Overview
The authors have thoroughly addressed the main concerns raised in the previous review. The revised article has been significantly improved in terms of methodological transparency, analytical depth, and clarity of the comparative analysis between the Flood Forecasting and Early Warning Systems (FFEWSs) in the transboundary Meuse and Rhine river basins in Northwestern Europe. The authors have now reported a more systematic classification and synthesis of several important aspects of the FFEWSs, offering more insights into how and why the FFEWSs differ between countries, reflecting differences in forecasting methods (including in models ownership), crisis management procedures, and governance systems (federal vs. centralized mandated agencies). The revised summary table (Table 1) is now far more comprehensive and clearly structured, allowing the readers to find more clearly the key information retrieved by the authors for the different regions. This synthesis supports the comparative analysis of the key aspects of FFEWS across countries and regions that the paper aims to provide. Moreover, the authors added some useful information and insights on the technical improvements of the FFEWSs after the 2021 floods, on the remaining challenges and barriers (e.g., to develop impact-based forecasts), enriching the discussion as suggested. Finally, four insightful recommendations are provided and linked to the main limitations of the FFEWS.
However, the article requires several further minor revisions to enhance its quality, readability and clarity. In particular, here below a summary of the main points is reported followed by detailed comments.
In the introduction, the two research questions exposed (Section 1.3, Research gap and aim) are too narrow with respect to what the paper actually achieves. They are valid but incomplete, as they do not anticipate the analysis of all the main operational and institutional characteristics of the FFEWSs, as they limit the focus on warning levels, which is only one of the points analysed in detail by the authors. Other findings are not introduced, including how these systems have evolved in response to the July 2021 flood event and which technical limitations remain (e.g., lack of impact-based forecasts). These aspects could be introduced as a third question addressed, to improve alignment between the paper’s introductory framing and its findings.
The authors should reduce overlaps and repetitions between the Introduction and the Case study section (see detailed comments below).
On the methodological side, the interview methodology has been clarified (see Section 2.1), including how the interviewed experts were selected, and their role. The literature review approach is better contextualized, but is still not very transparent in terms of the search procedure and selected materials, neither providing the full set of strings for the search, nor stating explicitly the number of documents found (only 16 are mentioned, and the reader is left to guess if they are all - probably not). The authors should further clarify these points for a fully systematic, reproducible literature search.
To summarize, significant improvements have been made, but further minor revisions are required to make the manuscript suitable for publication, enhancing readability and quality (see detailed comments below). These issues should be fixed through a full revision and proof-editing. If all the recommended minor revisions are addressed, the manuscript will become suitable for publication without further review needed.
In the comments below, line (L.) numbers refer to the revised manuscript with tracked changes.
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Minor and technical comments
-- Main comments --
- Section 1.3 - Introduction: The two research questions are valid and reflect some in-depth analyses conducted, but are too narrow relative to the broader scope of the paper. The two questions emphasize warning levels, but do not fully capture the comprehensive analysis of operational and institutional characteristics of FFEWSs across the transboundary basins that the authors conducted. Additionally, the paper explores how these systems have evolved after the July 2021 flood and identifies remaining technical limitations, none of which are reflected in the current framing of questions dealt with in the introduction. Including one (or two) additional research question(s) to address these aspects would improve alignment between the study’s aims and its actual contributions.
- Section 2.1 - Case study region and approach: there are some overlaps and references between these section and Section 1.2 (Current challenges in FFEWS) in the Introduction: I think that the challenges in FFEWS in the Introduction (Section 1.2) should be presented in a more synthetic and general way, without details on the 2021 flood event (including early warning challenges) which would better fit Section 2.1. This would help avoiding repetitions (E.g., “In the Vesdre, Ahr and Geul catchments, a significant share of the local population did not receive warnings (see Introduction).”)
- Section 2.2 - Approach: The “combinations of keywords” used for the literature search and mentioned here (Lines 205-207) should be reported explicitly (at least as an Appendix or Supplementary information) for full transparency of the methods. Also, the number of resulting documents from this literature search seems to be not reported or it is unclear if it’s only the 16 selected articles mentioned in the following sentence (“for an in-depth analysis”). The number of official reports analysed should be clarified too.
- Section 3.1 and Table 1: Differences in lead times of hydrological forecasts are reported in Table 1 but seem to be not commented on in the text. It would be good to highlight these differences and if available to provide insights on the origin or reasons behind such differences. A spontaneous question arises: Why do forecast lead times differ so much (from 1-2 days in Luxembourg to 15 days in The Netherlands)? Are these large differences somehow linked to catchment sizes?
- Section 3: The spatial resolution of the operational hydrological models would be a relevant piece of information but is not reported in the text (nor in Table 1). Can the authors briefly report if there is any information available on this? It would be interesting to see if there are differences among countries also on that aspect.
