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<br />1 <br /> <br />Background and Experience: The pacific Southwest Inter-Agency <br />I" Committee, October, 1968, Report of the Water Management <br />~ Subcommittee, Factors Affecting Sediment Yield and Measures for <br />~ the Reduction of Erosion and Sediment Yield is generally known by <br />~~ its acronym the PSIAC (pronounced si-ak) procedure. This <br />procedure has been tested against other sediment yield procedures <br />in measured field conditions and found to be a reliable model of <br />sediment yield (Renard, 1980) and (Shown, 1970). The Department <br />of Interior, Bureau of Land Management (BLM)' and the Department <br />of Agriculture, Soil Conservation Service (SCS) have authorized <br />the use of the PSIAC procedure since 1968 for modelling the <br />sediment yield rates on rangeland in the intermountain west. The <br />ses in utah has been using the PSIAC procedure extensively in <br />watershed planning and emergency fire response work since 1983. <br /> <br />USGS, Circular 256 (King and Mace, 1953), entitled SEDIMENTATION <br />IN SMALL RESERVOIRS ON THE SAN RAFAEL SWELL, gives detailed <br />hydrologic studies, geologic formations, rock weathering, <br />climate, sediment trapping efficiency and measured sediment yield <br />for 15 reservoirs and debris basins over a period of 8-12 years <br />beginning in 1936. The data presented in Table 5 (King and Mace, <br />1953) relates the average annual sedimentation rates in acre- <br />feetper square mile to rock weathering categories. These data <br />form an excellent basis for evaluating PSIAC procedure sediment <br />yield rates in the same or comparable rock units throughout the <br />upper Colorado Plateau. None of the SCS modelled rangeland <br />sediment yield rates using the PSIAC procedure exceeded the <br />ratings givenin King and Mace (1953). It should be noted that <br />some of the SCS fire related ratings did exceed the King and Mace <br />(1953) rates. These high ratings occurred on areas where fire <br />had been extremely hot, burned almost all vegetation to the <br />ground, burned the root mass to a depth of a few inches and <br />located on the very steep Wasatch Mountain Front, and therefore, <br />warranted the high sediment yield ratings. <br /> <br />PROPOSED REVISION <br /> <br />A revised PSIAC (1968) procedure would serve as a documentation <br />tool for quantification of present and future impacts of <br />conservation plans on rangeland. This revised procedure could <br />also be used to document, quantify, characterize and predict <br />environmental impacts for rangeland planning in the preparation <br />of National Environmental Protection Act documents. <br /> <br />Modifications 1 through 7 relate to the over all character of the <br />PSIAC sediment yield procedure. Modifications 8 through 16 are <br />proposed updates to the actual rating of the individual PSIAC <br />sediment yield factors (a through i). Modification 17 is a <br />proposed revision of the PSIAC Sediment Yield Factor Rating <br />Sheet. <br /> <br />Modification 1: It is proposed that the title of the PSIAC <br />procedure be restructured for consistent reference. The original <br />document has a title page that presents either a misleading title <br />