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5. HORTON'S METI~OD. In the 1930's Robert E. Horton, a consulting Hydraulic Engineer, found that infiltration capacity curves approximate the form: Fp = Fc + ( Fa - Fc ) . e .. ( - k . t ) (14) where, Fp = Infiltration rate in inches per hour Fc = Minimum constant infiltration rate Fa = Maximum infiltration rate e = Napierian base k = empirical constant t = time from beginning of rainfall In the Stor.m Water Management Model (SWMM), the Horton equation is used for pervious areas. Additional parameters can be specified for initial abstractions for both pervious and impervious areas. If the Maximum and minimum infiltration rates are set equal in Horton's equation, a uniform infiltration rate is achieved. 6. GREEN-AMPT. The Green-Ampt method expresses the relationship between cumulative infiltration and Infiltration rate as: F = wf . ( P - wc) I ( ( flk ) - 1 ) (15) where, F = Cumulative infiltration wf = wetted front suction p = total porosity or water content at saturation wc = initial water content f = infiltration rate k = hydraulic conductivity An advantagel of the Green-Ampt method is that it gives a direct means for including the initial soil moisture condition through the parameters wf and wc. Disadvantages in applying the method are that (1) it is not applicable until surface ponding occurs which is calculated as the time from the beginning of the storm to the time average rainfall intensity is equal to the infiltration rate, and (2) estimation of the parameters is difficult. Techniques have been developed to overcome the problem of determining infiltration prior to poneling. For example, in the HEC1 program, equation (15) is used with an infiltration rate equal to the average rainfall rate over the period prior to ponding to determine the depth of infiltration prior to ponding, Once ponding occurs, equation (15) Colorado Flood Hydrowgy Manual DRAFT 7.18