How do we identify flash droughts? Analysis tool and Central European Croplands analysis

Flash droughts are often characterized as events of rapid and unusually large depletion of root-zone soil moisture, in comparison to average conditions, caused by climatic compound conditions over short periods (weeks). We compared six flash drought identification methods and analysed their functioning using measured data from FLUXNET2015 stations across Central Europe. All methods were implemented in an R package and are available as a Shiny app for the public, where the user can visualise the different results of flash drought identification for each method. An in-depth analysis and cross-comparison of methods for co- and misidentification for cropland sites showed a large degree of synchronicity among them, although some divergence was detected, related to four intrinsic differences in the underlying flash drought definitions associated to each identification method: (1) type of critical variable, (2) velocity of drought intensification, (3) pre-set threshold values for final depletion, and/or (4) minimum length of the duration of flash droughts. To balance strengths and weaknesses of the individual methods, we suggest the use of an ensemble approach for each event identification. To balance such strengths and weaknesses of the individual methods we propose an ensemble approach for event identification, allowing the detection of the current unclearly defined sub-types of flash droughts, related to the different combinations of compound drivers and differences in intensification dynamics.