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0016~7 25 Chance Exceedance of Monthly Base Flow at Streamflow Stations The method of defining base-flow duration curves for each of the 12 months at 84 long-term stations was described in the previous subsection. Instead of presenting these 1,008 curves in their entirety, graphs for each of the 84 stations (fig. 12-95) are presented which show the monthly base- flow discharge that may be expected to be equalled or exceeded 5, 10, 20, 50, 80, 90, and 95 percent of the time, on the average. The smooth curve for each chance exceedance is the line fitting actual discharges read from plots such as for Soldier Creek near Topeka (fig. 11). Thus, from figure 11 tne discharge of 7.0 cfs has been observed to be equalled or exceeded 80 percent of the time in May. This value was converted to 0.026 cfs per sq. mi., and in consideration with values for other months was smoothed to 0.020 cfs per sq. mi., as shown in figure 47. The value may also be inter- polated by the right hand scale as a ratio of 0.23 of the yearly mean base flow. The illustrations for the 84 stations (fig. 12-95) are shown in down- stream order. The sites may be located by station number on the map of figure 1. All curves are for the standard base period, which is 1923-62 except for stations in northwestern Kansas where it is 1929-32, 1937-62. The lower ends of the curves have generally been drawn no lower than that corresponding to 0.1 cis; thus, for months where no curves are shown, the value for a given chance is less than 0.1 cfs and may be considered prac- tically zero. The probability curves represent essentially unregulated base flow. For this report it was arbitrarily decided that records for all years at the 84 stations could be used in developing unregulated base-flow probability curves, except when affected by regulation at the following major reser- voirs: Bonny, Lovewell, Kirwin, Webster, Toronto, and Fall River. The years of record not used after closure are identified by cross-hatched lines in table lof Technical report 6A. Storage in ponds and detention res- ervoirs is considered as not materially affecting base flow. From the curves in figures 12-95, information such as the following may be determined at each of 84 stream locations in Kansas: (1) the aver- age base flow of any month that has a chance of being equalled or exceeded 5, 10, 20, 50, 80, 90, or 95 percent of the time; (2) the ratio of this base flow to yearly mean base flow; (3) the chance exceedance in any month (by interpolation) of any selected rate of base flow. As a word of caution, the curves in figures 12-95 are for separate and individual months. They cannot be used to define the probability for longer periods. For example, the 12 monthly values of base flow for 20- percent chance cannot be averaged to determine the mean 20-percent chance for the year. Also these curves cannot be used to predict the likely base flow of the following month if the base flow for the current month is known.