Ground Water
Ground water represents a major subterranean reservoir of fresh water. In general. it is not tapped by plants as it is too deep and does not suffer from loss due to evaporation. It slowly flows through the underground porous substratum from one place to another under the influence of gravity. Its flow may vary from a few millimetres to as much as a metre or so per day. At some places ground water reappears at the surface in the form of a spring. The zone of sub-surface formation that provides water to wells is called an aquifer. Some aquifers exhibit an unusually high hydrostatic pressure to force water upto the soil surface when a well is dug. Such a well from which water is pushed automatically to the soil surface is called an artesian well. In areas devoid of lakes and rivers ground water is the most important source of water.
A close examination of the global distribution of water resources reveals three important features. First, much of the water is stored on more or less permanent basis. The largest quantity of water is in the oceans. The Arctic and Antarctic ice masses, glaciers and lakes are also major water reservoirs not available to man. Second, some of the Earth's water is in constant flux represented by snow, rainfall, cloud drift and river flow towards the sea. Third, the water on land is very unevenly distributed.