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- J - <br />Equation [8] is the general rainfall-runoff relation with the <br />initial abstraction taken into account. The initial ahstraction~is <br />comprised of interception, infiltration, and surface storage, all of <br />vihich occur before runoff begins. Each of these processes has been <br />discussed previously in this handbook. <br />In computing storm runoff, it is assumed that a fraction of the <br />potential maximum retention S, is the initial abstraction Ia, as the <br />result of interception, infiltration, and detention storage. These compo- <br />~- <br />j reAtsmust be satisfied before rur•off begins. The residual retention <br />i- is primarily the infiltration which occurs after runoff begins. It is <br />' controlled: (1) by the rate of infiltration at the soil surface, or <br />I (Z) by the rate of transmission through the Soil mantle, or (3) by the <br />viater-holding capacity of the soil mantle, whichever is the limiting <br />. factor. The SCS Handbook (SCS, 1972) discusses the relationship of <br />these processes to a succession of rainfall events as follo::s: <br />- A succession of storms, such as one a day for a week, reduces <br />the magnitude of 5 each day because the limiting factor does <br />not have the opportunity to completely recover its rate or <br />capacity through weathering, evapotranspiration, or drainage. <br />But there is enough recovery, depending on the soil-cover <br />complex, to limit the reduction. During such a storm period <br />the magnitude of S remains virtually the same after the second <br />- or third day even if the rains are large so that there is, <br />from a practical viev:point, a lower limit to S for a given <br />soil-cover complex. Similarly there is a practical upper limit <br />to S, again depending on the soil-cover complex, bayond erhieh <br />the recovery cannot take S unless the complex is altered. <br /> <br /> <br />