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- 4 - <br />• The parameter S' in e~,uation [1] does not contain the <br />initial abstraction and differs from the parameter S to be <br />used later. The retention S' is a constant for a particular <br />storm because it is the maximum that can cccur under the <br />existing conditions if the storm continues without limit. <br />The retention F varies because it is the difference between P <br />and Q at any point on the mass curve or <br />F=P-Q [2] <br />Ii <br />Equation [1] can therefore 6e re~:rritten: <br />P s g = 4 [~] <br />Solving for Q produces the equation: <br />z <br />Q = P P S ~ [a] <br /> <br /> <br />vrhich is a rainfall-runoff relation in which the initial <br />abstraction is ignored, <br />The initial abstraction is brought into the relation by <br />subtracting it frc~ the rainfall. The equivalent of <br />eyuatien [1] beco~;~es: <br />F = Q L5] <br />S P- Ia <br />where Ia is the initial abstraction, F S, and Q (P - Ia). <br />The paramter S includes Ia; that is, S = S' + Ia <br />Equation [2] becomes: <br /> <br />,__ • <br />r <br />~_ <br />equation [3] becomes: <br />(P - Ia) - Q Q <br />and equation [4] becomes: <br />Q` P- Ia)+S <br />