| 20 Mar 2026
The complex effect of climate change and urbanization on streamflow in small–medium Mediterranean catchments
Abstract. Future floods in the Mediterranean region are influenced by the dual pressures of accelerated climate change and rapid urbanization. Yet the small spatial scale and complexity of hydrometeorological processes make it difficult to project their joint effects. Intra-basin flood projections, in particular, remain absent. This study examines how these drivers affect peak discharge and flood volume in small to medium-sized Mediterranean basins, using high-resolution weather and hydrological models. We analyze 32 rainstorms under historical (late 20th century) and future (late 21st century) climate scenarios, incorporating projected urban expansion. Results show that while short-duration and high rain rates increase, accumulated precipitation, rainfall area, storm duration, and soil water storage significantly decrease. The combined effect of changes in rainfall patterns, soil water storage, and urbanization produces contrasting trends: urbanization alone leads to a substantial increase in mean peak discharge (+43 %) and flood volume (+41 %), especially when soil water storage is undersaturated and the influence of impervious surfaces is greatest. Conversely, considering only projected rainfall scenario yields decreases in mean peak discharge (-21 %) and flood volume (-30 %), despite higher rain intensities. However, during extreme events, when soil moisture approaches saturation, flood intensification can occur. Combined climate and urban scenarios demonstrate that urbanization dominates, resulting in increased mean peak discharge (+13 %). The high-resolution modeling reveals substantial intra-basin variability, with peak discharge intensification concentrated in upstream and urbanized areas. These localized and contrasting effects highlight the need for integrated high-resolution modeling and future land-use planning to support effective flood mitigation and water-management strategies.
This manuscript presents an investigation of the combined effects of climate change and urbanization on flood characteristics in small Mediterranean catchments, using high-resolution climate and distributed hydrological modeling. The study addresses an important and timely question, and the use of convection-permitting simulations represents a clear strength. The focus on intra-basin variability and the distinction between saturated and unsaturated event responses is a very strong contribution of this manuscript. For instance, the analysis of intra-basin variability demonstrates that localized increases in runoff may not translate into increased outlet discharge, with important implications for hazard assessment.
The manuscript is generally well structured and clearly written, and the results are scientifically interesting. In particular, the finding that projected rainfall changes tend to reduce flood magnitude while urbanization increases it, and that the latter can dominate the combined signal under certain conditions, is an important contribution. However, some aspects of the methodology and generalization require clarification and strengthening before the manuscript is suitable for publication.
The study relies on a pseudo-global warming approach based on a multi-model mean signal from CMIP5 (RCP8.5), yet the spread across climate models is not explored. As a result, the projected rainfall changes, which are central to the conclusions, are presented in a somewhat deterministic manner. This limits the robustness of the findings, particularly given that Mediterranean precipitation projections are known to be highly uncertain. The manuscript would benefit from a more explicit discussion of this limitation, including the implications of using an ensemble mean forcing rather than a range of future changes in rainfall. At minimum, the authors should clarify that the results are scenario-based, for the worst-case scenario (RCP8.5) and do not represent probabilistic projections, and discuss how model uncertainty might influence both the sign and magnitude of the reported changes.
A second important issue concerns the representation of urbanization. While the study appropriately highlights the hydrological impact of increasing impervious surfaces, the implemented scenario includes a substantial and partly hypothetical expansion of urban areas. This choice appears to strongly influence the results, particularly the conclusion that urbanization dominates over climate change in controlling flood response. However, the realism of this scenario is not sufficiently justified, and key processes such as stormwater management, drainage infrastructure, and mitigation measures are not represented or discussed. These omissions may lead to an overestimation of the hydrological response to urbanization. The authors should better justify the chosen land-use scenario, clarify whether it represents an upper-bound case, and at least discuss how the inclusion of urban drainage systems or green infrastructure could alter the results.
The generalizability of the findings is another aspect that requires careful consideration. Although the manuscript frequently refers to the Mediterranean region as a whole, the analysis is based on a single study area in the eastern Mediterranean. Also, I do not quite understand why the results of only one basin are presented, and the results for the other ones are kept in supplementary materials. While the process-based insights are likely transferable, the quantitative results are strongly dependent on local climatic, topographic, and land-use conditions. The manuscript would benefit from a clearer distinction between site-specific findings and more general conclusions. Yet, I fully understand that this type of study cannot be conducted at the large scale.
Some specific comments along the manuscript:
Line 31. It would be helpful to include a few references that specifically address urban development and how it can alter hydrological responses, particularly in Mediterranean or semi-arid regions.
Line 62: I think it is necessary to clarify here that it is primarily the magnitude frequent floods that are decreasing. On the other hand, there are indications of an increase in the most extreme and rare floods linked to the intensity of precipitation. See:
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
Line 82. There is a study that clearly show that while the signal for relatively frequent floods is very close to zero, there is a fairly systematic increase in the most extreme floods projected for the future. Nevertheless, the scope of these conclusions must be limited due to the short length of the time series available in these high-resolution simulations =
Poncet N., Tramblay Y., Lucas-Picher P., Thirel G., Caillaud C., 2025. Projections of extreme rainfall and floods in Mediterranean basins from an ensemble of convection-permitting models, Climatic Change, 178, 141. https://doi.org/10.1007/s10584-025-03983-8
Line 114: It might be interesting, if possible, to quantify the change in the percentage of urbanized area over time within the basin during the historical period.
Line 122. I'm not sure I understand this part =’with an average of 7 flow events’ does this mean it's an intermittent river where flow is observed only seven times a year?
Line 209. +876% seems like a lot. It might be interesting to put that into perspective by comparing it with other projections in different contexts or with other statistics in Israel that would make these trends seem more plausible.
Line 255: Why are two tests used at the same time? In particular, the ttest is questionable, it is more suited to normal distributions. Figure 4 show that the two test give similar results.
Line 265. It isn't made clear enough here that this isn't a continuous forecast for rain, but rather for individual episodes. The study focuses on a subset of heavy precipitation events exceeding a threshold associated with flood generation, rather than a continuous or climatologically representative sample. While this approach is appropriate for analyzing flood-producing storms, it may bias the results toward certain types of events and does not allow conclusions about changes in flood frequency or seasonal behavior.
Line 290. It is important here to explain what it is “saturated”. Soil storage above a threshold value? it is not clearly explained in section 2.3. For example, in Tarasova et al. cited above, there a procedure to estimate the saturated soil conditons.
Line 355. I don’t understand ‘Urbanization has a limited effect on peak discharge ‘ in this context. Seems contradictory with the main results.
Section 4.4.3. This type of analysis is really interesting, and I haven't come across any other examples in the literature where this kind of analysis has been conducted. I think there should be more clear explanation of the implications of this type of analysis and its main conclusions in a slightly less technical way.
Line 410 and after: Not clear if it is literature or findings of the present study. Please clarify.
Line 428: Yes, your results show a decrease in precipitation during events, but that is not the case at all in other projections or in other regions of the Mediterranean (see Poncet et al 2025 above), so you really need to put this result into perspective.
Line 465: The reason high-resolution RCM models are not yet widely used is that they still have significant limitations. They cover only a few regions; there are few simulations available; and, most importantly, these simulations cover only short time periods.
Line 496 It would be interesting to clarify whether the river discharge used for the reservoirs affect the flood event described in this basin.
Line 515. It is really important to say here that this applies to small to medium-sized basins in Israel. These projections do not necessarily apply throughout the Mediterranean.