The following review of some basic facts and concepts about ground water serves as background for the discussion of ground-water sustainability.
Ground water occurs almost everywhere beneath the land surface.
Areal recharge ranges from a tiny fraction to about one-half of average annual precipitation. The unsaturated zone, capillary fringe, water table, and saturated zone. Water beneath the land surface occurs in two principal zones, the unsaturated zone and the saturated zone. In the unsaturated zone, the spaces between particle grains and the cracks in rocks contain both air and water. Although a considerable amount of water can be present in the unsaturated zone, this water cannot be pumped by wells because capillary forces hold it too tightly.