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t = time from beginning of rainfall In the Storm Water Management Model (SWMMI. 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. 7. GREEN-AMPT. The Green-Ampt method expresses the relationship between cumulative infiltration and infiltration rate as: F = wf * ( P - wc) / ( ( f/k ) - 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 advantage 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 ponding. 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) is .integrated to obtain the post-ponding infiltration function which is expressed mathematically as follows: . (F/Sf) -In(1 + (F/Sf)) = (K/Sf)*(t+te+tp) (16) where, te = (Sf/K)*(Fp/Sf)-ln(1 + Fp/Sf))) (17) Sf = Product of wetting front suction and soil moisture deficit at the beginning of storm ( wf * (p-wc) ) t = time in hours tp = ponding time in hours 7-7