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based on the physical characteristics of the watershed and is simple to apply since it requires only one parameter to be estimated, it has been increasingly used in applications that its authors had not intended. In development of the method, the basic assumption was that during a storm event, there is a threshold which must be exceeded before runoff occurs which satisfies interception, depression storage, and the infiltration quantity before the start of runoff. This amount of rainfall is termed the initial abstraction, or la. After the initial abstraction is satisfied, the total actual retention increases with increasing rainfall up to the maximum retention. Since runoff also increases as the rainfall increases, the SCS hypothesized that the ratio of actual retention to maximum retention is assumed to be equal to the ratio of runoff to rainfall minus initial abstraction. This assumed relationship is expressed mathematically as follows: Q / ( P - la ) = F / S (1) where, Q = Runoff in inches P = Precipitation in inches la = Initial abstraction in inches F = Total retention in inches S = Maximum retention in inches A second equation, based on the water balance equation, was developed and is presented as follows: P = Q + la + F (2) When equation (1) and (2) are solved simultaneously for Q, they yield: Q = ( P - la ) ** 2 / {( P - la) + S ) (3) Since equation (3) requires two parameters (Ia and S), the SCS further simplified it by developing an empirical relationship between la and S based on field data. la = 0.2 * S (4) When equation (4) is substituted into equation (3), the result is the SCS one parameter infiltration equation: - Q = ( P - 0.2 * S) ** 2/( P + 0.8 * S) (5) 7-4