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28 I I I I I I I I I I I I I I I I I I drainage area was substituted in the Qa equation for the variables used by James and the resulting standard error was the same. The equations for qlO and qso were recomputed because of an error found in the original data. I\lso, the symbol Iv was substituted for the variabil ity index (v) used by James. These equations are appl icable to drainage areas between 100 and 9,000 square miles. Use of the equations will be illustrated by an example for station 9175 on the Marmaton River near Fort Scott, which has a drainage area of 408 square mi les (see Figures 12, 13). If no streamflow records were available q50 would be computed in the following manner: The mean discharge interpolated from figure 12 is 0.78 cubic foot per second per square mile, and the variability index from figure 13 is 1.00 log unit. Substituting values in the equation for q50 gives 46 cfs. The 1921-56 computed value from the report by Furness (1959) is 33 cfs. Applying the discharge of 46 cfs to the relation line on each graph for 50-percent duration of discharge (Figure 9), the stream width is about 40 feet, the average depth is about 0.9 foot, and the average velocity is about 1.3 feet per second. The 1 imitations on the accuracy of the values determined are shown by the confidence limits. REAERATION COEFFICIENT Values of hydraul ic-geometry parameters can be estimated, within the stated confidence 1 imits, for any location on streams in Kansas. The reaeration coefficient (measure of the ability of water to absorb oxygen) then can be calculated from the estimated values of mean velocity and mean depth. Several equations are available for computing this coefficient, but one of the simplest is the equation developed by Langbein and Ourum (1967) and used by O'Brien and Angino (1968, p. 89) for the Kansas River: k2 = 3.3 v/dl.33 (6) where k2 is the reaeration coefficient, and v and d are as previously defined.