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* I I ~ I ! t I !~ Figure 4. 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. In contrast to the unsaturated zone, the voids in the saturated zone are completely filled with water. The approxi- mate upper surface of the saturated zone is referred to as the water table. Water in the saturated zone below the water table is referred to as ground water. Below the water table, the water pressure is high enough to allow water to enter a well as the water level in the well is lowered by pumping, thus permitting ground water to be withdrawn for use. Between the unsaturated zone and the water table is a transition zone, the capillary fringe. In this zone, the voids are saturated or almost saturated with water that is held in place by capillary forces. . Ground water commonly is an important source of surface water, The contribution of ground water to total streamflow varies widely among streams, but hydrologists estimate the average contribution is some- where between 40 and 50 percent in small and medium-sized streams. Extrapolation of these numbers to large rivers is not straight- forward; however, the ground-water contri- bution to all streamflow in the United States may be as large as 40 percent. Ground water also is a major source of water to lakes and wetlands. 25 percent is estimated to be stored as ground water. Freshwater stored in rivers, lakes, and as soil moisture amounts to less than 1 percent of the world's freshwater. The reservoir aspect of some large ground-water systems can be a key factor in the develop- ment of these systems. A large ratio of total ground-water storage either to ground- water withdrawals by pumping or to natural discharge is one of the potentially useful characteristics of a ground-water system and enables water supplies to be maintained through long periods of drought. On the other hand, high ground-water use in areas of little recharge sometimes causes wide- spread declines in ground-water levels and a significant decrease in storage in the ground-water reservoir. , . Ground water serves as a large subsurface water reservoir. Of all the freshwater that exists, about 75 percent is estimated to be stored in polar ice and glaciers and about - r-..fililIi< -- 7