In the manuscript “Rain-on-wet-soil compound floods in lowlands: the combined effect of large rain events and shallow groundwater on discharge peaks in a changing climate” authors modeled flood response in 12 lowland catchments in the Netherlands and studied the relationship between effective rainfall, groundwater depth, and flood volume. In addition to studying current conditions, an observed climate forcing dataset was transformed to mimic future climate scenarios, and the change in the rainfall, depth, and flood relationship was evaluated as well.

The study finds that high effective rainfall in combination with shallow groundwater table depth led to higher flood flows. This relationship is consistent across all catchments, although flashy catchments are more sensitive to effective rainfall. The dependence on groundwater depth means that seasonal changes in groundwater depth, such as drying during the summer months leads to a delayed reduction of flood peaks in fall, despite increased effective rainfall. In winter and spring groundwater depth recovered and lead to higher peak volume. These relationships are similar under future climates. The study mostly analyses frequent flood events (10 per year), but also studies rare events (0.1 per year). These rare, most extreme events are increasing in frequency the most under future scenarios with the predicted increases similar between catchments.

Overall, I find the focus on lowland catchment and the role of groundwater depth in flooding very relevant, as it has not been studied in detail much so far in Europe. Since groundwater depth can be measured much easier knowledge about the relationship to flooding can advance flood prediction. However, there are a few open questions that need to still be addressed.

All sections:

The authors oscillate between the terms “(initial) catchment wetness”, “groundwater depth”, and “soil wetness” (e.g. Section 4.2). These terms are not interchangeable and should not be used as such. Especially soil wetness is not equal to groundwater depth and should not be used as synonym. While you define at the end of section 2.3 that you assume groundwater depth to be representative of topsoil wetness, conceptually these are different terms and need to be more clearly defined early on if you are using them in a different context. I would argue to continuously use groundwater depth. Although groundwater depth depends on soil wetness within the model, the variables still mean different things.

Abstract:

The first sentence states that the severity of floods is determined by initial wetness, while the next sentence (and the entire study) then declares that this relationship needs to be studied more. It should be more explicit, that the relationship between soil moisture and floods is well studied, but between groundwater depth and floods has not, especially in lowland catchments.

Section 1

The summary of previous flood trend and flood type studies is very long. Please consider if you can make the three paragraphs more concise or summarise the findings in a table. I would also recommend focusing more strongly on previous studies that show a relationship between groundwater and flooding, e.g.

Berghuijs, W. R., & Slater, L. J. (2023). Groundwater shapes North American river floods. Environmental Research Letters, 18(3), 034043.

Shamsudduha, M., Taylor, R. G., Haq, M. I., Nowreen, S., Zahid, A., & Ahmed, K. M. U. (2022). The Bengal water machine: quantified freshwater capture in Bangladesh. Science, 377(6612), 1315-1319.

Section 2.1

The results of the study rely on a well-performing model. Currently, the reader is given no information besides the declaration by the authors that the model was validated. Besides the location of the catchments, no information is given in regard to the observed discharge time series that must have been used both to calibrate the model for some catchments and that have been used for evaluation. How long are the time series? Are they at an hourly resolution and was the model evaluated at an hourly resolution? How well did the model perform compared to observed values? What evaluation measure is used? Although the authors mention in Section 4.1 (Line 379) that the model was validated for this current study, no validation information has been given. Against what was the model validated? Only discharge or groundwater depth measurements as well, as the model performance in that regard is quite relevant for this study? What does it mean when you say model simulations were validated “qualitatively by assessing the realism of internal model variables” (line 380)?

It is unclear why some catchments have their model parameters calibrated while others use previously determined parameters. Were there no prior parameters for the calibrated catchments available? Does it make a difference in performance if a model has been calibrated or used prior determined parameters?

Table 1 is apparently sorted by discharge threshold. It would be helpful to have that value given as a column as well.

Section 2.2.

I do not find the abbreviations for the climate scenarios very intuitive. This might be my preference, but since they are only explained once and then continuously used throughout the rest of the paper, can they be slightly expanded (e.g. mod/low, mod/high, warm/low, warm/high or something similar?)

