**2. Aridity**

Aridity is the imbalance between the long-term average water supply and the long-term average water demand [1]. Unlike drought, which is defined as a period of abnormally dry air long enough to cause a serious hydrological imbalance, aridity is permanent, not temporary. Aridity is a condition in which the amount of usable water in an area is reduced to such an extent that it hinders or prevents the growth and development of plants and animals. Regions with arid climates lose or tend to lose their vegetation. These regions, which are generally located close to the equator, are called xeric, arid, or desert according to their aridity levels. In the more extreme areas, called extreme arid deserts, the average annual precipitation is below 25 mm, under which conditions microorganisms must cope with not only by water scarcity but also by deadly UV radiation, high and low temperatures, high evaporation rates, prolonged drying times, oligotrophic conditions, and high salinity levels. Aridity is often evaluated with the aridity index (**Table 1**) and aids in determining whether there is a water shortage in the region and deciding the measures to be taken in case of a possible arid climate [3].


### **Table 1.**

*Climate classification and dry land subtypes based on the Aridity Index.*
