32 CONSERVATION OF GROUND WATER 



areas where the rocks are buried beneath less permeable 

 materials but yield usable water to wells. 



The blank areas of the map are significant because they 

 indicate that about half the country is not known to be under- 

 lain at any depth by rock materials which can yield moderate 

 quantities of water to wells. So far as known, the rocks under- 

 lying these blank areas are generally incapable of receiving 

 or transmitting large quantities of water, either by infiltration 

 from the land surface or from streams or by underflow from 

 adjacent areas underlain by permeable rocks. These areas are 

 of course not devoid of ground water, and many will yield 

 adequate supplies for domestic or stock use. Dug wells in these 

 areas may tap materials so impermeable that water drains into 

 the well very slowly. The success of these wells is dependent 

 largely upon the storage in the well itself, which permits pump- 

 age of several hundred gallons at a time to meet occasional and 

 limited demands. Because of the over-all low permeability of 

 the rocks or overlying mantle in these areas, the ground-water 

 phase of the hydrologic cycle carries a smaller than average 

 proportion of the total water, and probably for shorter dis- 

 tances. 



CHANGES OF STORAGE IN A GROUND-WATER RESERVOIR 



An aquifer capable of yielding a perennial supply to wells, 

 that is, where the water is a renewable resource, is an under- 

 ground unit of the hydrologic cycle, with a recharge area 

 higher than the area of discharge, so that water moves through 

 the aquifer by the force of gravity. Generally the recharge areas 

 are those where water can enter the aquifer from the land 

 surface or from a stream bed by percolation through perme- 

 able materials. With additions from percolation, storage of 

 water in the recharge area is increased, and the hydraulic gra- 

 dient toward the discharge area also increases so that there is 

 a greater rate of movement through the aquifer. Studies in 

 many areas have shown how the water table — the top of the 



