Standardized drought indices in ecological research: why one size does not fit all

Defining and quantifying drought is essential when studying ecosystem responses to such events. Yet, many studies lack either a clear definition of drought, and/or erroneously assume drought under conditions within the range of “normal climatic variability” (c.f. Slette et al., 2019). To improve the general characterization of drought conditions in ecological studies, Slette et al. (2019) propose that drought studies should consistently relate to the local climatic context, assessing whether reported drought periods actually constitute extremes in water availability.While we generally agree with their proposal, we argue that standardised climatic indices, such as the Standardized Precipitation and Evapotranspiration Index SPEI (Sergio M Vicente‐Serrano, Beguería, & López‐Moreno, 2010) as highlighted in Slette et al., cannot be recommended as stand‐alone criteria for drought severity, especially when applied in a global context. We base our critique on three major points: (1) standardisation can lead to a misrepresentation of actual water supply, especially for moist climates, (2) standardised values are not directly comparable between different reference periods, (3) spatially coarsely resolved data sources are unlikely to represent site‐level water supply.