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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 simpl'9st to understand. It consists of an initial 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 condiltions without producing runoff, It represents losses attributable to <br />surface retention whi ch 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 Cheny 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 both the soil and the litter <br />and duff cover. For conditions normally preceding major floods, the initial loss generally <br />ranges from about O.:;! 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 additional volume computed from depression storage areas. For the uniform <br />infiltration 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 Missouri River basin, the uniform <br />rate should be based on the minimum value from the generalized infiltration map in the <br />Missourf River FramElwork study. For areas located outside the Missouri River basin, <br />uniform infiltration rates can be based on the following general classifications in the <br />absence of soil surveys. . <br /> <br />Colorado Flood <br />Hydrology Manual <br /> <br />DRAFI' <br /> <br />7.14 <br />