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<br />I <br />I <br />I <br />I <br />I' <br /> <br />S = the slope of the plane, ft/ft; and <br /> <br /> <br />FRIG = a roughness parameLer similar to Manning's n, but scaled to <br /> <br /> <br />reflect roughness elements quite different than those <br /> <br /> <br />for typical open-channel flows. <br /> <br />The finite-difference numerical techniques developed by Leclerc and Schaake <br />(1973), and described by Dawdy and others (1978), are used to approximate q(x,t) <br />at discrete locations in the x-t plane. A rectangular grid of points spaced <br />at intervals of time, at, and distance, ax, is used. <br /> <br />Two additional features are accounted for in the routing of overland flow: <br />(1) The effect of irregular surface features causing the impoundment of small <br />pockets or puddles of water that collectively produce a surface-retention- <br />storage capacity; and (2) the effect of diversion dikes forming the downslope <br />boundary of the plot. Overland flow can only occur when the effective surface- <br />retention capacity is exceeded. Magnitude of this retention capacity is small <br />(on the order of a few hundredths of an inch), but has been observed to exert <br />an important influence on the time to runoff. Diversion or training dikes <br />effectively pond water immediately above the measuring flume. This impoundment <br />of surface water causes a slight but recognizable attenuation of the rising <br />hydro graph, and also sustains the recession hydrograph after overland flow <br />ceases. Ponded storage attenuation is modeled by a reservoir-routing technique <br />that utilizes a storage-outflow relation of the form: <br /> <br />KN <br />Storage = KS . Outflow ; <br /> <br />(8) <br /> <br />where KS and Kn are determined from a detailed survey of the plot area within <br />the diversion dikes. The parameters used in the simulation model of plot run- <br />off and the method of determination are summarized in table 2. <br /> <br />Table 2.--PalLameteM CLIled .i..n. l>hnu.la-t.i..on model. 06 plo;t /Lww66 <br /> <br />Method of <br /> <br />Parameter <br /> <br />Meaning <br /> <br />determination <br /> <br />KSAT <br />P <br />WWET <br />WINT <br />FRIC <br />S <br />m <br />SURF <br />KS, NS <br /> <br />Hydraulic conductivity <br />Effective pressure head at wet front <br />Moisture content of transmission zone <br />Initial moisture content <br />Surface roughness <br />Flow-plane slope <br />Turbulent flow-routing parameter <br />Surface-retention storage, in inches <br />Reservoir-routing parameters <br /> <br />Fit. <br />Do. <br />Sampled. <br />Do. <br />Fit. <br />Measured. <br />Fixed (1.67). <br />Fit. <br />Measured. <br /> <br />11 <br />