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Accordingly, the runoff curve number approach proposed in $CS and Forest <br />• Service manuals for making these evaluations is de-emphasized in favor <br />of process hydrology previously discussed by Leaf (1977), <br />Theory <br />The following material is excerpted from Chapter 10 of Section A in <br />the Soil Conservation Service National Engineering Handbook (1972). It <br />summarizes the basic Hydrologic principles that must be considered in <br />evaluating storm runoff. <br />If records of natural rainfall and runoff for a large storm <br />over a small area are used, a plotting of accumulated runoff <br />versus accumulated rainfall will show that runoff starts after <br />some rain accumulates (there is an "initial abstrac*_ion" of <br />rainfall) and that the double-mass line curves, becoming <br />asymptotic to a straight line. On arithmetic graph paper and <br />with equal scales the straioht line has a 45-degree slope. <br />The relation between rainfall and runoff can be developed from <br />this plotting, but a better understanding of the relation is <br />• had by first studying a storm in which rainfall and runoff <br />begin simultaneously (the initial abstraction does not occur). <br />For the simpler storm the relation between rainfall, runoff <br />and retention (the rain not converted to runoff) at any point <br />on the mass curve can be expressed as: <br />~ _ ~ [l] <br />where F =actual retention <br />S' = potential maximum retention (S'F) <br />Q =actual runoff <br />P =potential maximum runoff (P Q) <br />Equation [1] applies to on-site <br />there is a lag in the appearanc <br />gage, and the double-mass curve <br />But if storm totals for P and Q <br />apply even for large watersheds <br />lag are removed. <br />runoff; for large watersheds <br />__ of the runoff at the stream- <br />produces a different relation. <br />are used equation [1] does <br />because the effects of the <br />i• <br />