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'. 1otlole, several hundred years would be required to replenish the stored ground water, whereas streams are replenished seasonally. J.n outstanding example of large storage and small replenishment is offered by the southern High Plains in Texas and New MeJdco. Overall, the ground water in storage in the Texas portion is about 200 million acre-feet, but if exhausted it would take considerably more than 1,000 years to replace (U. S. Senate Select Ccmmittee on National Water Resources, 1960a. p. 15). The rate of movement of ground water alBo contrasts sharPlJ" with ths.t of surface water. Generally, ground water flows inches or feet per day, whereas the now in streams is measured in feet per second or minute. The contrast in rates of .now is illustrated by the movement of groun1 water in the aquifer supplying Houston, Tex. Houston, the largest city of the United States dependent mainly on wells for lllIInicipal supPlJ". is served by an aquifer haVing a better- than-average capability for transmitting water. Even so, a cross section of the aquifer 45 miles wide and 600 feet thick is required to transmit 80 mgd (million gallons per day) to the Houston area at a h,ydraulic gradient of 10 teet per mile. New York City obtains three times as much water from Croton Reservoir through a pipeline less than 14 feet in diameter (Thomas, 1951, p. 98). The flow in streams is such that water from a large area can be gaged at a single location. Surface reservoirs also can be gaged at a single location. However, permeable earth materials otfer signiti- cant impedance to flow of ground water, with the result that a ground-w&ter reservoir (aquifer) must be gaged at maror points if the statue of the resource is to be evaluated. Water-level measurements in a single observation well may indicate conditiomr in only a flIIlall part of an aquifer. Though stream channels are not simple conduits, they can be easilJ' mapped and measured. The same is not true of aquifers. Aquifers are composed of a wide range of earth materials deposited by ma~ geologie processes. The ver,y nature ot geologic processes insures that the materials canposing aquifers will vary in character both laterally and vertically. Formations that are water bearing (aquifers) in OIl8 locality mllO' change lsteral4r to becalle nonwater bearing (aquicludes) in another area. Several aquifers may be present in a particular area, separated vertic~ by aquicludes, and both water-table and artesian conditions mllO' eJdst. The source of water to and the area of discharge from an aquifer may be distant or nearby. Therefore, aquifers are not easilJ' mapped. However, they must be mapped, their water-bearing characteristics detennined, and their hydrologic regimen evaluated before decisions can be made as to optimum development and management of the ground-water resource over a long period. of time. . 2