Changes in Mediterranean flood processes and seasonality
Abstract. Floods are a major natural hazard in the Mediterranean region, causing deaths and extensive damages. Recent studies have shown that intense rainfall events are becoming more extreme in this region, but paradoxically without leading to an increase in the severity of flood events. Consequently, it is important to understand how flood events are changing to explain this absence of trends in flood magnitude despite increased rainfall extremes. A database of 98 stations in Southern France with an average record of 50 years of daily river discharge data between 1958 and 2021 was considered, together with a high-resolution reanalysis product providing precipitation and simulated soil moisture. Flood events, corresponding to an average occurrence of one event per year (5317 events in total), were extracted and classified into excess rainfall, short rainfall and long rainfall event types. The evolution through time of the flood event characteristics and seasonality were analyzed. Results indicated that, in most basins, floods tend to occur earlier during the year, the mean flood date being on average advanced by one month. This seasonal shift can be attributed to the increased frequency of southern-circulation weather types during spring and summer. An increase in total and extreme event precipitation has been observed, associated with a decrease of antecedent soil moisture before rainfall events, linked to a smaller contribution of base flow during floods. The majority of flood events are associated with excess rainfall on saturated soils, but their relative proportion is decreasing over time with a concurrent increased frequency of short rain floods. Therefore, this study shows that even in the absence of trends, flood properties may change over time and these changes need to be accounted for when analyzing the long-term evolution of flood hazards.
Yves Tramblay et al.
Status: open (until 13 Apr 2023)
RC1: 'Comment on hess-2023-46', Anonymous Referee #1, 10 Mar 2023
AC1: 'Reply on RC1', Yves Tramblay, 24 Mar 2023
- AC3: 'Reply on AC1', Yves Tramblay, 24 Mar 2023 reply
- AC1: 'Reply on RC1', Yves Tramblay, 24 Mar 2023 reply
RC2: 'Comment on hess-2023-46', Anonymous Referee #2, 10 Mar 2023
- AC2: 'Reply on RC2', Yves Tramblay, 24 Mar 2023 reply
RC3: 'Comment on hess-2023-46', Anonymous Referee #3, 16 Mar 2023
- AC4: 'Reply on RC3', Yves Tramblay, 24 Mar 2023 reply
Yves Tramblay et al.
Yves Tramblay et al.
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The manuscript “Changes in Mediterranean flood processes and seasonality” by Y. Tramblay et al. is an interesting study analysing changes in flood event characteristics, flood types and their seasonality in 98 catchments in Southern France. The results presented are coherent with other recent literature about flood changes in the Mediterranean and demonstrate and confirm that soil moisture is the primary driver of flood changes in this region. The manuscript is overall well written and logically organised. Please find my comments below.
My main concern is about the reliability/suitability of the reanalysis product used for the retrieval of precipitation and soil moisture information for the catchments (L135-136). What is the spatial resolution of this product? Is the spatial resolution fine enough for the relatively small catchments in the analysis? At L394-395 the authors state “[…] despite the large sample of basins considered, the patterns are consistent and homogeneous across different basin sizes and locations”. Could this be due to the (coarse) spatial resolution of the reanalysis data compared to the (small) size of the catchments?
L46-47: “[…] the mean flood date being on average advanced by one month”. Please specify that the shift refers to two sub-periods.
L116-117: I suggest adding in the introduction (and discussion) one recent study by Tarasova et al. (2023) about changes in flood processes in Europe. Tarasova, L., Lun, D., Merz, R. et al. Shifts in flood generation processes exacerbate regional flood anomalies in Europe. Commun Earth Environ 4, 49 (2023).https://doi.org/10.1038/s43247-023-00714-8
L155: in other words, did you adopt a peak-over-threshold (POT) approach?
L163-167: it’s unclear how the maximum precipitation is calculated. Is it the maximum daily precipitation within the same time interval where total precipitation is calculated?
L213: does the first period start in 1959 or in 1950 (as stated in the abstract)? Please check.
L214-215: it is not clear how the pivot year is selected and used in the analysis. Is the extension of the two periods always the same in all catchments (as also shown in all figure legends) or does it vary? Please clarify.
L230: I suggest renaming this section “Results and discussions” as it also contains, alongside with the results, a considerable amount of interpretation of the findings in the context of the literature.
L234: please specify that the changes refer to the difference between the two periods. This also applies to subsequent occurrences in the manuscript, especially in the caption of the figures, where it is not always clear what exactly “changes” refers to.
L256: how is the runoff coefficient calculated for each event?
L258-261: Correlations between antecedent soil moisture and runoff coefficients are analysed and reported in the text. I suggest adding a table or adding these results as a panel of figure 3 (or modifying figure 3) to make it easier to follow. The same suggestion (i.e. adding a table/plot) for L395-399 and L401-405.
Figure 2: I suggest inverting the colours of the colorscale and adding the units of the relative change to the axis.
L293: a mountain range is mentioned. To facilitate reader that are not familiar with this area I suggest adding a label to the map of Figure 1 to locate the mountain range.
Figure 4: the coloured dots look all a bit brownish and therefore the map is not so easy to read. I suggest making the colours brighter.
L330: “Association between flood occurrence and weather patterns”. How is the association done? Is the WT selected based on the date of occurrence the flood peaks or the preceding days? Please specify it here or in the method section.
L340: “Change in seasonality (of what? Of floods?) can be ascribed to changes in the seasonal occurrence of the weather types”
L332-347: please check coherence of WT numbers and names in the text and in figure 6. The WT numbers and names seem to be different in some occurrences in the text and in the figure. E.g., WT2 is “Atlantic circulation” in the text but “Steady Oceanic” in figure 6, where Atlantic is WT1 instead.
L340-347: the described changes are tiny in figure 7 and therefore do not seem very significant in the context of the description and interpretation of the results. There are other larger changes in figure 7 that are instead not described. Perhaps figure 7 could be further discussed.
L402-404: “For short rain and long rain, the maximum contributions observed are 36% and 32%, respectively, but these maximum values are only found in small basins.” Do these findings refer to the same 30 basins mentioned above?
Figure 7: please add a label to the vertical axis
L513: “[…] related to higher evapotranspiration rates” could you add a reference?
L470: please specify how the regional distributions in fig 12 are obtained.
L419 and L426: the word “flood drivers” is here introduced and I believe it refers to the flood types mentioned in the rest of the manuscript. Please use consistent terminology to avoid confusion.
L423: is the peak in January or February? (I think February is correct, fig 9)
L426: where are the long-term changes shown?