Oases and Life in Arid Environments

Oases form wherever groundwater reaches the desert surface, whether through a natural spring where the water table intersects a topographic depression, artesian pressure forcing water upward through a confined aquifer, or a hand-dug or drilled well. Many of the largest oases draw on "fossil water," ancient groundwater recharged during past wetter climatic periods, such as the African Humid Period, and effectively non-renewable on human timescales. The most significant such reserve, the Nubian Sandstone Aquifer System, underlies Libya, Egypt, Sudan, and Chad and is estimated to hold roughly 150,000 cubic kilometers of water, making it one of the largest known fossil aquifers on Earth.