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.J <br />Accordingly, the runoff curve number approach proposed in SCS dnd Forest <br />• Service manuals for making these evaluations is de-emphasized in favpr <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 Platicnal 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 accu~,~ulated rw~off <br />versus accumulated rainy"all will show that runoff starts after <br />same rain accumulates (there is an "initial abstraction" of <br />rainfall) and that the double-mass line curves, becoming <br />asymptotic to a straight line. un arithmetic gr'apii paper and <br />with equal scales the straight line has a 45-degree slope. <br />The relation bet~~;een rainfall and runoff can be developed from <br />this plotting, but a better understanding of the relation is <br />• toad by first studying a storm in which rainfall and runoff <br />begin Simultaneously (the initial abstraction does net occur). <br />rr-or the sir,pler 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 />5 P [lJ <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 runoff; for large watersheds <br />there is a tag in the appearance of tiro runoff at the stream- <br />- nape, and the double-mass curve produces a different relation. <br />`..'ut if storm totals for .' and Q are used equation [lJ does <br />rpply even for large ~:;atersheds because the effects of the <br />lag are removed. <br />• <br />