An isolated fertile area, usually limited in extent and surrounded by desert. The term was initially applied to small areas in Africa and Asia typically supporting trees and cultivated crops with a water supply from springs and from seepage of water originating at some distance. However, the term has been expanded to include areas receiving moisture from intermittent streams or artificial irrigation systems. Thus the floodplains of the Nile and Colorado rivers can be considered vast oases, as can arid areas irrigated by humans. Desert

Oases are restricted to climatic regions where precipitation is insufficient to support crop production. Such regions may be classified as extremely arid (annual rainfall less than 2 in. or 50 mm), arid (annual rainfall less than 10 in. or 250 mm), and semiarid (rainfall less than 20 in. or 500 mm). Many African and Asian oases are in extremely arid areas. Most oases are found in warm climates. Oasis soils are weakly developed, high in organic matter but often saline, and have been strongly affected by human occupation.