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22 of curves were found not to be the same value above the mimmum experi- enced flow because regardless of the number of days when base flow and total flow were equal, there were other days elsewhere in the long record having the same range in discharge but with total flow exceeding base flow. Base-flow duration curves, such as in figure 9, provide an excellent portrayal of basin characteristics (Kunkle, 1962), but do not evaluate chron- ological sequence. This deficiency may be partly overcome by delineating base-flow frequency for continuous periods, such as 7 or 30-days duration in any year. Figure 10 shows a comparison of base-flow frequency curves in comparison with total-flow frequency curves. (See Technical report 2 for procedures in computing low-flow frequency relations.) However nei- ther flow-duration curves for the complete period nor low-flow frequency curves for groups of days indicate that part of the year when these proba- ble base flows are available for use. This deficiency has been overcome in this report by developing duration curves of base flow for each month of the year, as described in the following subsection. Monthly Base-Flow Duration Curves at Streamflow Stations Monthly values of base flow for any of 84 gaging stations may be ob- tained from Technical report 6A for each year in the standard base period, which was usually 1923-62. Thus for most of the 84 stations,40 essentially independent experiences of base flow are available for any month. For each separate month at each separate station, the individual base-flow values were arrayed in order of magnitude and plotted on loga- rithmic probability paper. Figure 11 shows the plot of the 40 arrayed dis- charges for the month of May in the period 1923-62 at station no. 65, Sol- dier Creek near Topeka, Kans. The plotting position of these discharges on the scale of percent-of-time probability is computed from the formula usually used by the Geological Survey of 100M/N+l, where M is the order number of the arrayed discharges with the highest designated as number one and N is the total number of years in the base period. The significance of the data as presented in figure 11 is that, to the extent that experience in the long-term base period is representative of future expectancy, the chance of monthly base flow equalling or exceeding a desired magnitude can be defined.