Section 2.4.

Why do you compute baseflow using the approach by Gustard and Demuth? Is the modelled flow between the groundwater-vadose zone reservoir to the surface reservoir (Gs) not a representation of baseflow?

Section 3.3

How do the findings on flood occurrence with climate change relate to the catchment properties, such as the discharge dynamics (Figure 3), or the special conditions mentioned for five catchments (upward drainage, supplied surface water, line 100-101)?

Why does the Radewijkerbeek catchment show a much larger increase in frequency and peak volume of floods in the WH scenario?

Technical comments:

L18-19: check sentence structure. Some duplication.

Figure 3: No colour is necessary in this figure as every line is labelled. There is no explanation for the meaning of the colour given anyway.

All figures with seasonal depictions: The chosen colours for the seasons cannot be read by people with colour vision deficiency. Please think about using alternative colour scales (Stoelzle & Stein 2021)

Figures 4, 5, 7, 8: You chose to focus on a specific catchment in each of these figures, but it is always a different one. One improvement would be to have one constant example catchment in combination with a chosen catchment that illustrates the point you are trying to make.  

Figure 7: While a circular diagram is technically correct for a depiction of year round values, the zero point is very difficult to see. Furthermore, it is unclear why in Figure 7a the shape of the colour filled areas and the lines for the different scenarios not match (e.g. in July and August). I would recommend testing a visualisation as bar plot with a clear zero center value.

Stoelzle, M., & Stein, L. (2021). Rainbow color map distorts and misleads research in hydrology–guidance for better visualizations and science communication. Hydrology and Earth System Sciences, 25(8), 4549-4565.

This study investigates how initial groundwater depth interacts with rainfall to influence pluvial flood volumes in lowland catchments under current and future climates. Using the WALRUS hydrological model and 109 years of hourly climate data, the authors simulate discharge and groundwater depth for 12 lowland catchments in and around the Netherlands. The work contributes to understanding compound flood drivers in flat, groundwater-dominated systems, which are often overlooked in large-scale or mountainous catchment studies.

However, the findings about the groundwater-rainfall-flood relationship is not supervising. Additionally, the quantified findings have notable uncertainty, as they rely heavily on model outputs without thorough validation of the groundwater component.

General comments

• The study emphasizes the importance of shallow groundwater storage in controlling flood severity in lowlands. However, the groundwater-related variables are entirely estimated by the WALRUS model, which is calibrated only against streamflow data. This could introduce bias, especially when groundwater depth is the key factor for explaining flood generation mechanisms. How much confidence do the authors have in their estimates of groundwater depth?
• Moreover, the calibration period is short (e.g., one year), which is insufficient to capture the full range of hydrometeorological variability (e.g., droughts, wet years, seasonal shifts, extremes). This raises concerns about the generalizability of the model to future climates, which may introduce conditions absent in the calibration year. Please provide quantitative uncertainty assessments in the main text or supplementary materials.
• The lowland catchment is a region closedly related to human activities. How were these human impacts on groundwater depth and fluxes considered or approximated in the modeling framework?
• The study includes 12 catchments. Are their hydrogeological characteristics homogeneous? If not, how do these differences influence the groundwater–rainfall–flood interaction?
• The WALRUS model includes two-way exchange between groundwater and surface water. Did this feedback mechanism significantly influence flood generation in the simulations? If so, please discuss and quantify where possible.
• The WALRUS model includes bilateral flow exchange between groundwater and surface water. Did the flow direction between these water modules influence the flood generation in the simulations?

Specific comments:

• L18-19: Typo
• L205: How much the difference between the estimated baseflow by Gustard and Demuth (2009) and the flow from the modelled groundwater?
• Disucssion 4.2: The authors try to mix up the definition of soil wetness and groundwater in the discussion. It is conceptually in
• Figure 7: Better use full name of scenarios instead of acronym
• Figure S2: The range of values in the legend for Qpeak does not cover the full range of Qpeak values shown in the plot.