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<br />3. SCS CURVE NUMBER METHOD. <br /> <br />Probably the most widely used and misused method for estimating infiltration <br />losses is the SCS Curve Number method. This method was developed by the SCS in the <br />1950's in order to pmdict 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 increasin,gly used in applications that its authors had not intended. <br /> <br />In developmont 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 ret,antion 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 foi:lows: <br /> <br />Q/(P-la)=F/S <br /> <br />(1) <br /> <br />where, <br /> <br />Q = Runoff in inches <br />P = Precipitalion 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 />(2) <br /> <br />When equation (1) and (2) are solved simultaneously for Q, they yield: <br /> <br />Q = ( P - la ) .. 2 I (( P - la) + S ) <br /> <br />(3) <br /> <br />Since equation (3) requires two parameters (la and S), the SCS further simplified <br />it by developing an empirical relationship between la and S based on field data. <br /> <br />la = 0..2 . S <br /> <br />(4) <br /> <br />Colorado Flood <br />Hydrology Manual <br /> <br />DRAFT <br /> <br />7.16 <br />