274 CONSERVATION OF GROUND WATER 



be pumped out chiefly when precipitation and stream flow 

 are above normal, and therefore when most water users need it 

 least. Conversely in times of drought less water would be 

 available when farmers and cities need water most. Thus a 

 ground-water reservoir is not being used effectively if its oper- 

 ation is on the basis of a balance each year between recharge 

 and discharge. If the balance is achieved during a climatic 

 cycle of several years' duration, however, the ground-water 

 reservoir can provide holdover storage from wet years for the 

 drier years when the need for water is greatest. 



Experiments in artificial recharge of ground-water reser- 

 voirs have been successful in many places, and the techniques 

 have been used by numerous industrial plants and agricul- 

 tural areas to increase their available perennial water supply 

 substantially. In view of the proved economic possibility of 

 artificial recharge in many areas, the dependence upon natu- 

 ral replenishment may be considered a limited objective, to 

 which the public should resign itself only if there are no un- 

 appropriated waters available for artificial recharge or if no 

 feasible method of artificial recharge can be devised. In many 

 regions there are opportunities for man to store water under- 

 ground for use when needed, with minimum loss by evapora- 

 tion. 



It does not follow that all ground-water problems will be 

 solved by putting water underground everywhere. Over most 

 of the Nation's area such practices would have no direct effect 

 upon the more productive ground-water reservoirs. Artificial 

 recharge can be deemed successful only if it augments the 

 storage in a ground-water reservoir where it can be used, and 

 therefore a knowledge of the hydrologic characteristics of that 

 reservoir is prerequisite. 



The problems of flood control are related to those of artifi- 

 cial recharge of ground- water reservoirs, for floods commonly 

 provide the surplus waters needed for the artificial recharge. 

 Along many of the rivers subject to damaging floods there are 

 ground-water reservoirs of large capacity, in some cases large 

 enough to hold more water than the river discharges in a year. 



