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<br />-_.._...:....:-~.-......;.~- <br /> <br />Basin Characteristics Investi2ated <br /> <br />Necessary preconditions for development of useful transfer equations are <br />that the basin characteristics used be limited in number and easily determined <br />from maps. The basin characteristics (independent variables) investigated for <br />this analysis were contributing drainage area, main chaI)nel slope, percent <br />area of storage, percent area of lakes and of forest cpver, basin shape, mean <br />annual runoff, and 30-year-normal annual precipitation (Baker and Kuehnast, <br />1978). Regression analysis indicated that both runoff and precipitation were <br />significant characteristics in some regions. . However, runoff generally pro- <br />duced a more linear fit than did precipitation. For that reason, mean annual <br />runoff was used rather than normal annual precipitation in the regression <br />equations. <br /> <br />Definitions and procedures for calculating selected basin characteristics <br />are given in the glossary. Figure 4 shows the mean annual runoff for the <br />State of Minnesota. <br /> <br />ESTIMATING FLOOD FREQUENCY OF UNREGULATED STREAMS <br /> <br />The most reliable estimates of flood-flow magnitudes for specific recur- <br />rence intervals are based on an analysis of recorded floods at the site under <br />consideration. Such records are not available at most places of interest. and <br />,estimates of floods must be obtained by transfer of information from gaged <br />sites or by analysis of generalized flood-frequency relations. The following <br />is a discussion of both techniques along with example computations. <br /> <br />Analvses of Un2a2ed Sites usin2 Generalized Relationships <br /> <br />Equations, obtained from multiple-regression analyses of gaging-station <br />data in each hydrologic region, can be used to obtain flood-frequency <br />estimates for ungaged sites on unregulated streams. Peak discharges for <br />selected recurrence intervals can be computed from the empirical equations <br />that relate flood magnitude to basin characteristics. A set cf equations to <br />estimate flood peaks for 2-, 5-, 10-. 25-, 50-, and 100-year recurrence <br />intervals (identified as Q2' Q5' QIO' etc.) are provided in table 1 for each <br />of the four hydrologic regions in Minnesota. The four regions of the State <br />are outlined on figure 3. <br /> <br />The regions represent areas in which each of the stations in the region <br />exhibits an unbiased response to the equations for that region. However, <br />regional boundaries cannot be precisely defined and particular care should be <br />exercised when the site in question has basin characteristics that differ from <br />the general characteristics of a region. The limiting basin characteristics <br />for each region are discussed in the section on Accuracy and Limitations of <br />Estimating Techniques. If an estimate is to be made downstream from a <br />regional boundary that a stream crosses (the St_ Louis River is the only such <br />stream in the State). the discharge at the site should be determined by <br />transfer from the gaged site. <br /> <br />8 <br />