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Rainfall Losses Hydrologic Design Manual for Maricopa County Land-use and/or Surface Cover (1 ) SurfaCt~ Retention Loss IA,lncheis (2) I I I. I I I I lable 4.1 Surface Retention l.oss for Various Land SUrfaces In Maricopa County Natural ________ ____ Desert and ran eland. flat slope ______~D.35---- Hillslo es. Sonoran Desert 0.15 Mountain. with vegetated sUrface ______. 0.25___ Develo ed Residential and Commerciall.___. Lawn and turf ~0.20 Desertlandsc IE' _________ 0.1~_,_ Pavement ___,_____ 0.05 A ricultural Tilled fields and irrigated paSture ---:=--\ :~~_._ I Infilttation can be controlled by percolation if the soil does not have a sustained drainage capacity to provide access for more infiltrated water. However, before percolation can be assumed to restrict infiltration for the design rainfalls being considered in Maricopa County, the extent by which percolation carl restrict infiltra- . tion of rainfall should be carefully evaluated. SCS soil scientists have defined hydrologic soil group D as: "Soils having very stow infiltration rates when thoroughly welled and consist- ing chiefly of clay soils with a high swelling potential, soils with a pennanent high water table, soils with a daypan or clay layer at or near the surface, and shallow soils over nearly imP~rvious material." This definition indicates. that hydrCllogic soil groups A, 13, or C could be classified as D if a near imperviolU' strata of clay, caliche, or rock is beneath them. When these soils are considered in regard to long-duration rainfalls (the design events for many parts ofthe United State-) this defil\ition may be valid. However, when considered for short-duration and relatively small design rainfall depths in Maricopa County, this definition could result in underestimation of the rainfall losses. This is because even a relatively shallow horizon of soil overlaying an impervious layer still has the ability to store a significant amount of infiltrated rainfall. For~ample, consider the situation where only 4 inches of soil COVers an impervious layer. If the effective pc,rosity is 0.30, then 1.2 inches (4 inches x 0.30) of water can be infiltrated and stored in the shallow soil horizon. For design rainfalls in Maricopa County, this represents a significant storage volume for infiltrated rainfall and so when using ~~ for Maricopa County that contain significant areas claSSifi:detemuned. ~,.,.b l'<,p~r'15 <Jr' .w.. '''''',.,.."..,',.,.,''''".,.,."...",."x,.,',.,.".e,~.~.t~~,'::~,..,~f'(~(ot::,"~{q/..(~.,"',., 42 . ..:.:::::....::.:-~:.::: :.: ... . .." '.::: :'- :,.: :<-:'....:-:.:-:-::'~:.:':.;.:.