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6 LOW-FLOW FREQUENCY DATA FOR MULTIYEAR PERIODS The storage required to sustain a selected gross reservoir outflow is a function of the amount and distribution of the inflow. Therefore the chance of deficl ency in outflow from a reservoir can be evaluated from the frequency and duration of periods of natural low flow. The magnitude and frequency of natural low flow for periods of from one day to 12 consecutive months have been defined in Technical Report No, 2 of this series. These periods alone are inadequate for defining storage requirements where long periods of stor- age are needed to develop the optimum use of the water, such as in Kansas. Accordingly, drought flows for longer periods of consecutive months must be evaluated. The frequency of multiyear low flow must be developed by a rather extensi ve process of applying statistical principles to streamflow records. As a knowledge of the process is not required in order to understand the develop- ment of storage requirement data, the methodology is presented in the appen- dix of this report. Methodology as described in the appendix has been followed to compute low-flow frequency curves as 113 gaging stations for periods enduring for 24, 48, and 96 months. Figure 2 shows such curves for a typical gaging station. These curves show how often in years, on the average, that the average rate of flow of specified duration may be expected to fall below the indicated rate. For example, at Valley Falls the flow for 24 consecutive months may be ex- pected to fall below 0.06 cubic feet per second (cfs) per square mile once every 20 years on the average. Low-flow frequency data are listed in table 1 for each of 113 gaging sta- tions for which 6 or more years of record are available. The information is expressed in cfs per square mile for five values of recurrence intervals be- tween 2 and 50 years. Each gaging station is identified by a numb er that is shown on the map of figure 1 and on other maps of this report. Also given is the location, years of record used for analysis, and drainage area. The user may find mo re detailed descriptions of these sites in the issues of water-supply papers of the U. S. Geological Survey (issued annually). Frequency curves for periods of 1 day to 12 months are shown in Technical Report No, 2 of this ser- ies of repo Ms (p. 39-157), but for convenient reference, data for 183-day and 12-month periods have been included in table 1.