Dry spells and flash droughts – a comparative analysis of definitions, co-occurrence, trends, and impacts

With climate change, new forms of droughts have emerged and/or gained major interest, such as flash droughts and dry spells. Contrarily to the classical concept of droughts, which are often defined as slow-evolving and long-lasting extremes with no definite start and end, these rapidly emerging droughts have rapid onset and clear duration.

In this study, we analyze trends of frequency and duration of rapidly emerging droughts using four dry spell (DS) and four flash drought (FD) definitions, as well as the co-occurrence of DS and FD. We also evaluate the impact of DS and FD occurrence on crop yields. To achieve that, we use 52 DWD weather stations with daily measurements across Germany with no missing data between 1980 and 2023. The DS and FD definitions require precipitation, temperature, soil moisture and actual and potential evapotranspiration series. ETP is computed using the Penman-Monteith equation. ETA and SM are obtained from the WOFOST crop simulation model using maize as the default crop.

Results show strong positive trends across Germany on both duration and frequency for both DS and FD, with particularly intense trends on compound dry-hot events (all latitudes), in short to mid-length dry spells (7 to 20 days – all latitudes), and in southern Germany (most FD and DS event definitions). We also observe a high co-occurrence rate (synchronicity) between dry spells and flash droughts in northern Germany, which could assist in developing early warning systems. Finally, results indicate strong correlations between rapidly emerging drought occurrence and significant crop losses, particularly when FD and DS are concentrated in the early summer months.