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<br />6. GREEN-AMPT. <br /> <br />The Green-Ampt method expresslls the relationship between cumulative infiitration <br />and infiitration rate as: <br /> <br />F= <br /> <br />wf( p-wc) <br />(f) -1 <br /> <br />(11-10) <br /> <br />where, <br /> <br />F = Cumulative infiltration <br />wi = wetted front suction <br />p = total porosity or water content at saturation <br />wc = initial water content <br />f = infiitration rate <br />k = hydraulic conductivity <br /> <br />An advantage of the Green-Ampt method is that it gives a direct means for including the <br />initial soil moisture condilion through the parameters wi and wc. Disadvantages in <br />applying the method are that (1) it is not applicable until surface ponding occurs which is <br />calculated as the time from the beginning of the storm to the time average rainfall intensity <br />is equal to the infiltration rate, and (2) estimation of the parameters is difficuit. Techniques <br />have been developed to overcome the problem of determining infiitration prior to ponding. <br />For example, in the HEC1 program, equation (15) is used wilh an infiltration rate equal <br />to the average rainfall rate over the period prior to ponding to determine the depth of <br />infiitration prior to ponding. Once ponding occurs, equation (11-10) is integrated to obtain <br />the post-ponding infiitration function whic~1 is expressed mathematically as follows: <br /> <br />SFf -In (1 + _F ) = J5.. ( t +te +tp ) <br />Sf Sf <br /> <br />(11-11) <br /> <br />where, <br /> <br />te =(~)(~)-In (1 + ~) <br /> <br />(11-12) <br /> <br />. <br /> <br />Sf = Product of wetting front suction and soil moisture <br />deficil at the beginnin~1 of storm ( wi . (p-wc) ) <br />t = time in hours <br />tp = ponding time in hours <br /> <br />Colorado Flood <br />Hydrology Manual <br /> <br />DRAFT <br /> <br />7.2S <br />