WATER STORAGE 687 



uniform rate of flow of 30 second-feet, then, by applying this 

 slope as a tangent to the summation curve at C, it is observed 

 that the stream from about October 1st began to discharge less 

 than this flow and did not rise above the same until the beginning 

 of April at D. The flow can be readily interpreted in this way by 

 drawing a datum and different slopes or tangents on a piece of 

 tracing linen and applying this at any point on the curve." 



Having a certain reservoir capacity and the mean daily dis- 

 charges of a stream for a period of years, the method of utilizing 

 the summation curve for finding the maximum regulated flow 

 which can be obtained for power purposes, is explained in the fol- 

 lowing paragraphs. 



" ABCDE represents the stream-flow in day-second-feet 

 (usually a period of minimum run-off when water power is con- 

 templated) ; FG is the desired regulated flow and H is the capacity 

 of the reservoir in day-second-feet. Starting with a full reservoir 

 on or about October 10, 1895 (the smaller units of time are pur- 

 posely omitted), the summation curve shows that the stream-flow 

 is below the desired regulated flow AB\, parallel to FG, and that 

 the ordinates JK, LM, etc., represent the amounts of storage 

 required to maintain the regulation. Plotting these ordinates 

 below the high-water level of the reservoir in the storage diagram 

 as JiKi, L\Mi, etc., the storage curve H\J\L\ is obtained, showing 

 the behavior of the reservoir during the uniform rate of dis- 

 charge for power purposes. At B, about April 10, 1896, the sum- 

 mation curve shows that the stream-flow is above the desired 

 regulated flow; consequently, the ordinates NO, PQ, etc., show 

 the amount of water which can be stored and these ordinates are 

 plotted as NiOi, PiQi, etc., for the remaining portion of the stor- 

 age curve until the reservoir fills about June 1. By continuing 

 the plotting of these ordinates RS, TU, etc., as RiSi, TiUi, etc., 

 in the storage diagram, the curve S\UiWi is obtained, showing 

 the quantity of water which passes over the reservoir spillway. 

 This process is then repeated and in this way is ascertained the 

 behavior of the reservoir from year to year while a continuous 

 draft is being made on it. The ordinate X, showing the water 

 left in the reservoir at the end of the drawing period, enables one 

 to experiment with differently regulated flows to ascertain just 

 how much draft the reservoir can stand. Frequently two or three 

 exceptionally dry years in succession in a long period of obser- 



