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84 CHAPTER TWO <br />being the proportion of the total area represented by the individual <br />curve numbers. <br />Table 2 ~2 Factors for Converting CN's to Antecedent Condition. <br />Curve Number Factor to Convert Curve Number <br />for for Condition 11 to <br />Condition fl Condition 1 Condition 111 <br />10 0.40 2.22 <br />20 0.45 1.85 <br />~.I '~~ 30 0.50 1.67 <br />' 40 0.55 1S0 <br />50 0.62 1.40 <br />60 <br />~ <br />~ 0.67 1.30 <br />70 0,73 1.21 <br />' <br />i 80 0.79 1.14 <br />j. ~ 40 0.87 1.07 <br />j 100 <br />~! 1.00 1.00 <br />i4, As an example oC computing rainfall excess, assume the rain- <br />storm of Table 2.13 falls on a watershed that is 3S percent bare <br />spoil in hydrologic soil group D and has 30 percent of its remaining <br />{,, soils in hydrologic soil group B under grass and 3S percent in hydro- <br />logic soil group C under forest. The appropriate CN can now be cal- <br />culated by noting the spoil area has a CN of 91, the B soil has a CN <br />of 58, and the C soil, a CN of 77. Thus, the weighted CN is <br />CN =.35(91) + .35 (77) +.30(58) = 77 <br />In this example, it is assumed that the various soils are randomly <br />and somewhat uniformly scattered throughout the watershed and <br />an unknown antecedent condition exists. Considering that the 2S- <br />year, 3-hour rain is likely to be a part of a longer storm, it is assumed <br />that antecedent condition III applies. The CN of 77 corresponds to <br />an antecedent condition of II so a conversion is required. Selecting <br />the proper value from Table 2.22, the final CN is 1.16(77) or 89. <br />Table 2.23 shows the calculations required to arrive at the effective <br />rainfall pattern. <br />In some cases the weighting of infiltration indices, whether <br />they be CN's or ~ indices or some other index, may be inappropriate. <br />