WATER STORAGE 685 



increase may both be done justice by the correct solution of this 

 factor. As a rule, however, the development of a power site 

 usually also involves the consideration of an auxiliary power 

 source, such as a storage reservoir or a steam plant. 



If the secondary power can be sold without an auxiliary steam 

 plant, the amount of secondary power which may be developed 

 economically depends only upon whether or not the price received 

 for such power will cover interest and profit on the investment; 

 that is, the investment which is over and above that for develop- 

 ing primary power. If a steam plant has to be maintained the 

 amount of secondary power to be developed depends also upon 

 the cost of the steam power. 



WATER STORAGE 



In order to increase the capacity of a hydro-electric plant 

 at times of low water, the question of storage is one of vital im- 

 portance, and the extent to which the irregular stream-flow can 

 be equalized depends upon the quantity of storage. It is also 

 obvious that no considerable amount of money can be judiciously 

 expended in the construction of storage reservoirs under average 

 conditions unless the head available at the plant is considerable, 

 and this question must be largely determined by local conditions 

 surrounding each individual development. 



Water-storage problems are most readily solved graphically 

 by means of " mass-curves," and the most economical solution is 

 fixed by balancing the value in the increase in output as against 

 the cost of securing the same. From the mass-curve, the available 

 water for power is obtained and this, under given net heads will 

 determine the power available. 



The application of the " flow-summation " or " mass-curve " 

 to problems of water storage is clearly explained by Mr. E. C. 

 Jansen in the Engineering News for December 25, 1913, as follows: 



" To plot the stream-flow for any period of time, the mean 

 daily discharges in any convenient unit are added day by day and 

 plotted as ordinates, the units of time being represented by 

 abscissas, so that the sum total or ordinate on any date repre- 

 sents the total quantity of water which has flowed past the gauging 

 station up to that date (see curve ABCDE, Fig. 400). Second- 

 feet (cubic feet per second) are now most commonly used as units 

 of flow and, when the mean daily discharges are expressed as such, 



