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<br />- Using climatological data and actual gauging records, hydrologically <br />separate the peak flows in each year into rain or snowmelt events, <br />then tabulate and compute their logarithms. <br /> <br />- Compute statistical parameters for each type event fitting the gauge <br />records to a log-Pearson type III distribution including: <br /> <br />(a) mean <br />(b) standard deviation <br />(c) coefficient of variation = standard deviation/mean <br />(d) skew <br /> <br />- Test the region for basin homogeneity by performing statistical tests <br />(t-tests) on the means, coefficients of variation, and skews. <br /> <br />- Re-define the region if statistical parameters fail the tests. <br /> <br />- Assign weights to each station in the region equal to their length of <br />record. <br /> <br />- Perform regression analysis to find relationships of drainage area, <br />mean watershed elevation, or other basin variables to the statistical <br />parameters. <br /> <br />- Test significance of each independent variable (sequential F-tests) <br />used in the regression analysis for inlcusion in the final regression <br />equations, <br /> <br />- Determine significance of independent variables (drainage area, mean <br />watershed elevation, etc,) at the desired ungauged site. <br /> <br />- Using the final regression equations, compute the mean, coefficient of <br />variation, standard devation, and skew for the desired location. <br /> <br />- Statistically combine snowmelt and rain frequency-discharge curves <br />into a composite frequency curve at the ungauged site. <br /> <br />PRACTICAL APPLICATION <br /> <br />The regional approach presented herein has been used in a flood flow fre- <br />quency analysis for the report entitl ed "Flood Insurance Study of the Upper <br />San Juan River Basin in Southwestern Colorado" prepared for the Federal <br />Insurance Administration by Camp Dresser & McKee, Inc. of Denver, Colorado, <br />in 1977. <br />The Upper San Juan River Basin in Colorado comprises two subbasins, the <br />San Juan River mainstem subbasin and Animas River subbasin, as shown in <br />Figure 5, The San Juan River flows in a generally southwest~ard direction and <br />is a typical mountain stream throughout its course in Colorado. Its head- <br />waters start from the southern slopes of the San Juan Mountains at about <br />13,000 feet above mean sea level (MSL), and fall to about 7,100 feet MSL, at <br />Pagosa Springs, Colorado, resulting in an average fall of approximately 250 <br />feet per mile, Below Pagosa Springs, its fall averages 27 feet per mile in <br />the reach extending to Arboles, Colorado. <br />