| 03 Feb 2026
The influence of lakes and reservoirs on flood peaks at hourly vs. daily timescales in Switzerland
Abstract. Water bodies such as lakes and reservoirs can play a crucial role in reducing flood peaks both on daily and hourly timescales. While the effect of water bodies on flood peaks at different time resolutions has been demonstrated in the past, it remains unclear how they affect the ratio between daily and hourly peaks. Here, we analyse how water bodies attenuate flood peaks at daily and hourly time resolution and the relationship between flood peaks at these two time scales using two approaches: (1) four local case studies with gauges upstream and downstream of reservoirs, and (2) a large-sample hydrological dataset covering Switzerland. Our results show that hourly flows are dampened much more strongly than daily flows, which leads to similar daily and hourly flood peaks downstream of reservoirs. Specifically, our case study analysis highlights that (sub-)hourly flows are attenuated by up to 70 percent downstream of reservoirs during flood events with a 10-year return period. We also find that the attenuation effect is particularly pronounced in catchments that are heavily influenced by water bodies, i.e. those catchments where more than 60 percent of the area contributes to water body inflow. We conclude that considering water body influence on flood peaks is crucial to understand the similarity between daily and hourly flood peaks and that it should be considered in large-sample analyses using suitable metrics.
Competing interests: At least one of the (co-)authors is a member of the editorial board of Hydrology and Earth System Sciences.
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The manuscript “The influence of lakes and reservoirs on flood peaks at hourly vs. daily timescale in Switzerland” (i) analyzes the effect of water bodies (i.e., reservoirs and lakes) on flood peak at both hourly and daily timescale (i.e., the ratio daily/subdaily) in Switzerland, (ii) identifies parameters/catchment features that exhibit a significant effect on the D/H ratio, and (iii) concludes, if I correctly interpret the main message, that the ratio is particularly high (i.e., tends to one, hence daily and subdaily flood peak tends to converge) when the contributing water body area exceeds about 60% of the catchment area. Hence, if only the catchment area is considered to assess the ratio D/H, which is usually 0.8 for catchments larger than 5000 km2, there is a risk of underestimating this ratio in smaller basins that are strongly influenced by water bodies (at least for the small return periods considered here).
Overall, I find the general idea of assessing the water bodies effect on the daily/subdaily ratio quite interesting. In my view, the study suggests that for gauges highly influenced by waterbodies, flood frequency analysis on daily peak flows could, in some cases, provide a reasonable approximation of instantaneous peak flows (IPFs), even in relatively small catchments.
However, I have some concerns regarding the framework setting, the general writing, and, most importantly, the discussion and interpretation of the findings.
In particular, I think that the framework setting, especially the inclusion of different catchment descriptors in the random forest model, is not sufficiently discussed from a physical perspective. A deeper interpretation of the results would be necessary. Catchment area and contributing area remain the main parameters discussed, although two additional descriptors are included in the model (biogeographical region and geological permeability). Their role is not clearly interpreted in the discussion, and they seem to disappear from the narrative once the model results are presented.
In addition, I believe there is room for improvement in the writing, which is sometimes repetitive and slightly dense.
Finally, I do not find that the results and conclusions are fully clarified in terms of their implications. For example, it is not entirely clear what is meant by “large-sample analyses”, which could refer to several different methodological frameworks (e.g., regional flood frequency analysis, IPF estimation, attribution studies). I recommend that the authors be more explicit and precise about the practical message and utility of their findings.
Overall, I recommend publication of the paper, since the topic is highly relevant and I appreciate the general idea, but only after a major revision, where I would expect improvements in the clarity of the text and a deeper interpretation of the findings, especially regarding the role of the selected catchment descriptors and the implications for large-sample studies
Below, I provide specific comments and technical corrections.
(lines 3 to 6): Could you please rephrase this sentence (especially points (1) and (2))? Since you refer to “approaches”, the points would be clearer if presented as “(1) by comparing upstream and downstream gauges over four local case studies, and (2) …”;
(line 6): I suggest using “hourly peak discharge” instead of “hourly flows”, which would be more precise;
(lines 10 to 12): What exactly do you mean by “... that it should be considered in large-sample analyses using suitable metrics”? The term “large-sample analysis” is quite general. It would be useful to specify which type of large-sample study is concerned and how your results would concretely affect such analyses;
(Line 17): “However, information on IPFs is often not available and IPFs can differ substantially from daily flows.” I think this is essentially the core motivation of the study and should be emphasized more strongly, also by highlighting that subdaily data are often less available than daily data;
(line 31): Consider “varying depending on reservoir characteristics” instead of “between reservoirs”;
(line 67): “(2) to which degree can the consideration of water bodies improve the analysis of flood flows in large-sample analyses”
Could you clarify this research question? How exactly do you expect your analysis of the daily/subdaily peak ratio under water body influence to improve large-sample analyses? Which specific aspect of large-sample flood studies would benefit from this? This point should be clarified (also in lines 73–74);
(line 68): ‘first’ instead of ‘First’;
(lines 80-81): This part seems partially repetitive;
(lines 89-90): I do not fully understand the configuration of the Walensee case study. From Figure 1b it appears that both gauge stations are located downstream of the lake, or that the “upstream” gauge drains a different catchment. Could you clarify this hydrological configuration? Also, considering the 77% increase in catchment area between the two gauges, how does this affect the interpretation of the attenuation results?
(lines 93-94): I do not fully understand this sentence, specifically why those early periods were excluded from the samples. Please clarify the reasoning;
(line 125): When more than one water body is located upstream of a gauge station, how was the contributing area evaluated?
(lines 224 to 226): Here you describe the random forest model used to identify the most relevant catchment descriptors for explaining the D/H ratio. You identify contributing area percentage and catchment area as the most important descriptors, together with biogeographical region and geological permeability. Regarding the latter two variables, why is no physical interpretation provided? In the discussion, these descriptors are not considered anymore, although they were selected as relevant by the model. I think a deeper interpretation of their role is necessary, especially in terms of hydrological processes controlling the daily/hourly peak relationship. Also, the analysis is conducted only for the 10-year return period, correct? What do you expect would happen for higher return periods, for which the attenuation effect of water bodies is usually smaller? This limitation should be discussed more explicitly.
(line 236): Should this refer to Fig. 6a instead of Fig. 6b?
(lines 273-275): “Therefore we conclude that catchment area is a crucial factor influencing the peak ratio – unless much of the catchment area lies above water bodies.” If I understood correctly, you are highlighting that catchment area generally controls the D/H ratio, but when a large portion of the catchment is drained by water bodies, this control is overridden and the ratio approaches unity due to strong attenuation of hourly peaks. I suggest reformulating this conclusion more explicitly, as it represents one of the key messages of the manuscript.