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<br />The basin characteristics used in the estimating equations include <br />drainage area (A), slope (S), percent storage (St), percent lakes (Lk) , and <br />mean annual runoff (R). Values for the basin characteristics should be <br />determined by the methods described in the glossary. <br /> <br />Various combinations of basin characteristics define the significant <br />independent variables in the equations for the regions (table 1). Years of <br />record is used to determine a weighting factor for calculating peak discharges <br />at sites on gaged streams. The standard error of the estimate and equivalent <br />years of record are discussed in the section "Accuracy and Limitations of <br />Estimating Techniques". <br /> <br />The use of regression equations to estimate flood discharges on ungaged <br />streams is explained in the example below. The technique is similar for all <br />regions and all recurrence intervals for which equations are provided. If an <br />estimated discharge is required for a recurrence interval not defined by an <br />equation, plots of frequency-curve values for the site can be obtained by <br />solution of equations for all intervals. The desired discharge then may be <br />estimated graphically. <br /> <br />Example 1 <br /> <br />Estimate the 25-year peak discharge for an ungaged site on Spring Creek <br />in Swift County, at the crossing of State Highway 9, 3 1/2 miles west of <br />Sunburg. <br /> <br />1. Inspection of figure 3 shows that the site is located in Region D. <br /> <br />2. Inspection of table 5 indicates that no gaging-station data are available <br />for this stream; therefore, flow-frequency estimates should be derived <br />from regional equations. The appropriate equation for the 25-year flood <br />is found in table 1. <br /> <br />3. <br /> <br />Drainage area above the point <br />7 -1/2 -m2nute topographic map. <br />1. 28 mi . <br /> <br />of interest is outlined on the De Graff SE <br />The drainage area (A) is p1animetered as <br /> <br />4. Total lake, pond, and swamp area is de termined from the map by the grid <br />method described in the discussion on storage in the glossary. Fifteen of <br />the small grid squares are counted as storage area; the area computation <br />follows: <br /> <br />15 squares x 0.00144 mi2 - 0.02 mi2 <br /> <br />Percent storage is computed by dividing the storage area by the drainage <br />area and multiplying by 100: <br /> <br />Storage - 0.02/1.28xl00 - 1.6 percent <br /> <br />5. The mean annual runoff, estimated from figure 4 is 3.0 inches. <br /> <br />6. The main channel slope is computed as follows: <br /> <br />12 <br />