Drylands are defined by their scarcity of water. They are zones where precipitation is counterbalanced by evaporation from surfaces and transpiration by plants (evapotranspiration). UNEP defines drylands as tropical and temperate areas with an aridity index of less than 0.65. The drylands can be further classified into four sub-types: dry sub-humid lands, semi-arid lands, arid lands, and hyper-arid lands. Some authorities consider Hyper-arid lands as deserts (UNCCD) although a number of the world’s deserts include both hyper arid and arid climate zones. The UNCCD excludes hyper-arid zones from its definition of drylands.