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I v:a., SCf' ~~~ ~~~ <br />,SrCj~D/~ ~ (f~J02D~0(r~j 10.5 <br />• wLiclr i:', thc.• r.~infnii-runof:' r i~~li~r.'•:ith t.h~. iritinl abr:Lr•,ction <br />taken info acc:ouut. <br />The .initial abstraction con.;i:,t_; m'~inly of int~rc=otion, infiltra- <br />tion, and surface storage, ali of which occur boa"ore runoff begins. <br />Wit,. i.n:,~ rt on fi;;ur '_0.1 s'..:..:.; tl,~ i csiLion of .a in n tyr,ical <br />stox-m. To remove t}::: n~•cescit,/ 1'cr ~-::timating t':ese variabi~:; in <br />equation 10.8, the relat.icn betwe<'n Ia and S (which include; Za) <br />was developed by means of rainfall and rxrloff data from exr.~rim~n'_al <br />small watersheds. i~:e relation is discussed later .n connection <br />with figure 10.2, The ertipirical relationship is: <br /> <br />Substituting 10.9 in 10.8 give;: <br />Q P + 0.2 S (10.10) <br />which is the rainfall-runoff relation used in the SCS method of es- <br />timating direct nmoff from storm rainfall. <br />Retention Parameters <br />• " Equation 10.9 states that 20 percent (an average, as figure 10.2 <br />shown) of the potential maxi¢'xm retention S is the initial abstrac- <br />ticn Ia, which is the interception, infiltration, and surface stor- <br />age occurring before runoff begins. The remaining 80 percent is <br />mainly the infiltration occurring after runoff begins. This later <br />infiltration is controlled by the x•ate of infiltration at the soil <br />surface or by the rate of transmission in the soil profile or by <br />the water-storage capacity oP the profile, whichever is the limit- <br />ing factor. A succession of storms, such as one a day for a wee};, <br />reduces the magnitude of S eacY. day because the limiting factor <br />does not have the opportunity to completely recover its rate or <br />capacity through weathering, evapotranspiration, or drainage. But <br />there is enough recovery, dependir~ on the soil-cover complex, to <br />limit the reduction. DurinS such a stox2n period the r.~agnitude of <br />S remains virtually the same after the second or third day even <br />if the rains are large so that there is, from a practical viewpoint, <br />a lower limit to S for a given soil-covex• complex. Similarl; there <br />is a practical upper limit to S, again depending on the soil-cover <br />complex, beyond which the recovery cannot take S unless the complex <br />is altered. <br />In the SCS method the change in v (actually in•CI•~) is based on an <br />antecedent moisture condition (.4.1C) detex-mined h': the total rainf <br />• <br />