Return periods of areal precipitation during the Central European Floods 2024 in Czechia

The September 2024 flood affected large areas of Central Europe, particularly Czechia, Poland, Slovakia, Austria, and Germany. The meteorological cause of the flood was the low-pressure system "Boris", which moved from the Mediterranean towards Central Europe, where its progression was stopped by a blocking anticyclone over Eastern Europe. The frontal boundary associated with this cyclone exhibited intense rainfall activity. This mechanism is consistent with the one observed during the major floods in Central Europe in 1997 and 2002. The most severely impacted area was the Jeseníky Mountains, where a Central European record 1-day precipitation of 385.6 mm was observed. The objective of this work is to evaluate the 2024 flood in Czechia, focusing on the exceedance of return periods of areal precipitation.

The return periods are analyzed for four selected catchment size classes, based on the official catchment classification system used in Czechia. For each catchment, areal averages of precipitation totals are calculated using 10-minute precipitation intensities during the flood event. These intensities are derived from radar reflectivity data at a height of 2 km above sea level (pseudo-CAPPI 2 km) with a spatial resolution of 1 km, and subsequently adjusted using 1-day precipitation totals from ground-based rain gauges.

The second input dataset consists of parameters of the Generalized Extreme Value (GEV) distribution of areal design precipitation. These data are also derived from adjusted radar data, covering the 20-year period from 2002 to 2021. From the resulting precipitation intensities, areal precipitation totals are computed for accumulation durations ranging from 30 minutes to 3 days. Based on annual maxima, GEV distribution parameters are estimated using L-moments. These parameters are then used to calculate the return periods of the areal precipitation totals observed during the flood event.

The spatial distribution of return periods across the catchment size classes is analyzed, with particular attention given to the influence of topographic factors on return period values. Special emphasis is placed on orographic effects, such as the orographic enhancement of precipitation. Our analysis confirms extreme values of return periods over the affected area for long accumulation durations and at particular catchments even for short accumulation durations.