Urban runoff response to climate-change-driven heavy precipitation and urbanization

Excess runoff from heavy precipitation events (HPEs) in urban environments often leads to urban flooding, a severe hazard with significant implications for human life, property, and infrastructure. Modeling runoff response in complex and heterogeneous urban areas, while accounting for rainstorm and surface characteristics, remains a significant challenge. Climate change and urbanization are key drivers of increased future urban runoff intensity. Research on the interaction between these factors and urban runoff in the eastern Mediterranean region is particularly limited. Previous studies using high-resolution models have projected an increase in short-duration rainfall intensities, alongside a decrease in long-duration intensities, rainfall coverage area, and total event rainfall during HPEs in the eastern Mediterranean under the RCP8.5 scenario. The current study examines the implications of these changes on peak discharge and volume of urban runoff by the end of the 21st century and evaluates the influence of varying urbanization scenarios, providing insights into the interplay between climate change and urban development. Using high-resolution radar-rainfall and surface data, we developed and calibrated a SWMM-based urban rainfall-runoff model for the Nahal Ra'anana basin (13 km²) on Israel's coastal plane. This Mediterranean-climate region encompasses most of the city of Ra'anana and has approximately 40% impervious surfaces. The model was developed using 23 runoff events utilizing leave-one-out cross-validation and a multi-objective optimization approach, and demonstrated robust performance with KGE values of 0.80 for runoff peak discharge and 0.83 for total runoff volume. A variance-based sensitivity analysis identified three primary factors influencing urban runoff: rainstorm intensity distribution, impervious surface coverage, and basin water storage. Analysis of HPEs under historical and future climatic conditions revealed that, at the current urbanization level of the city, climate change alone is unlikely to alter peak or total runoff discharge significantly. This is attributed to the decrease in total event rainfall and coverage area, alongside an increase in short-duration rainfall intensities. However, with substantial urbanization (e.g., increasing impervious surface to 52% or more), future climate HPEs are expected to exhibit a noticeable shift in the trend, leading to increased peak discharge. Further analysis indicates the elevated importance of rainfall intensities in determining runoff peaks in future climate conditions. In historical HPEs the maximum rainfall intensities over a 60-minute duration strongly correlate with peak runoff discharge (R²=0.75), where in future climate HPEs, correlations of shorter and longer rainfall durations are improved compared to historical HPEs with the maximum obtained for 60–120-minute durations (R²=0.81). The non-linear discharge response to climate change underscores the importance of integrating climate projections into urban planning to mitigate future flooding risks and highlight the potential for short-term peak discharge forecasting under both current and future climatic conditions.