HYDROLOGIC PRINCIPLES 29 



man can extract water. Because there is usually movement of 

 water through a ground-water reservoir, the connotation is 

 not quite the same as for surface reservoirs, which are con- 

 structed to halt and accumulate the flow of streams. But move- 

 ment underground is generally so slow, compared with that 

 in streams or in the atmosphere, that it represents a definite 

 retardation in the hydrologic cycle. Ground-water reservoirs 

 thus provide slow-moving storage from which man may obtain 

 water as he requires. If an analogy is permitted with the dis- 

 tribution system for manufactured goods, the ground-water 

 reservoirs might correspond to giant warehouses in which the 

 movement of goods in ton-miles is far slower than the rail, 

 plane, or trucking phases of the system. 



Nevertheless, an essential characteristic of ground-water res- 

 ervoirs is movement of water through them. Most of the satu- 

 rated materials underground are dense rocks, shales, clays, or 

 glacial tills. They are not suitable reservoirs at all, as far as 

 man is concerned, for they hold water in pores so small that 

 it cannot be transmitted in usable quantities to wells or 

 springs. 



Wells will yield a perennial supply only to the extent that 

 water can be transmitted to them through the entire course of 

 the aquifer from the place where the water enters the ground. 

 Even without wells, the ground-water phase of the hydrologic 

 cycle is one of movement from the places where water enters 

 the aquifer — the "recharge" areas — to the place where the 

 water is discharged from the ground, either by evapotranspira- 

 tion, by springs, or by seepage to streams or lakes or directly 

 into oceans. Thus, as a rule, usable ground water does not re- 

 main at rest under a piece of land until the owner is ready to 

 use it but is moving continually to some point of discharge at 

 the surface. 



DELINEATION OF GROUND- WATER RESERVOIRS OF THE 

 UNITED STATES 



This country unquestionably offers an endless variety of 

 prospects for obtaining usable water from wells, both geo- 



