ALBERT

Description: 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. A better understanding of flood mechanisms and its potential drivers across the country is therefore of high interest to decision-makers. To meet this objective, we employed the dataset covering components of the water balance with a daily time step at the sub-basin level over the country for 1952-2020. The data set was derived from the previously calibrated and validated Soil & Water Assessment Tool (SWAT) model for over 4,000 sub-basins. We applied circular statistics-based approach on annual maximum floods and various potential flood drivers to estimate the relative importance of each driver. In addition, two sub-periods (1952-1985 and 1986-2020) were considered in order to detect changes in flood mechanisms in the recent decades. We show that snowmelt is a primary driver of flooding across the country, followed by soil moisture excess and precipitation. The latter seemed to be the dominant driver only in a small region, particularly mountain-dominated region in the south. Soil moisture excess gained importance mainly in the northern part, 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.