- Sections 3 and 4: Several statements describing the current issues of FFEWSs (post-2021 floods), needs and wishes of stakeholders and authorities would benefit from more explicit sourcing of information (a more direct reference or quotes).
To enhance transparency and strengthen the qualitative grounding of the analysis, the authors should consider including anonymized quotes from interviewees or at least indicate whether insights stem from interviews (with which authority) or literature (which report or article). For instance, in Section 3.5 (Hydrological Forecasts – The Netherlands, lines 519–520), the following claim would carry greater weight if supported by a direct quote or attributed source: “Predictions are deterministic, although a strong need and wish exist to deploy probabilistic forecasting approaches”. Similarly, in Section 4, where issues raised by interviewees are summarized in Table 2, brief excerpts from interviews could enrich the discussion and bring practitioner voices more directly into the analysis.
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Detailed technical comments, presentation quality, and English language issues
- The authors should use consistently either the present tense or the past simple and not alternate between the two as done now in most sections (especially in the Abstract and Results). For example, in the abstract: “Expert interviews across the region reveal that …”; “The assessment of warning systems showed …”; “The interviews also revealed …”. I would suggest sticking to the present when talking about what the interviews and analysis show.
- Abstract: I would suggest briefly reporting in the abstract all the four main recommendations and avenues for further research (those outlined in Section 5.2), as only one out of four does not seem the most appropriate summary of the findings.
- Table 1: The caption should clarify that the information reported is relative to the time of the authors’ analysis (present day), i.e., after the 2021 flood event. Otherwise, given the focus of the paper, it can lead to misunderstanding on whether it reports the state of the systems during the 2021 events. Also, the authors should check and clarify the text in the first column (5th row): "Main hydrological and model(s) …”; probably there is a typo (the word ‘and’ should be removed).
- Section 1 - Introduction: check the use of verb tenses; at the moment, the authors often move from present to past tense in an inconsistent way, even within the same section; for example, at Lines 67-69 when describing general concepts.: “For example, uncertainty in the collection and processing of meteorological data (Fig. 1, Left) may lead to a ‘missed’ forecast event, where thresholds in the system were not surpassed, while observed water levels reached extreme heights … By contrast, uncertainties can also lead to so-called false alarms, where the predictions suggest that warning thresholds may be exceeded, while effectively they are not.”. Here I would suggest consistently using the present tense everywhere.
- Section 1 - Introduction (Line 44): the authors should revise the following sentence: “The 2021 flood further showcased the potential of early warning systems”. I do not think that this sentence (especially the wording “showcased the potential”) is in line with the analysis provided by the authors and with the literature (e.g., Da Costa et al., 2026). The 2021 flood event seems to expose more significantly the shortcomings in the early warning system chains rather than their potential, given the deadly outcome of the flood event and the fact that the forecasts and early warnings did not lead to timely and effective anticipatory actions like evacuation orders. Recent additional literature on this should be cited, e.g., Da Costa et al. (2026).
- Section 1 - Introduction (Line 87): the sentence introducing post-processing techniques should be improved as post-processing is generally used for other objectives, e.g. improving forecast skill, more than for estimating the uncertainty.
- Section 1 - Introduction (Line 89): the sentence applies also to other variables rather than precipitation, so the authors should explicitly write that heavy precipitation is just a relevant example here: “in case some (or all) ensemble members exceed a certain threshold of heavy precipitation intensity”; for example, the same applies to hydrological ensemble forecasts or temperature.
- Section 1 - Introduction (L. 92): this is an inaccurate statement as it does not reflect the current or most up-to-date situation: ensemble forecasts are already a standard in hydro-meteorology and widely used in operational systems; the statement sounds outdated and indeed it refers to a quite old reference from 2016. Since then, ensemble flood forecasting has gained significant momentum also in operational contexts, e.g., see Wu et al. (2020) and Speight et al. (2021). The authors should revise this sentence and refer to more recent studies than a 2016 one for such a statement aiming to provide a present-day perspective: “It is expected that ensemble forecasts will be widely integrated in operational forecasting chains in the near future (Pappenberger et al., 2016).” Actually, ensemble forecasts are already commonly made at most major operational weather prediction facilities worldwide (Speight et al., 2021). Most operational hydrological forecasting chains in Europe already integrate probabilistic forecasts, as also shown by the authors in this study in Northwestern Europe.
- Section 3.1 - Lines 245-247: the sentence can be improved for clarity and flow, e.g.: "Flanders is the only region where flood inundation forecasts are run operationally for short-term (48-hour) lead times, whereas all other regions rely on discharge forecasts and thresholds."
