HYDROLOGIC PRINCIPLES 27 



they will apply to the current year is no greater than the chance 

 that the next person one meets will be of average height. Be- 

 cause of the vagaries of precipitation, far larger portions of the 

 country may suffer from water deficiencies in some years, and 

 on the other hand, the normally arid states may in some years 

 develop moderate water surpluses. 



Nature's Great Reservoirs: Ground Water 



Although water is continuously passing on through the hy- 

 drologic cycle, man as a living thing must store enough or de- 

 pend on natural storage to meet continuing or recurring de- 

 mands. The major reservoirs of the hydrologic cycle are, of 

 course, the oceans, which hold water unsuitable for most uses, 

 but which, thanks to the energy of the sun, provide the great 

 bulk of the fresh water falling upon the continents. 3 Coastal 

 areas worried about their water supplies have the comforting 

 (though not very comforting) thought that whenever water 

 becomes sufficiently valuable to justify the expenditure for 

 energy, they can imitate this process and derive fresh water 

 directly from the ocean. On the continent, the Great Lakes 

 and many smaller fresh-water lakes form excellent reservoirs 

 for communities in their vicinities, and the reservoirs created 

 by man in all sections of the country serve to hold back the 

 flood flows of streams for later use. Without such reservoirs, 

 a stream cannot be counted on for more than the quantities 

 available during periods of minimum flow — water that is de- 

 rived largely from ground-water reservoirs. 



Soil moisture constitutes a reservoir of major economic im- 

 portance to agriculture. Water received by infiltration may 

 remain in the soil for days or weeks, until it is used by plants 

 or evaporated. Especially in humid areas this storage may be 

 adequate for the needs of crops between rainstorms, but crops 

 can experience shortage of water anywhere if the interval be- 



3 Benton, G. S., R. T. Blackburn, and V. D. Snead, The Role of the At- 

 mosphere in the Hydrologic Cycle, Trans. Am. Geophys. Union, vol. 31, pp. 

 61-73, 1950. 



