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<br />00lSgg <br /> <br />I <br /> <br />I <br />I <br />I <br />I <br /> <br />The composite statewide hydraul ic equations from the least-squares <br />regressions are: <br />Oa (Average discharge) w = 3.1QO.58 d = O.36QO.30 v = 0.99QO.14 <br />qlO (la-percent durat ion) w = 3.4QO.54 d = 0.35QO.31 v = 0.82QO.16 <br />q50 (50-percent durat ion) w = 3.4QO.65 d = 0.31QO.29 v = 0.86QO.l0 <br />q80 (BO-percent duration) w = 7.0QO.53 d = 0.24QO.32 v = 0.64QO.15 <br />The departure from 1.00 of the product of the coefficients and the sum <br />of the exponents results from rounding of the data. <br /> <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br /> <br />The discharge for the selected duration must be computed for deter- <br />mining the parameters for any location on a stream. A method of <br />determining the discharges for the desired durations for streams <br />with drainage areas between 100 and 3,000 square miles is through <br />the development of a synthetic duration curve aided by a few <br />appropriately timed low-flow current-meter measurements, as <br />described by Furness (1959, p. 209-211). The percentage duration <br />of flow of the low-flow measurements is determined as the percentage <br />duration of the simultaneous flow at a continuously operated station <br />on a stream whose basin has a cl imate (and geology) similar to that of <br />the ungaged basin. To complete the duration curve, discharges are <br />computed from ratios shown on maps and curves, and an average slope <br />factor (Furness, 1959). The standard error of estimate (measure of <br />scatter about the 1 ine of regression) of a duration curve obtained <br />by this method is not expected to exceed 27 percent for the <br />discharges at the 10, 50, and 80 percent points. <br /> <br />A simpler, but often less accurate, method of determining the <br />discharge for the selected frequency is by using the following <br />equations developed by James (1968): <br /> <br />Oa = 2.2 AO.89 qmO.91 Standard error 18 percent <br />qlO = 1 .4 AI. 05 qm 1 .03 Standard error 22 percent <br />q50 = 0.135 Al.Ol 0.97 Iv -1.59 Standard error 37 percent <br />qm <br />q80 = 0.0009 Al.4 qml.2 Iv -5.18 Standard error 96 percent <br /> <br />in which A is the drainage area, in square miles; qm is a mapped <br />value of mean discharge, in cubic feet per second per square mile; <br />and Iv is a mapped value of variability index, in log units (see <br />Figures 12, 13). To retain the same variables in the equations, <br /> <br />25 <br />