WATER STORAGE 691 



The economic limit of capacity for the particular development in 

 question, for a steady twenty-four-hour power after regulation, is 

 thus seen to be approximately 40,000 horse-power, based on a 

 228-foot head. Such a development would run twenty-four 

 hours per day 58 per cent of the time, or seven months per year 

 on the average. The energy furnished by the natural flow each 

 year would be 29,000 horse-power-years, from stored water 8400 

 horse-power-years, and from the auxiliary source 3100 horse- 

 power-years. 



The diagrams also bring out the fact that full economic advan- 

 tages of the stream cannot be secured even after regulation without 

 auxiliary power. They also show that a small auxiliary plant 

 will render more additional energy available from the stream-flow 

 after regulation than the same amount of auxiliary capacity would 

 render available before regulation; i.e., after regulation auxiliary 

 power is more essential to the best economic results than before 

 regulation. 



All the above has been based on a steady twenty-four-hour use 

 of power; i.e., a load factor of 100 per cent. The general con- 

 clusions are not, however, affected by a smaller load factor, and 

 where there is pondage a low load factor simply permits a larger 

 economic installation. Thus, in the above case, with a load factor 

 of 62 per cent the economic development would be about 64,000 

 horse-power. 



A point in connection with water-storage problems which is not 

 always realized is, that while a given quantity of water in stor- 

 age will raise the minimum flow of the stream a certain definite 

 amount, a further addition of that same quantity of storage, when 

 put into the stream, will not raise the minimum flow by any- 

 thing like the first quantity, because its use will have to be distrib- 

 uted throughout a longer period of time in the year. Therefore, 

 as storage reservoirs continue to be built out, the increment in 

 the minimum flow becomes less and less, which means that as the 

 development of storage reservoirs progresses, the economical 

 outlay per unit of storage becomes less and less, and the time 

 comes when it becomes cheaper to increase the minimum flow 

 by means of an auxiliary steam plant. 



This may be illustrated by the diagram, Fig. 403, which rep- 

 resents a typical hydrograph or river-flow curve. It will be noted 

 that the minimum flow shown by this curve is 470 cu. sec. ft. 



