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I CHAPTER TWO HYDROLOGIC PRINCIPLES 83 '.he runoff poten- treatment combinations. Other similar tables are available in various ous land use and ,.~ references (Soil Conservation Service, 1972). Impervious azeas and' water surfaces are assigned a CN of 98-100. Recognizing that abstrac- uburban, and Urban .4~ lions from rainfall depend on the antecedent conditions that exist i{ at the time a rainstorm occurs, three antecedent conditions are de- ~1C 501L G101P `.` fined as in Table 2.21. The curve numbers given in Table 2.20 are e c n j for antecedent condition II. To convert to antecedent condition 1 at ee 91 - or [I1, the factors given in Table 2.22 may be used. Antecedent con- Tl Te e1 dition I is used when there has been little rainfall preceding the tainfall in question and condition IiI is used where there has been r9 ab e9 considerable rainfall prior to the rain in question. 4 T4 80 j: se Tt re at Originally the CN values were assigned by plotting observed runoff versus measured rainfall For a number of experimental plots ee TT ea scattered throughout the U.S. The CN's were then correlated with 55 TC TT land use. Again, note that the CN approach is a runoff approach and -' not an infiltration approach. Certainly infiltration is a factor, but ~ T4 eo so is quick return Flow and initial abstractions. Combining the CN ~ 79 04 approach with infiltration approaches such as minimum retention { 9z 94 9s rates carries the CN concept beyond it original intent and beyond 1 the data on which the CN's are based. ' e3 91 9] { I Table ?.'_ I Definition of Antecedent Condition. es 90 9z 5-Day Antecedent Rainfall Ts as eT in inches rz e1 e6 Dormant Growing n so es Condition General Description Season Season 6e T9 B4 1 Optimum soil condition from about lower < 0.5 < 1.4 B 9a ye 9e plants limit to wilting point 11 Average value (or annual hoods 0.5 - I.1 1.4 - ?. I s 9e 9e 9a ey eq yr 111 Heavy rainfall or light rainfall and low tempera- > I.I > ?.1 2 e2 BT e9 lures within 5 days prior [o the given storm yv~ nrer co "- 1911' Once the proper CN is obtained, equations 2.38 and 2.39 can ~'- be used to estimate the accumulated rainfall excess as a function R Y4w~Y m+r^ 1wu of total accumulated rainfall. Figure 2.26 has been prepared to sim- .,~4,0. cmaT:tai Plify the solution of equation 2.38. Curve numbers for complex areas can be calculated by a weighting procedure with the weighting factor ~ ..e.