FUTURE NEEDS FOR DEVELOPMENT 273 



no question that this can be done, and there are some deserts 

 close enough to the California coast to make it reasonable to 

 consider augmenting their meager natural water supplies in 

 this way. But in general the purification of sea water, contain- 

 ing the soluble wastes from the continents through several 

 geologic ages, is a more formidable job than the cleaning up 

 of water supplies available on the continent, which have been 

 contaminated largely by our use. 



The relative scarcity of water in some areas has led many 

 industries to develop techniques which permit use of small 

 quantities of water over and over again, rather than large 

 quantities of water once. By recirculation some large users 

 have been able to cut their needs for "new" water to as little 

 as 5 or 10 per cent of the quantities used in similar industrial 

 operations in other areas. In some states multiple use of ground 

 water is effected by regulations requiring the return of water 

 after use to the ground-water reservoir via diffusion wells. 



POSSIBILITIES OF MANIPULATION OF STORAGE IN GROUND- 

 WATER RESERVOIRS 



It should be evident to all that surface reservoirs would be 

 useless for most purposes if they were maintained at constant 

 level. A constant level would indeed provide an attractive lake 

 for recreational purposes, where the shores could be lined with 

 cabins and docks and where an unsightly mud bottom would 

 never be exposed. But such controls would negate all other 

 purposes for which a reservoir might be constructed, because 

 the water flowing from it must then be equal to the inflow, 

 and there could be no storage of water from times of excess 

 to times of need. Thus a reservoir held at constant level would 

 be of little value for municipal or industrial use, power, irri- 

 gation, navigation (except on the reservoir), or flood control. 



The same conditions apply to ground-water reservoirs. They 

 receive their greatest replenishment when precipitation and 

 stream flow are most abundant, and least in times of drought. 

 But if storage were held at a relatively constant quantity (by 

 prohibition of pumping below a certain level), water would 