- Figure 3: The water-level symbol summarizing the number of warning levels for fluvial floods is not very clear. The authors should consider footnoting the symbols or explaining them in the caption to help readers' comprehension. Moreover, the font size of the text labels appearing beside this symbol (e.g., “>100 year”, etc.) is too small and in these labels the word return period or the abbreviation RP (e.g. “>100 year RP”) should be added for clarity.
- Section 3.5: typo in “a dedicated emergency management plans” - a single plan?
- Section 3.6 (Lines 566-568): improve and clarify the following sentence: “Warnings for a specific color can also be issued if there is a small chance (< 65% or < 25% of the area) on precipitation amounts belonging to the following color.” - clarify and correct: smaller chance (or area) of precipitation amounts belonging to the next, more severe warning class? How much smaller? I guess there must be a minimum probability threshold. Also, the wording “chance on” should better read “chance of” in the following parenthesis: “a small chance on rainfall belonging”.
- Section 3.6 - L. 583: 10-day forecasts should be defined as medium-range forecasts and not as ‘long-term’
- Section 4 - Lines 693-694: It would be important to clarify what the following statement means: “Luxembourg implemented a new alerting system (LU-alert), harmonizing the warning levels of the meteorological forecasts alerts and the crisis management.”
- Section 4.3 - Lines 817-820: the paragraph title “Best practices of impact-based forecasting” could be removed or better phrased to represent the content, as in addition to best practices the plans and initiatives for future developments are reported.
- Section 5 - Line 939: the word 'systems' might be missing after 'national-scale cell broadcasting'
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References
Da Costa, J., Ebert, E., Hoffmann, D., Cloke, H. L., & Neumann, J. (2026). Signals without action: A value chain analysis of Luxembourg’s 2021 flood disaster. Natural Hazards and Earth System Sciences, 26(1), 343–366. https://doi.org/10.5194/nhess-26-343-2026
Speight LJ, Cranston MD, White CJ, Kelly L. Operational and emerging capabilities for surface water flood forecasting. WIREs Water. 2021; 8:e1517. https://doi.org/10.1002/wat2.1517
Wu W, Emerton R, Duan Q, Wood AW, Wetterhall F, Robertson DE. Ensemble flood forecasting: Current status and future opportunities. WIREs Water. 2020; 7:e1432. https://doi.org/10.1002/wat2.1432
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Summary and general comments
This study reviews the status of Flood Forecasting and Early Warning Systems (FFEWSs) in transboundary river basins in the Northwestern Europe Countries that were hit by the July 2021 flood (Germany, Luxembourg, Belgium, and The Netherlands). Following the deadly and costly flood event of 2021, such analyses are essential for improving flood risk management and early warning systems chains, and to foster increased regional cooperation in transboundary river basins.
The study uses semi-structured expert interviews and literature review to analyze and compare FFEWS characteristics in the different countries, including forecast types, warning levels, communication protocols, emergency response plans and institutional coordination. Expert interviews from the region reveal that all systems are under a significant and rapid development after the 2021 flood event, which brought attention to some limitations of the FFEWS at the time. The main findings include the identification of key differences between countries and challenges, especially around harmonization and impact-based forecasting, which is still underused in the region, as only Flanders has operational inundation forecasts. Moreover, the authors find a lack of harmonization in protocols and inconsistencies in warning levels and communication protocols, which hinder cross-border coordination in transboundary river basins.
The paper is well written and contains many interesting details about the FFEWS structure in the four Countries studied, which are of interest for the community and the readers of NHESS.
However, it has some analytical limitations that should be addressed to enhance its clarity (in terms of organization of material and description of methods), rigor (in terms of systematic comparisons), and practical relevance (in terms of enhanced discussion and solid recommendations). In terms of clarity, the organization of the material can be improved as some key information is dispersed and difficult to find (see detailed comments below). A better synthesis should be made to be able to better compare all the key FFEWS characteristics across countries and regions (as further detailed in the comments below). Moreover, the discussion of some developments, limitations and barriers of the current systems should be enhanced, to better connect the analytical review of the FFEWS to the recommendations for their improvement.
Major comments
Table 1 only reports that now all countries have probabilistic hydrological forecasts, but it is unclear when these have been established. It might be beneficial for sake of clarity to have an additional table or scheme, listing or summarizing all the recent developments in FFEWS, or including some information about recent changes in the current Figure 3 or Table 1. The information to highlight and summarize should include: (i) when and how the probabilistic FFEWS were developed (from deterministic to probabilistic or increase in ensemble size?), being this one of the key findings, (ii) when the online platforms were improved, (iii) the emergency response plans were updated, (iv) the communication protocols changed, e.g. national-scale phone-based alerts, etc. This information is only hinted at in different parts of the paper.
Minor comments