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<br />6. GREEN-AMPT. <br /> <br />The Green-Ampt method expresses tile relationsllip between cumulative infiltration <br />and infiltration rate as: <br /> <br />I" wfCiJ we) <br />f <br />Ii <br /> <br />(11-10) <br /> <br />where, <br /> <br />F = Cumulative infiltration <br />wf = wetted front suction <br />p = total porosity or water content at saturation <br />wc = initial water content <br />f = infiltration 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 condition througrl the parameters wf 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 tile storm to the time avera(Je rainfall intensity <br />is equal to the infiltration rate, and (2) estimation of 'the parameters is difficult. Techniques <br />have been developed to overcome the pl'Oblem of deterrnining infiltration prior to ponding. <br />For example, in the HEC1 program, equation (15) is used with an infiltration rate equal <br />to the average rainfall rate over the period prior .to ponding to determine the depth of <br />infiltration prior to ponding. Once ponding occurs, equation (11-10) is integrated to obtain <br />the post-ponding infiltration function which is expressed mathematically as follows: <br /> <br />F <br />Sf -In <br /> <br />(11 <br /> <br />F <br />S'f ' <br /> <br />K <br />-- ( I ,te ~ Ip ) <br />Sf <br /> <br />(11-11) <br /> <br />where, <br /> <br />Ie <br /> <br />. Sf,.' rp\ <br />III <br />1<, Sf <br /> <br />In 11' Fp I <br />Sf ' <br /> <br />(11-12) <br /> <br />Sf = Product of wetting front suction and soil moisture <br />deficit at the beginning of storlTl ( wf . (p-wc) ) <br />t = time in hours <br />tp = pondin!l time in hours <br /> <br />Colorado Flood <br />Hydrology Manual <br /> <br />DRAFT <br /> <br />?25 <br />