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<br />2. INITIAL AND UNIFORM METHOD. <br /> <br />Of all the methods available to analyze infiltration losses, the initial and uniform <br />method is the simplest to understand. It consists of an inilial volume loss, normally <br />expressed in inches of rainfall, and a uniform infiltration rate usually expressed in inches <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 and surface depression storage. <br /> <br />Initial loss values may range from a few tenths of an inch during relatively wet <br />seasons to over 2 inches during dry summer and fall months. SCS observations made <br />during the course of field examinations in the Cherry Creek, Colorado basin noted that <br />small rivulets would build up to 0.25 to 0.5 inch in depth and still not penetrate the surface <br />layer due to apparent surface tension effects associated with beth the soil and the litter <br />and duff cover. For conditions normally preceding major floods, the initial loss generally <br />ranges from abeut 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 high <br />uniform rates, higher initial losses can be justified. <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 initial loss from pervious areas <br />plus any addilional volume computed from depression storage areas. For the uniform <br />infiitration loss rate, it is recommended to base 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 the Platte River basin, the uniform rate <br />should be based on the minimum value from the generalized infillration map in the <br />Missouri River Framework study. For areas located outside the Platte River basin, uniform <br />infiitration rates can be based on the following general classifications in the absence of soil <br />surveys. <br /> <br />Table 7.3 <br /> <br />RECOMMENDED UNIFORM INFilTRATION RATES <br />SOil TEXTURE CLASS INFILTRATION RATE (inlhr) <br />Sand 4.00 - 8.00 <br />loamy Sand 1.20 - 2.40 <br />loam 0.50 - 1.00 <br />Si~ loam 0.15 - 0.30 <br />Sandy Clay loam 0.10 - 0.20 <br />Clay loam 0.05 - 0.10 <br /> <br />Colorado Flood <br />Hydrology Manual <br /> <br />DRAFT <br /> <br />7.16 <br />