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{] U 1 '11. (. 25 thetical storage reservoir at that site. Accordingly, the graphical analysis procedure that was used in figure 3 may be applied in figure 4 to compute the gross outflow for a selected storage capacity, except that in this case the maximum storage will be defined with the draft lines of outflow starting at the origin. The lines need not be investigated at other trial starting points. As with observed conditions but starting at the origin, the slope of a straight line defines a certain constant gross outflow. Draft lines of outflow, as illustrated in figure 4, are drawn for rates of 1.17, 1.9 and 4.6 cfs per square mile requiring 120, 60 and 15 days of detention, respectively, to reach the points of maximum ordinate intervals between the accumulated inflow and outflow lines. A computation on figure 4 illustrates the conversion of ordi- nate interval to storage requirements. The storage requirements can be ex- pressed in many terms; however for this report cfs-days per square mile of storage was converted to an equivalent 11.0 inches of runoff. Also, the ac- cumulated release or outflow volume of 140 cfs-days per square mile, read at 120 days on figure 4, may be converted to an outflow rate of 1.17 cfs pe r square mile by dividing by 120 days. The days duration to the time of maximum ordinate is the effective de- tention period in days and indicates the time required to fill the reservoir to its maximum storage requirement starting wi th an empty reservoir and dis- regarding evaporation and seepage losses. If the outflow lines in figure 4 were extended, their intersection with the inflow line indicates the duration in days to complete the cycle from an empty re servoir to full and return to empty reservoir. From computations for selected detention periods such as illustrated in figure 4, a storage-outflow curve may be developed at each site as shown in figure 5 for the Delaware River station. This curve shows a principalobjec- tive of this report, namely storage requirements and maintained gross out- flow rates at a streamflow station for a selected chance probability of ex- ceedance. The flagged points along the curve represent selected detention periods ranging from 1 to 120 days. The upper numeral on the flag line rep- resents the critical detention time from empty to full reservoir; the lower numeral represents the complete cycle of storage time from empty to full reservoir and return to empty. The latter time is not shown when it exceeds 183 days. The storage-outflow curve in figure 5 has not been extended to detention periods of less than one day because there is little likelihood of building such reservoirs. The 2-percent chance storage curve in figure 5 has an upper limit equal to the instantaneous peak discharge having a 2-percentchance of recurrence, and becomes asymptotic to this magnitude as storage approaches zero.