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<br /> <br />land covers, this method is difficult to apply. However, regional relationships may be <br />developed between the derived parameters and watershed characteristics. The <br />parameters of the method represent the effects of depression storage, infiltration <br />rates, and the nonlinearity in the loss rate process. The general form of the HEC loss <br />rate function is expressed mathematically as follows: <br /> <br />L = A * P ** E <br /> <br />(9) <br /> <br />B = Ao / ( R * * ( . 1 * C ) ) <br /> <br />(10) <br />(11 ) <br /> <br />A = B + I <br /> <br />I = .2 * 0 * ( 1 - C / D ) ** 2 <br /> <br />(12) <br /> <br />R = Ao / A 1 0 <br /> <br />(13) <br /> <br />where, <br /> <br />L = Loss rate in inches per hour <br />A = Represents combined effect of interception and infiltration and varies <br />as a function of accumulated loss <br />E = Exponent regulating the effect of the precipitation intensity term <br />Ao = Starting value of loss coefficient on exponential recession curve <br />A 10 = Value of loss coefficient' A' for cumulative loss of 10 inches <br />C = Cumulative loss in inches <br />o = Amount of accumulated loss to satisfy interception, depression <br />storage, and infiltration during the initial loss phase. <br />I = Increased losses during initial loss phase representing interception <br />and depression losses, only used until accumulated losses exceed D. <br />R = Ratio of starting value of loss coefficient to value when cumulative <br />loss equals 10 inches. <br /> <br />6. HORTON'S METHOD. <br /> <br />In the 1930's Robert E. Horton, a consulting Hydraulic Engineer, found <br />that infiltration capacity curves approximate the form: <br /> <br />Fp = Fc + ( Fo - Fc ) * e * * ( - k * t ) <br /> <br />(14) <br /> <br />where, <br /> <br />Fp = Infiltration rate in inches per hour <br />Fc = Minimum constant infiltration rate <br />Fa = Maximum infiltration rate <br />e = Napierian base <br />k = empirical constant <br /> <br />7-6 <br /> <br />