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<br />Rainfall Losses <br /> <br />Hydrologic D'eslgn Manual <br />for Maricopa County <br /> <br />-~ <br /> <br />where <br /> <br />Ll.t = the computation interval <br /> <br />F = accumulated depth of infiltration at the start of Ll.t. <br /> <br />The average infiltration rate is: <br /> <br />M <br />f=- <br />tot <br /> <br />(3) <br /> <br />Use ofthe Green and Ampt equation as coded in HEC-1 involves the simulation of <br />rainfall loss as a two phase process, as illustrated in Figure 4.2. TIle first phase is <br />the simulation of the sw'face retention loss as previously described; this loss is called <br />the initial loss (IA) in HEC-1. During this first phase, all rainfall is lost (zero rainfall <br />excess generated) during the period from the start of rainfall up to the time that the <br />accumulated rainfall equals the value of Ii\. It is assumed, for modeling purposes, <br />that no infiltration of rainfall occurs during this first phase. Initial loss (lA) Is <br />primarily a function of liond-use and surface cover, and recommended values of IA <br />for use with the Green and Ampt equation are presented in Table 4.1, For example, <br />about 0,35 inches of rainfall will be lost to runoff due to surface retE:ntion for desert <br />and rangelands on relatively flat slopes in Maricopa County. <br /> <br />The second phase of the rainfall loss process is the iniiltration of rainfall inlo the soil <br />matrix. For modeling purposes, the infiltration begins immediately after the surface <br />retention loss (IA) is ccmpletely satisfied, as illustrated in Figurl~ 4.2. The three <br />Green and Ampt equation infiltration parameters as cuded in HEC-1 are: <br /> <br />>> hydraulic conductivity at natural satul,ltion OCKSA 1') equal to I<s in Equation 1; <br /> <br />>> wetting front capillary suction (l'SIF) equal to'!' in Equation 1; and <br /> <br />>> volumetric soil moisture deficit at the start of rainfall (DTHET A) equal to <br />e in Equation ], <br /> <br />The three infiltration parameters are functions of soil characteristics, ground surface <br />characteristics, and land. management practices. The soil characteristics of interest <br />are particle size distribution (soil texture), organic matter, and bulk density. The <br />primary soil surface characteristics are vegetation canopy cover, ground cover, and <br />soil crusting. The land management practices arE: identified as various tillages as <br />they result in changes 1:0 soil porosity. <br /> <br />Values of Green and Ampt equation parameters as a function of soil characteristics <br />alone (bare ground condition) have been obtained from publishl~ reports CRawls <br />and others, 1983; Rawls and Brakensiek, 1983). Average values of XKSA T and l'SIF <br />for each of the soil texl:ure classes from Rawls and Brakensiek (1983) are shown in <br />Columns (2) and (3) ofTable 4.2. Values ofXI<SA T and l'SIF (as a function of percent <br />of sand and percent of clay for soil with 0.5 percent organic matter and base value <br />[unaltered] soil porosity) are shown in Figures 4.3 and 4.4, respe<:tively (Rawls and <br />Brakensiek,1983). Th,e values of XKSAT and PSIF from Table 4.2 should be used if <br /> <br />. -.' ," ::::.::;.:.;.:-::.;:.:.::-::~~.:{.:-: ".. ":::-::-::'::';':':'::':':':0;:':':':';-'" ..............,:::.::.:..:.:.:.-......... <br /> <br />. ".' - :.: :':',:- -:;.:.;.:.:.:,:.::-;-:-:-:-~:-:.:,:~-~ <br /> <br />46 <br />