PERMFILE57973

• Permit No: C1981038
• IBM Index Class Name: Permit File
• Doc Date: 12/11/2001
• Section_Exhibit Name: VOLUME 8- RUNOFF PREDICTIONS
• Media Type: D
• Archive: No

Equation [8] is the general rainfall-runoff relation with the

initial abstraction taken into account. The initial abstraction is
co„~prised of interception, infiltration, and surface storage, all of
which occur before runoff begins. Each of these processes has been
discussed previously in this handbook.
In computing storm runoff, it is assumed that a fraction of the
potential maximum retention S, is the initial abstraction Ia, as the
result of interception, infiltration, and detention storage. These compo-
re~ts must be satisfied before runoff begins. The residual retention
is primarily the infiltration which occurs after runoff begins. It is
controlled: (1) by the rate of infiltration at the soil surface, or
(2) by the rate of transmission through the soil mantle, or (3) by the
water-holding capacity of the soil mantle, whichever is the limiting
factor. The SCS Handbook (SCS, 1972) discusses the relationship of
these processes to a succession of rainfall events as follows:
A succession of storms, such as one a day for a week, reduces
the magnitude of S each day because the limiting factor does
not have the opportunity to completely recover its rate or
capacity through weathering, evapotranspiration, or drainage.
But there is enough recovery, depending on the soil-cover
complex, to licit the reduction. During such a storm period
the magnitude of S remains virtually the same after the second
or third day even if the rains are large so that there is,
from a practical viewpoint, a lower limit to S fora given
soil-cover complex. Similarly there is a practical upper limit
to S, again depending on the soil-cover complex, beyond which
the recovery cannot take 5 unless the complex is altered.