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CHAPTER TWO ~ HYDROLOGIC PRINCIPLES 83 1te runoff poten- ~ treatment combinations. Other similar tables are available in various ous land use and references (Soil Conservation Service, 1972}. Impervious areas and water surfaces are assigned a CN of 98-100. Recognizing that abstrac- 'uburban, and Urban + bons from rainfall depend on the antecedent conditions that exist at the time a rainstorm occurs, three antecedent conditions are de- sacrc spry °aarv fined as in Table 2.21. The curve numbers given in Table 2.20 are e c ° ~ for antecedent condition Il. To convert to antecedent condition I ~ ~ 91 or iii, the factors given in Table 2.22 may be used. Antecedent con- r1 ,a e4 dition I is used when there has been little rainfall preceding the rainfall in question and condition III is used where there has been r9 B6 89 considerable rainfall prior to the rain in question. A r4 B° u r1 ,e Originally the CN values were assigned by plotting observed runoff versus measured rainfall for a number of experimental plots ~ n "' scattered throughout the U.S. The CN's were then correlated with ~ 70 " land use. Again, note that the CN approach is a runoff approach and not an infiltration approach. Certainly infiltration is a factor, but v ,4 eo so is quick return flow and initial abstractions. Combining the CN s9 r9 a4 approach with infiltration approaches such as minimum retention rates carries the CN concept beyond it original intent and beyond '~ 94 9s the data on which the CN's are based. OC 91 97 Table 2.? I Definition o(Antecedent Condition. to 9° qp S-Day Antecedent Rainfall rs e~ e, in inches rx B1 e6 Dormant Growing ~ eo es _ Condition General Descnption Season Season I fa 79 94 ;, I Oplvnum soil condition from about lower < 0.5 < 1.4 " ~ qa qa '-+ plants limit to wilting point s 11 Average value for annual hoods 0.5 - 1, I 1.4 - 2. I g ~E 9B 9B '~=~ ,; ~, e9 91 ;! 111 Heavy rainfall or light rainfall and low tempera- > 1.1 > 2.1 _ ~ ar °9 lures within 5 days prior to the given storm ~e refer co ~•'~' `• Once the proper CN is obtained, equations 2.38 and 2.39 can °1' be used to estimate the accumulated rainfall excess as a function ~~ _ of total accumulated rainfall. Figure 2.26 has been prepared to sim- aP.~e eonale,a~ ~ plify the solution of equation 2.38. Curve numbers for complex areas ~' can be calculated b a wei htin y g g procedure with the weighting factor IY M".