Detection of trends in observed river floods in Poland

Study Region Poland (representative sample of 146 flow gauges free of major human modifications, located in 12 major river basins) Study Focus The objective was to present trends in selected river flood indicators (magnitude, frequency and timing) in Poland, using the annual maximum daily flow and peak-overthreshold approaches. Two periods (1956–2019 and 1981–2019) were analyzed. The first period maximized the temporal coverage and featured 58 gauges, while the second period maximized spatial coverage and featured 146 gauges. Trends were computed using the non-parametric Mann-Kendall test (MK) and Sen slope for changes in magnitude and timing, while flood frequency trend was detected by chisquare test on parametric Poisson regression. New Hydrological Insights for the Region We found a general decreasing trend in flood magnitude, with frequently occurring statistical significance, for the majority of river basins (particularly in north-eastern Poland). The strongest of these trends exceeded 15% per decade. A positive trend was detected in the southern part, particularly in the Upper Vistula Basin. No substantial and significant trends were detected in flood frequency, where identified changes mostly did not exceed values of ± 0.2 events per decade. Trend in flood timing showed a strong pattern of earlier flood occurrences in the southern half of the country for both periods, and an opposite behaviour in the north-eastern and northwestern regions.