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<br />20 <br /> <br />a vertical line on figure 2 at the 50-year recurrence interval. At its inter- <br />section with each curve the rate of flow can be read for a 50-year frequency. <br />This rate of flow is then converted to volume of flow by multiplying by days <br />and by 1.98 to convert to acre-feet. Volumes of runoff for periods shorter <br />than 12 months are determined from values in figure 64 of Technical Report <br />No.2. The volume s are plotted against corresponding days as circles in fig- <br />ure 4, A smooth curve through these points defines a mass curve of accumu- <br />lated runoff equal to the minimum volume expected once in 50 years on the <br />average. It is analogous to the mass curve of observed inflow shown in figure <br />3 and represents the minimum 50-year inflow to a storage reservoir at that <br />site. Accordingly, the graphical procedure in figure 3 may be applied in fig- <br />ure 4 to compute the gross outflow for a selected storage capacity, except that <br />in this case the gross ,outflow or draft rate is represented by a straight line <br />through the origin. '-', <br />',-~-.. <br /> <br />Draft lines of computed ou tflow rates are illustrated on figure 4 for stor- <br />age capacities of 200, 400, and 500 acre-feet per square mile. The storage of <br />500 acre-feet per square mile is shown tobe withdrawn in1,810 days by a sus- <br />tained gross outflow of 0.248 cfs per square mile. This period, which is the <br />most probable period of detention, represents the critical length of time that <br />the body of water would be subjected to evaporation and seepage losses. <br /> <br />From computations illustrated in figure 4 it is possible to develop an out- <br />flow-storage curve at each site as shown in figure 5 for the Valley Falls sta- <br />tion. The points shown as x's were computed from a variety of draft lines in <br />figure 4. The curve in figure 5 defines the storage required to allow a con- <br />tinuous gross outflow rate of various amounts. The actual gross outflow rate <br />may fall below the indicated value one year out of every 50 years on the aver- <br />age, or when expressed in probability terms, the outflow rate has a 2-percent <br />chance of being deficient in anyone year, The conversion to probability terms <br />is made by dividing years of recurrence into 100 percent; thus, a 20-year re- <br />currence equals a 5-percent chance. <br /> <br />Outflow-storage relations for a 2-percent chance deficiency can now be <br />compared with those for the 1952-57 drought, As developed in the previous <br />section, the 1952-57 drought relations are shown as circles on figure 5. At <br />Valley Falls the storage requirements for the 1952-57 drought are fairly sim- <br />ilar to the 2-percent chance requirements. Similar comparisons are shown in <br />figures 6-118 at most gaging stations in Kansas and indicate that the 1952-57 <br />drought was severe but of variable intensity. <br /> <br />The method of developing outflow-storage curves with 2-percent chance <br />of deficiency has been used to develop similar curves with 5-, 10-, 20-, and <br />50-percent chance of deficiency in order to provide users with a wide selec- <br />tion of design criteria, <br />