The increasing flashiness in the Czech Republic: Natural variability or recent climate change?

This study examines catchments in the Czech Republic that have generated flash floods in the past, with a focus on hydrological response and physiographic parameters. The hydrological response of events with peak discharges exceeding the one-year return period during the summer half-year was evaluated using the flashiness index. Catchments were categorized into clusters I–III based on physiographic parameters, employing principal component analysis and k-medoids clustering. To evaluate Czech flash floods, a descriptive flashiness metric was computed for both the Czech and European flash-flood datasets, enabling cross-regional comparison. The results revealed an increase in 1-h flashiness during the recent period from 2018 to 2023 compared to 2005–2010, observed across all three clusters. The highest flashiness values were recorded in a group of small, steep catchments characterized by high terrain roughness, maximum elevations, a dense river network, and compact shape. A comparison of flash floods in the Czech Republic with those in Europe and the Mediterranean indicated that Czech flash floods generally exhibit lower unit peak discharge and 1-h flashiness values, although they can occasionally reach extreme intensities significant within the European and Mediterranean contexts. • 17 Czech flash-flood catchments with peak discharge >Q1 were analyzed. • Flashiness increased in recent years compared to earlier periods. • Small, steep catchments with rough terrain, dense river network are flashiest. • Czech flash floods can occasionally match extreme European-Mediterranean events.