FUTURE NEEDS FOR DEVELOPMENT 241 



but the unsuccessful attempts are numerous enough to indi- 

 cate that there is still much to be learned as to how and where 

 to operate. 



In the arid regions there is an increasing awareness of the 

 value of underground storage as an alternate to surface storage, 

 for it has been found that "no matter how large the (surface) 

 reservoir capacity, streams of erratic annual and cyclic flow 

 will yield for useful purposes no more than 50 to 60 per cent 

 of the average annual discharge because the remainder will 

 be lost, over the years, by evaporation from the excessive water 

 surface of the reservoir necessary to impound the water of the 

 infrequent years of large discharge." 10 Factors that tend to 

 make the use of ground-water reservoirs attractive, in addition 

 to the saving of water otherwise lost by evaporation, are the 

 increasing cost of gravity developments and the decreasing 

 cost of pumping due to lower power cost and increased effi- 

 ciency of pumps. 



The utilization of ground water may be increased signifi- 

 cantly above the limit set by natural replenishment if flood- 

 waters or other surplus waters can be used to increase the 

 replenishment artificially. Thus a coordinated program of 

 flood control and artificial recharge may serve the double pur- 

 pose of preventing damage from too much water and storing 

 it until it can be used beneficially. 



Characteristically the ground-water reservoirs along water- 

 courses receive their greatest replenishment in time of flood. 

 In places where the water table has been lowered by pumping 

 from wells, the replenishment of the ground-water reservoir 

 takes more water from the stream than would be required 

 under conditions prior to construction of the wells. Thus 

 pumping along a watercourse may subsequently cause some 

 reduction of stream flow in time of flood. Here again ground- 

 water development and flood control have a complementary 

 relationship. Watercourses offer some of the best opportuni- 

 ties for development of large additional quantities of water 



10 Conkling, Harold, Utilization of Ground-water Storage in Stream-system 

 Development, Trans. Am. Soc. Civil Eng., vol. Ill, pp. 279-280, 1946. 



