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.