Relationships between heat waves and soil drought in the Czech Republic

Heat waves represent meteorological phenomena with serious impacts on human society and environment. Coincidence of their increased occurrence and intensity with increased frequency and severity of the summer drought episodes in the Czech Republic, particularly in the last decade of the 2010s and early 2020s calls for the study of their long-term spatiotemporal variability and compound effects. As an example serve the year 2015, when the Czech Republic experienced a record number of hot days and during July and August the soil moisture was very low. Our analysis of these effects and relationships is based on homogenised data of climatological stations of the Czech Hydrometeorological Institute and on soil moisture outputs coming from calculations of the SoilClim water-balance model for the 1961–2022 period. Different methods of statistical analysis are used to find changes in long-term fluctuations, trends, severity and spatial features of heat waves and soil droughts over the territory of the Czech Republic. Anticyclonic, cyclonic and directional circulation types derived from objective classification taken in account flow strength, flow direction and vorticity are used to investigate possible drivers of related changes in both heat waves and soil droughts and their spatiotemporal relationships. It is also tested whether a stronger relationship exists between soil moisture and heat waves represented by at least three consecutive days with a maximum temperature above 30°C or defined by a certain percentile threshold (e.g., 90 or 95%). Results obtained are discussed with respect to their impacts on the occurrence of wildfires and numbers of related fatalities and injures in the Czech Republic as well as in the context of a broader European scale.