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<br /> <br />parameters can be determined from field and laboratory tests. When the infiltration <br />parameters are based on laboratory tests, it should be noted that for various reasons, <br />loss rates in natural drainage basins average much less than rates in controlled <br />laboratory experiments with similar soils, sometimes an order of magnitude smaller. <br /> <br />2. INITIAL AND UNIFORM METHOD. <br /> <br />Of all the methods available to analyze infiltration losses, the initial and <br />uniform method is the simplest to understand. It consists of an initial volume loss, <br />normally expressed in inches of rainfall, and a uniform infiltration rate usually <br />expressed in inches per hour. The initial loss is defined as the maximum amount of <br />precipitation that can occur under specific conditions without producing runoff. It <br />represents losses attributable to surface retention which include interception and <br />surface depression storage. <br /> <br />Initial loss values may range from a few tenths of an inch during relatively <br />wet seasons to over 2 inches during dry summer and fall months. SCS observations <br />made during the course of field examinations in the Cherry Creek, Colorado basin <br />noted that small rivulets would build up to 0.25 to 0.5 inch in depth and still not <br />penetrate the surface layer due to apparent surface tension effects associated with <br />both the soil and the litter and duff cover. For conditions normally preceding major <br />floods, the initial loss generally ranges from about 0.2 to 0.5 inch and is relatively <br />small in comparison with the flood runoff volume. For areas which have significant <br />depression storage areas with relatively high uniform rates, higher initial losses can be <br />justified. ' <br /> <br />In the absence of calibrated loss rate information for use in hypothetical <br />storm analysis, it is recommended to use a value of 0.5 inch for initial loss from <br />pervious areas plus any additional volume computed from depression storage areas. <br />For the uniform infiltration loss rate, it is recommended to base it on the minimum <br />permeability or saturated hydraulic conductivity values obtained from SCS soil <br />surveys. If SCS soil survey information is not available for basins located within the <br />Missouri River basin, the uniform rate should be based on the minimum value from the <br />generalized infiltration maP in the Missouri River Framework study. For areas located <br />outside the Missouri River basin, uniform infiltration rates can be based on the <br />following general classifications in the absence of soil surveys. <br /> <br />7-2 <br /> <br />---- <br /> <br />