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<br />Hydrologic Design Manual <br />for Maricopa County <br /> <br />Rainfall Losses <br /> <br />~~~11;~;~~~~?~~ <br />!l:~;~1 Surface Retention Loss <br /> <br />:::::~<".:,::::,>:~ <br />;;;;\';111; Surface retention loss, as used herein, is the summation of all raiirualllosses other <br />....... ..." than infiltration. Tl1e major component of surface retention loss is depression <br />storage; relatively millOI' components of surface retention loss are due to intercep- <br />tion and evaporation, as previously discussed. Depression storage is considered to <br />occur in two forms. First, in-place dl~pression storage occurs at, and in tl1e near <br />vienity of, tl1e raindrop impact. The mecl1anism for this depression storage is the <br />microrelief of tl1e soilmd soil cover. Tl1e second form of depression storage is the <br />retention of surface runoff that occurs away from the point of raindrop impact in <br />surface depressions such as puddles, roadway gu.tters and swale,s, roofs, irrigation <br />bordered fields and la wns, and so forth. <br /> <br />A relatively minor contribution by interception Ls also considered as a part of the <br />total surface retention loss. Estimates of surface retention loss are dUficl.ll1: to obtain <br />and are a nmction of ':he physiography and. land.use of the area. <br /> <br />The surface retention loss on imperviious surfaces has been estimated to be in the <br />range 0.0625 inch to 0.125 inch by Tholin a nd Keefer (1960), 0.11 inch for 1 percent <br />slope to 0.06 inch for 2.5 percent SIOPI'S by Viessman (1967), and 0.04 inch based on <br />rainfall-runoff data for an urban watershed in Albuquerque by Sabol (1983). Hicks <br />(1944) provides estimates of surface retention losses during int,~nse storms as 0.20 <br />inch for sand, 0.15 inch forioam, and (1.10 inch for clay. TIlOlin and K~fer (1960) <br />estimated the surface retention loss for turf to be between 0.25 to 0.50 inch. Based <br />on rainfall simulator studies on undeveloped alluvial plains in the Albuquerque <br />area, the surface retl,ntion loss was estimated as 0.1 to 0.2 inch (Sabol and others, <br />1982a) Rainfall simulator shldies in New Mexico result in estimates of 0.39 inch for <br />eastern plains rangdands and 0.09 inch for pinon-juniper hillslopes (Sabol and <br />others, 1982b). Surface retention losses for vaJious land-uses and surface cover <br />conditions in Maricopa County have been extrapolated from these reported es, <br />timates and these are shown in Tabl,~ ~,.1. <br /> <br />:-.-% <br /> <br />r;~l~l Infiltration <br />.-. .,--." ':'::.,::.;~ <br /> <br />.,',r':.:. <br />. ."illi; Infiltration is the movement of water from the land surface Into the 5011. Gravity <br />.. ." and capillary forCe!; drawing water Into and through the pOlre spaces of the soil <br />matrix are the two forces that drive infiltration. Infiltration iis controUed by soil <br />properties, by vegel:atlon influences on the soil structure, by surface cover of rock <br />and vegetation, and by tillage practlces. The distinction between infiltration and <br />percolation is that percolation is the move,ment of water through the soil subsequent <br />to infiltration. <br /> <br />. <br /> <br />. <br /> <br />. <br /> <br />/ <br /> <br />,. <br /> <br />.;.'.;;:.:.;.;.;.;.:.;;.;:.;.: <br /> <br />...-.............,..."...'-.-. <br />................... ... <br /> <br />.-;:-,;.,.:: <br /> <br />";.;;..;.;.;.::::': <br /> <br />....::w <br />41 <br /> <br />/ <br />