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00lHl 23 The major application is illustrated using the outflow of 5 cfs per square mile. Maximum storage in the reservoir would be reached when the inflow and outflow lines are most distant from each other. For a constant outflow of 5 cfs per square mile the greatest ordinate interval is found by starting the outflow line on June 20th and drawing the line to July 13th. The detention time is 23 days. The storage requirement, as computed on figure 3, equals 180,400 cfs-days, which is equivalent to a runoff of 7.27 inches depth of water over the entire basin. This relation is plotted as circle on figure 5 (subse- quently presented). By constructing lines for other selected outflow rates on figure 3, additional storage requirements were computed as shown by other circles on figure 5. The circled values roughly define a relation curve of storage that would be required to limit gross outflow for various rates during the period of maximum observed runoff volume at this site. Such in- formation is shown in subsequent illustrations for each of 14 long-term gag- ing stations. A minor application of figure 3 is illustrated using the outflow of 10 cfs per square mile. If a proposed reservoir at the site was empty on June 20th and a constant outflow of 10 cfs per square mile was maintained, the accumu- lated inflow would exceed the accumulated outflow until the two lines meet, when the reservoir would be empty again. Storage Requirements by Frequency Methods The maximum storage requirement for a specific period of record at a specific site, as described in the previous Section, does not show how fre- quently storage may be expected to be inadequa te. The U. S. Corps of En- gineers (1955, p.10) suggested development of a mass curve from frequency data. Martin and Hulme (1957, p.55) utilized low-flow frequency data to com- pute synthetic mass curves on a frequency basis. These two concepts were applied to develop the storage capacity needed to sustain the low flow of Kan- sas streams (Furness, 1962, p.19-22) and are similarly applied herein for storage requirements to control high flow. Frequency ma ss curves of maxi- mum runoff have been computed for selected probabilities of events from flood discharge frequency curves such as figure 2. On figure 2 imagine a vertical line at the 2-percent chance of exceed- ance. Where this vertical line intersects each frequency curve, the rate of unregulated flow can be read. This rate of flow can be converted to volume of flow by multiplying by the duration in days and then plotting against the corresponding days as circles in figure 4. A smooth curve through these points defines a mass curve of accumulated flood volume having a 2 percent chance expectancy. This mass curve is analogous to the mass curve of ob- served inflow shown in figure 3. The curve differs only by representing maximum 2 percent chance inflow instead of the observed inflow to a hypo-