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<br />.." <br /> <br />2. REGRESSION EQUATIONS. <br /> <br />Regional frequency analysis usually involves regression analysis of gaged <br />watersheds within the general region. Through this very powerful technique, <br />sufficiently reliable equations can often be derived for peak flow of varying frequency <br />given quantifiable physical basin characteristics and rainfall intensity for a specific <br />duration. Once these equations are developed, they can then be applied to ungaged <br />basins within the same region. <br /> <br />A regional analysis usually consists of the following steps: <br /> <br />. Select components of interest, such as mean and peak discharge. <br /> <br />. Select definable basin characteristics of gaged watershed: drainage <br />area, slope, etc. <br /> <br />. Derive prediction equations with single or multiple linear regression <br />analysis. <br /> <br />. Map and explain the residuals (differences between computed and <br />observed values) that constitute 'unexplained variances, in the statistical <br />analysis on a regional basis. <br /> <br />The equation can then be used in ungaged areas within the same region <br />and for data of similar magnitude to that used in the development process. Much <br />more detail on regression and regional frequency analysis is available in EM 1110-2- <br />1415, Hydrologic Variability. <br /> <br />Regional equations have already been developed by the U.S. Geological <br />Survey (USGS) and published for the various areas of the United States. An example <br />of this type of equation is the following: <br /> <br />0'00 = 19.7 A 0.88 pO.84 H,o.33 <br /> <br />(2) <br /> <br />where: <br /> <br />0'00 = the I % chance flood peak (cfs) <br />A = drainage area (sq. miles) <br />p = mean annual precipitation (inches) <br />H = average main channel elevation at 10% and 85% points <br />along the main channel length (1000 feet) <br /> <br />7-87 <br />