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The parameter S' incquation [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 occur 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 />Equation [1] can therefore 6e rewritten: <br />P ~ = P ~3] <br />Solving for Q produces the eouation: <br />z <br />Q = P P S. [4] <br />~vhich is a rainfall-runoff relation in which the initial <br />abstraction is ionored. <br />The initial abstraction is brought into the relation by <br />• subtracting it from the rainfall. The equivalent of <br />equation [1] becomes: <br />F = [5~ <br />S - Ia <br />where Ia is the initial abstraction, F 5, and Q (P - Ia).. <br />The paramter S includes Ia; that is, S = S' + Ia. <br />Equation [2] becomes: <br />F = (P - Ia) - Q ~6~ <br />equation [3] becomes: <br />S ZP - <br />and equation [4] becomes: <br />• <br />Q P- Ia +S <br />