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

While we generally agree with Slette et al. (Global Change Biol, 2019), that ecologists 'should do better' when defining drought in ecological studies, we argue against the uncritical use of a standardized drought index (such as the Standardized Precipitation and Evapotranspiration Index, SPEI; Vicente-Serrano et al. J Climate, 23: 1696-1718, 2010), as a stand-alone criterium for quantifying and reporting drought conditions. Specifically, we raise the following issues: (a) standardization can lead to a misrepresentation of actual water supply, especially for moist climates; (b) standardized values are not directly comparable between different reference periods; and finally, (c) spatially coarsely resolved data sources are unlikely to represent site-level water supply. This article is a commentary on Slette et al., 25, 3193-3200; See also the response to this Letter to the Editor by Slette et al., 26, e1-e3.