Nelson Venegas Cordero, Luis Mediero, Mikolaj Piniewski

Fluvial floods are one of the most severe natural hazards. Changes in flood behaviour result from an interplay of climatic and human factors. Urbanization, with increasing land imperviousness, is the most critical human factor. This study investigates the effect of urbanization vs climate drivers on river floods in Poland, using the paired catchment approach. We used daily river flow data from 1975 to 2020 for four selected urban catchments and their non-urban counterparts as well as extreme precipitation, soil moisture, and snowmelt data generated from the process-based SWAT model. Changes in impervious areas were assessed using two state-of-the-art Copernicus products, revealing a consistent upward trend in imperviousness across all selected urban catchments. We employed a range of statistical methods: the Pettitt test, the Mann Kendall (MK) multitemporal test, the Poisson regression test, multi-temporal correlation analysis and multiple linear regression to assess changes in the magnitude and frequency of floods and flood drivers. The MK test results showed a contrasting behaviour between urban (increases) and non-urban (no change) catchments for three of four analysed catchment pairs. Flood frequency increased significantly in only one urban catchment. Multiple regression analysis revealed complex that non-urban catchments consistently exhibited stronger relationships between floods and climate drivers than the urban ones, although the results of residual analysis were not statistically significant. In summary, the evidence for the impact of urbanization on floods was found to be moderate. The results emphasize the importance of considering both hydrometeorological and human-induced factors when assessing river flood dynamics in Poland.