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<br />Model Application to Plot Runoff <br /> <br />A simplified, mathematical model of surface-runoff response to rainfall <br />was developed from PRMS. components and programmed for desk-top computer <br />solution, to aid in the analysis of both naturally occurring and rainfall- <br />simulator runoff events. The model is a conceptualization of reality, in <br />that the plot is characterized to be a uniformly sloping infiltrating plane. <br />Infiltration and overland-flow computations are coupled to give a more <br />realistic simulation of the boundary conditions influencing infiltration, <br />and also to account for infiltration after rainfall stops. In addition, <br />a surface-retention storage effect, and the influence of ponded storage <br />attenuation (resulting from training the flow to concentrate behind low <br />dikes and thus pass through the measuring flume) are accounted for. <br /> <br />A simple, but widely used, approximation to the infiltration process <br />was suggested over 70 years ago by Green and Ampt (1911), and is used to <br />compute time- and space-varying infiltration rates. The consequence and <br />formulation of the Green-Ampt infiltration equation was reviewed by Philip <br />(1954); more recently, Morel-Seytoux and Khanji (1974) derived an infiltra- <br />tion equation of similar form without the stringent assumptions regarding <br />the exact nature of the wetted profile. For vertical infiltration, the <br />Green-Ampt equation is given as: <br /> <br />dI <br />dt = <br /> <br />( Ho + P + Lf ) <br />KSAT L ; <br />f <br /> <br />(1) <br /> <br />where <br /> <br />~~ = infiltration rate, [ ~ ] ; <br /> <br />KSAT = hydraulic conductivity of the transmission zone, [ ~ ] ; <br /> <br />H = pressure head at the entry surface (the depth of ponded <br />o <br />water), [L]; <br />P = effective pressure head at the wetting front, [L]; and <br />Lf = length of the wetted zone, [L]. <br /> <br />The equation can be transformed to express infiltration rate as a function <br />of accumulated infiltration, I, by assuming a uniform initial moisture content, <br />WINT, and a uniform moisture content of the transmission zone above the wetting <br />front, WWET: <br /> <br />1= L (WWET - WINT); <br />f <br /> <br />(2) <br /> <br />and <br /> <br />dI - K <br />dt - <br /> <br />(1 + <br /> <br />(H <br />o <br /> <br />+ P) (~T - WINT) ) <br /> <br />(3) <br /> <br />9 <br />