Groundwater in Arid and Semi-Arid Areas

MDPI has published two issues on groundwater in arid and semi-arid areas in Geosciences. This topic is important because, excluding the polar deserts of Antarctica and Greenland, arid and semi-arid regions occupy about 30 to 40% of the Earth’s land surface. They are found in all continents and dominate Australia, Asia (Middle East), and Africa (North Africa and Sahel). Approximately 2.5 billion people live in these regions, which are defined by their low precipitation. More specifically, arid regions are defined by either by their annual precipitation or aridity index. Arid zones receive an annual precipitation of less than 25 cm/yr and semi-arid regions receive 25–50 cm/yr. The aridity index, the
ratio of annual precipitation to annual potential evapotranspiration, delineates three zones: the hyper-arid zone (0.03), with rainfall of <10 cm/yr (these are sometimes called true deserts); the arid zone (0.03–0.20), with annual rainfall of between 10 and 30 cm/yr; and the semi-arid zone (0.20–0.50), with annual rainfall of over 30 cm/yr.