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<br />Hydrologic Design Manual <br />for Maricopa County <br /> <br />Rainfall Losses <br /> <br />parameters, or for other valid reasons. It should be realized, as explained later, that <br />the use of the Green and Arnpt equation and parameters, as defined herein, will <br />probably result in lower peak disch,lr,ges and runoff volumes than the use of the <br />IL+ULR. <br /> <br />Other methods should be used only if there is technical justification for a variance <br />from this recommendation and if adequate infOlmation is available to estimate the <br />necessary parameter.>. Use of rainfall loss methods other than those recommended <br />should not be unde11aken unless previously approved by the Flood Control District <br />and the local regulatory agency. <br /> <br />, ~- <br /> <br />4.4.1 Green and Ampt Infiltration Equation <br /> <br />This model, first developed in 1911 by W.H. Green and G.A. Ampt, has since the <br />early 1970s, received increased interest for estimating rainfall infiltralion losses. <br />The model has the ferm: <br /> <br />1__ <br /> <br />I... <br /> <br />f=Ks (1 +~) <br />F <br /> <br />for f <i <br /> <br />(1) <br /> <br />f=i <br /> <br />for f~ i <br /> <br />where <br /> <br />. <br /> <br />f <br /> <br />= infiltration rate (L/T), <br /> <br />= rainfall intensity a.IT), <br /> <br />. <br /> Ks <br /> 'I' <br />.- <br /> 0 <br />.-- <br /> <br />= hydraulic conductivity, wetted zone, steady-state' rate (LIT) <br /> <br />= average capillary suction in the wetted zone (I.), <br /> <br />= I;oil moisture deficit (dimensionless), equal to effective soil <br />porosity times .J <br />(.theaJ.fference in final and initial volumetric soil saturations, and <br /> <br />F <br /> <br />= depth of rainfall that has inJlltrated into the soil since the beginning of <br />rainfall (L). <br /> <br />.-- <br /> <br />-.. <br /> <br />A sound and concise explanation of the Green and Ampt equ.,tion is provided by <br />Bedient and Huber (1988). <br /> <br />... <br /> <br />It is important to nol:e that as rain continues, F increases and f approadles Ks, and <br />therefore, f is inversely related to time. Equation 1 is implicit with respect to f which <br />causes computational difficulties. Eggert (1976) simplified Equation 1 byexpand- <br />ing the equation in a power series and truncating all but the first two terms of the <br />expansion, The simplified solution (Li and others, 1976) is: <br /> <br />~. <br /> <br />F = -{J.5 (2F _. Ks 6/) + 0.5 [(2F - Ks 6/)2 + 8Ks6t (00/ + F)] 1.1 <br /> <br />(2) <br /> <br />r-' <br />i <br /> <br />.;-:-, :.: ".; :-.-.;~<-:-:v:.;.:.",.:-:{:: ;.: ;.;."., ;-:~ :-,-.-:-;-; :-; :",x.,:.:.:.; :-;.; ;.:{.:*:: ,-: :';;"v;,';':':'; :'..'~ ;.; .{O;;<.;; ,.:.::;:: <br /> <br />:,.".,.:,;-:-;,:,::: <br /> <br />.; :.: ,.:;.:~:~ ~:': .,., ,.,.,.;.:.,., >':<':~'.-'..:; ..;.' ;. <br /> <br />45 <br />