Urbanization vs. climate drivers: investigating changes in fluvial floods in Poland

Fluvial floods are a severe hazard resulting from the interplay of climatic and anthropogenic factors. The most critical anthropogenic factor is urbanization, which increases land imperviousness. This study uses the paired catchment approach to investigate the effect of urbanization vs. climate drivers on river floods in Poland. Long-term daily river flow data until 2020 was used for four selected urban catchments and their non-urban counterparts, along with extreme precipitation, soil moisture excess, and snowmelt data generated from the process-based Soil & Water Assessment Tool (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. A range of statistical methods were employed to assess changes in the magnitude and frequency of floods and flood drivers, including the Pettitt test, the Mann Kendall (MK) multitemporal test, the Poisson regression test, multi-temporal correlation analysis and multiple linear regression. The MK test results showed a contrasting behaviour between urban (increases) and non-urban (no change) catchments for three of the four analysed catchment pairs. Flood frequency increased significantly in only one urban catchment. Multiple regression analysis revealed 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 study highlights the significance of evaluating both climatic and anthropogenic factors when analysing river flood dynamics in Poland.