Preface: Hydrogeology of arid environments

Aridity is a complex phenomenon for which a number of definitions and related indices exist (e.g., FAO-UNEP aridity index, Köppen-Geiger climate classification). Relying on a combination of indicators, Spinoni et al. (2021) estimate that arid environments, which are spread over the entire globe, cover roughly a third of the land surface (Fig. 1). However, this is not a static value as arid areas have increased over the last decades (Spinoni et al. 2015) and are likely to grow further due to climate change. Hence, also the number of arid zone inhabitants (currently about 1.4 billion people) will grow, and likewise the cropland area in arid areas (currently ca. 4.4·10⁶ km²; Spinoni et al. 2021) will increase.