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w ", CHAPTER'ilVO HYDROLOGIC PRINCIPLES 83 :he runoff poten- treatment combinations. Outer similar tables are available in various ous land use and ,:I references (Soil Conservation Service, 1972). Impervious areas and" water surfaces are assigned a CN of 98-100. Recognizing that abstrac- aburban, and Urban .E~ lions from rainfall depend on the antecedent conditions that exist ~~ at the time a rainstorm occurs, three antecedent conditions are de- ::~c[c sott. snow fined as in Table 2.21. The curve numbers given in Table 2.20 are c o for antecedent condition II. To convert to antecedent condition I st ea 91 or III, the factors given in Table 2.22 may be used. Antecedent con- s re at 'p dition I is used when there has been little rainfall preceding the 1 rainfall in question and condition III is used where there has been t9 ~ e9 _ considerable rainfall prior to the rain in question. 5y 7Y BO .. se Tr re :ai Originally [he CN values were assigned by plotting observed runoff versus measured rainfall for a number of experimental plots ~ " s7 scattered throughouC the U.S. The CN's were then cortelated with ss ro Tr - land use. Again, note that the CN approach is a runoff approach and ;' not an infiltration approach. Certainly infiltration is a factor, but u rY eo so is quick return flow and initial abstractions. Combining the CN 6" r9 ev approach with infiltration approaches such as minimum retention n 9~ 9s rates carries the CN concept beyond it original intent and beyond the data on which the CN's are based. W 9] 97 Table 2.'_ I Definition of Antecedent Condition. es 90 9z 5-Day Antecedent Rainfall n e7 Bt in inches n ai e6 Dormant Growing a eo es Condition Genera! Descripnon Season Season se n eY 1 Optimum soil condition from about lower c OS < 1.3 i 9e 9e 9e plants Gmit to wilting point I I Average value for annual floods 0.5 - I.I 1.4 - =. I I 9e 9B 9B es e9 9s 11I Heavy rainfall or light rainfall and low tempera- > I.I > ~.1 z e3 Br B9 lures within $ days prior to [he given storm ~~ Rrff t 0 "~- "ra' 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 m aes...v a.. •~ 1-~ of total accumu]ated rainfall. Figure 2.26 has been prepared to sim- dT~o, sma, tlo~ Plify [he solution of equation 2.38. Curve numbers for complex areas can be calculated by a weighting procedure with the weighting factor :..a.