ABSTRACT
Deserts include arid and semi-arid lands where evaporation plus transpiration (water lost from plants to the air) exceeds precipitation. The 2005 Millennium Ecosystem Assessment categorized dryland climates as hyper-arid, arid, semi-arid and sub-humid (Table 12.1). This classification was based on an aridity index, defined by how much greater evapotranspiration is than precipitation. Hyper-arid lands have at least 20 times more evapotranspiration than precipitation, indicating a moisture deficit. For comparison, humid climates often have less evapotranspiration than precipitation.
