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<br />1 <br /> <br />Hydrologic Design Manual <br />for Maricopa County <br /> <br />Rainfall Losses <br /> <br />. <br /> <br />~ <br /> <br />. <br /> <br />. <br /> <br />. <br /> <br />.. <br /> <br />Hydrologic soil group D should be retained only for: <br /> <br />>> clay soils, <br /> <br />>> soils with a permanent high water table, and <br /> <br />>> rock outcrop. <br /> <br />Hydrologic soil group Dshould probably nol be retained in aU situations where the <br />classification is basl!d on shallow soils over nearly impervious layer:s; .site specific <br />studies and sensiti vity analyses should be performed to estimate the loss rates to be <br />used for such soils. <br /> <br />t!:!~{N{)\~i~~ <br />i~if\l Recommended Methods for Estimating <br />l(il'llIllll Rainfall Losses <br />~:::;::~:':::::.;.:- <br /> <br />:Jl <br /> <br />.. <br /> <br />r' <br />J <br /> <br />J <br />.f <br /> <br /> <br />'JL <br />j <br /> <br />... <br />j <br />r <br />r <br />I <br />I <br />f <br />f <br />~ <br />Ilr <br /> <br />Many methods have been developed for estimating rainfall losses; five are listed as <br />options in the HEC-1 Flood Hydrolo!;)' Packag;e. They are: <br /> <br />1, Holtan Infiltration Equation <br /> <br />2. Exponential Loss Rate <br /> <br />3. SCS Curve Numbers (CN) Loss Rate <br /> <br />4, Green and Ampt Infiltration Equation <br /> <br />5. Initial Loss Plus Uniform Loss Rate (lL+lJLR) <br /> <br />Of these five. however, only twe,--G:reen and Ampt and IL+Ul.R--.are recom- <br />mended for estiIru,ting rainfall losses in Maricopa County for the reasons discussed <br />below, <br /> <br />The Holtan Infiltration Equation is an exponential decay type of equation for <br />which the rainfall loss rate asymptotically diminishes to the minimum infiltration <br />rate (fc). The Holtan equation is not extensively used and there is no known <br />application of this method in Arizona. Data and procedures to estimate the <br />parameters for lIse in Maricopa County are not available. Therefore, the Holtan <br />equation is not re<:ommended for general USI~ in Maricopa County. <br /> <br />The Exponential Loss Rate Method is a four parameter ml~thod that is not exten- <br />sively used, but it is a method preferred by of the U.S. Army Corps of Engineers. <br />Data and procedures are not available to estimate the parameters for this method <br />for aU physiographic regions in Maricop.l County, but Exponential loss rate <br />parameters have be)'!n developed from the reconstitution of flood events for a flood <br />hydrology study .iIf(portion of Maricopa County (U.S. Anny Corps of Engineers, <br />1982), However, adequate data j6 not a valla bIe to estimate the necessary parameters <br />afl:: <br /> <br />..~. .-.-.' '.." ";':'::-:':~:;-,.,: .:.:,-.'-' ... . <br /> <br />. ...--'......-... <br />....-......"... <br /> <br />:..:..~,,:-..:.:.:.:>:h..::.: <br /> <br />,~:::.t:.:,:',:-::.:. <br /> <br />.;:,:,;.:.,<:;';.;::..0:.:.".:::::- <br /> <br />.:..:';';'''-:':':~';'::-:_:.:-:';':.:~:_M_'.:.Z':':' :::'":::';';'.'''';':-:'::''-'':<:''-'':':;-:.' <br /> <br />43 <br />