferrarin on behalf of the Authors (31 Oct 2025) Manuscript

Journal article(s) based on this preprint

03 Dec 2025

Modelling river-sea continuum: the case of the Danube Delta

Christian Ferrarin, Debora Bellafiore, Alejandro Paladio Hernandez, Irina Dinu, and Adrian Stanica

Ocean Sci., 21, 3291–3310, https://doi.org/10.5194/os-21-3291-2025,https://doi.org/10.5194/os-21-3291-2025, 2025

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Christian Ferrarin, Debora Bellafiore, Alejandro Paladio Hernandez, Irina Dinu, and Adrian Stanica

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Month	HTML	PDF	XML	Total
Feb 2025	39	15	2	56
Mar 2025	43	17	2	62
Apr 2025	24	14	1	39
May 2025	23	12	3	38
Jun 2025	37	29	1	67
Jul 2025	68	24	3	95
Aug 2025	123	21	5	149
Sep 2025	340	22	2	364
Oct 2025	46	16	2	64
Nov 2025	66	45	2	113
Dec 2025	1	4	0	5

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The requested preprint has a corresponding peer-reviewed final revised paper. You are encouraged to refer to the final revised version.

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We implemented a hydrodynamic model to the entire Danube Delta region consisting of the river network, coastal lagoons and part of the prodelta coastal sea. The model was applied to investigate the water distribution among the river branches, the dynamics of the coastal sea in front of the delta, the renewal capacity of the lagoons, the processes regulating the water exchange among the different water bodies and the potential impacts of lagoon-sea reconnection solutions.


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