Model-based flood attribution over Poland: the roles of precipitation, snowmelt and soil moisture excess

Poland is characterized by hydrometeorological variability, where conditions such as snowmelt, extreme precipitation, or soil moisture excess could be the main natural mechanisms causing fluvial flooding. The interplay of these factors may be additionally modified by climate change. Therefore, it is of high interest to attribute the occurrence of floods over Poland to single or multiple drivers as well as to analyse how this attribution evolved over time.

To meet this objective in the present study, we used the dataset covering components of the water balance with a daily time step at the sub-basin level over Poland for the period 1951-2020. The data set was derived from the previously calibrated and validated Soil & Water Assessment Tool (SWAT) model for over 4,000 sub-basins. The high spatial and temporal resolution of the dataset as well as its temporal continuity allowed us to comprehensively analyse the flood drivers over the country and their evolution over time. We used a method based on the circular statistics approach, using dates of occurrence of annual maximum floods and flood-generating mechanisms to estimate the relative importance of each flood driver. In addition, two sub-periods (1952-1985 and 1986-2020) were considered in order to detect the climate change signal.

The analysis of the relative importance of flood drivers showed that snowmelt is the most important cause of flooding throughout the country, followed by soil moisture excess and precipitation. The latter appeared to be the dominant driver only in a small, mountain-dominated region in the south. Soil moisture excess gained importance mainly in the northern part, although not in a uniform way, suggesting that the spatial pattern of flood generation mechanisms is also governed by other features. We also found a strong signal of climate change in large parts of northern Poland, where snowmelt is losing importance in the second sub-period in favor of soil moisture excess, which can be explained by the temperature warming and the diminishing role of snow processes. This study for the first time quantified the importance of different flood generating mechanisms over Poland, suggesting that more attention should be paid to soil moisture excess. This work also shows the potential of using high-resolution simulated water balance data sets in flood attribution studies.