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<br />3. SCS CURVE NUMBER METHOD. <br /> <br />Probably the most widely used anCl misused method for estimating infiitration <br />losses is the SCS Curve Number method. This method was developed by the SCS in the <br />1950's in order to predict the effects of proposed changes in land use and treatment on <br />direct runoff. It was based on observed daily rainfall and runoff data from field test plots <br />located in the Midwest in order to develop average relationships between rainfall and runoff <br />and was not intended to be used for the simUlation of individual events. Since the method <br />is the only one available for estimating loss rates based on the physical characteristics of <br />the watershed and is simple to apply since it requires only one parameter to be estimated, <br />it has been increasingly used in applications that its authors had not intended. <br /> <br />In development of the method, the basic assumption was that during a storm <br />event, there is a threshold which must be exceeded before runoff occurs which satisfies <br />interception, depression storage, and the infiltration quantity before the start of runoff. This <br />amount of rainfall is termed the initial abstraction, or la. After the initial abstraction is <br />satisfied, the total actual retention increases with increasing rainfall up to the maximum <br />retention. Since runoff also increases as the rainfall increases, the SCS hypothesized that <br />the ratio of actual retention to maximum retention is assumed to be equal to the ratio of <br />runoff to rainfall minus initial abstraction. This assumed relationship is expressed <br />mathematically as follows: <br /> <br />~-=!:. <br />(P-I,,) 5 <br /> <br />(11-1) <br /> <br />where, <br /> <br />Q= Runoff in inches <br />P = Precip~ation in inches <br />la= Initial abstraction in inches <br />F = Total retention in inches <br />S = Maximum retention in inches <br /> <br />A second equation, based on the water balance equation, was developed and is <br />presented as follows: <br /> <br />P=Q+la+F <br /> <br />(11-2) <br /> <br />When equation (11-1) and (11-2) are solved simuitaneously for Q, they yield: <br /> <br />Q = (p-Ia)2 <br />(P-Ia) +5 <br /> <br />(11-3) <br /> <br />Since equation (11-3) requires two parameters (Ia and S), the SCS further simplilied it by <br />developing an empirical relationship between la and S based on field data. <br /> <br />Colorado Flood <br />Hydrology Manual <br /> <br />DRAFT <br /> <br />7.22 <br />