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<br />2. INITIAL AND UNIFORM METHOD. <br /> <br />Of all the methods available to analyze infiltration losses, the initial and uni:orm <br />method is the simplest to understand. It consists of an initial volume loss, normally <br />expressed in inches of rainf311, and a uniform infiltration rate usually expressed in inctles <br />per hour. The initial loss is defined as the maximum amount of precipitation that Can occur <br />under specific conditions without producing runoff. It represents losses attributable to <br />surface retention which include interception ard surface depression storage. <br /> <br />Initial loss values may range from a few tenths 01 an inch during relatively wet <br />seasons to over 2 inches during dry summer and fall months. SCS observations macle <br />during the course of field examinations in the Cheny Creek, Colorado basin noted that <br />small rivulets would build up to 0.25 to 0.5 inch in depth ancl still not penetrate the surlace <br />layer due to apparent surface tension effects 3SS0Clated with boU, the soil and the litler <br />and duff cover. For conditions normally preceding major floods the initial loss generally <br />ranges from about 0.2 to 0.5 inch and is relatively small in comparison with the flood runoff <br />volume. For areaS which have significant depression storage areaS with relatively 11i~lll <br />uniform rates, higher initial losses can be jus1ified. <br /> <br />In the absence of calibrated loss rate information for use in hypothetical storm <br />analysis, it is recommended to use a value of 0.5 inch for i.1itialloss from pervious areas <br />plus any additional volume computed from depression storage areas. For the uniform <br />infiltration loss rate, it is recommended to Pase it on the minimum permeability or saturated <br />hydraulic conductivity values obtained from SCS soil surveys. If SCS soil survey <br />information is not available for basins located within tile Platte River basin, the uniform rate <br />should be based on the minimum value from the generalized infiltration map in the <br />Missouri River Framework study. For area.s located outside the Platte River basin, uniform <br />infiltration rates Can be based on the following general classifications in the absence of soil <br />surveys. <br /> <br />Table ?3 <br /> <br />RECOMMENDED UNIFORM <br />SOIL TEXTURE CLASS [- <br />Sand <br />Loamy Sand <br />Loam <br /> - <br />Silt Loam <br />Sandy Clay Loam <br />Clay Loam <br /> <br />~FIL~RA!ION RA~S -l <br />INFI:,TRATION RATE ~ <br />4.00 . 8.00 <br />120.2.40 <br /> <br />o 50 . 100 <br /> <br />O,1~j. 0.30 <br /> <br />0.10.0.20 <br /> <br />O.OS - 0.10 <br /> <br />Colorado Flood <br />Hydrology Manual <br /> <br />DRAFT <br /> <br />7.16 <br />