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.
The analysis of these effects and relationships was 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 were 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 were used to investigate possible drivers of related changes in both heat waves and soil droughts and their spatiotemporal relationships. It was 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 were 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.
Conclusions:
• Frequency of heat waves is quickly getting higher in the entire Czech Republic, as well as frequency of soil drought in most of its area.
• South-east of the Czech Republic shows the fastest heat wave frequency increase, while soil drought is getting worse particularly in middle altitudes in general.
• Compound events (involving a heatwave initiating or deepening soil drought) are clearly common in the Czech Republic and their frequency may be getting higher in some regions.
• Large variety of possible definitions, particularly concerning soil drought, makes it difficult to clearly estimate the frequency of compound events.
• In context of the global climate change, compound events should be studied as an event with potential amplify its effect particularly in summer.