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<br />., <br /> <br />1 <br /> <br />I\) <br />(Xl <br />00 <br />~ <br /> <br />APPENDIX II <br /> <br />PROPOSED REVISION OF SEDIMENT YIELD PROCEDURE <br /> <br />PACIFIC SOUTHWEST INTER-AGENCY COMMITTEE <br /> <br />REPORT OF THE WATER MANAGEMENT SUBCOMMITTEE <br /> <br />,OCTOBER, 1968 <br /> <br />by: RObertC.Rasely,'Geploc;rist, Soil Conservation Service <br />Salt Lake City, Utah <br /> <br />., <br /> <br />Backgrol,1nd and Experienqe: The Pacific Southwes,t Inter-Agency <br />Committee (PSIAC), October, 1968, Report of theWatei Management <br />Subcommittee,' Factors Affecting Sediment Yield and Measures for the <br />Reduction of.Erosion and Sediment Yield is generally known by its <br />acronym the PSIAC (pronounced si-ak) procedure. This procedure has <br />been tested against other sediment yield procedures in ' measured <br />field conditions and found to be a reliable model of sediment yield <br />,(Renard, 198Q') and. (Shown, 1970). The Department of ,!nterior, <br />Bureau of Land Management (BLM) and the DepartmentofAg~iculture, <br />Soil Conservation Service (SCS) have authorized the use of the <br />PSIAC procedl,1re since 1968 for modellinc;r the sediment yield rates <br />on' ranc;reland in the intermountain west. The SCS in Utah has been , <br />using the PSIACprocedure extensively in watershed planning'and <br />emergency fire response work since 1983. <br /> <br />USGS, Ciroular 256 (King and Mace, 1953) ,entitled SEDIMENTATIon IN " <br />SMALL IU:SERVOIRSON 'l'HESAN RAFAEL SWELL, givesdetailedhy-drolog'ic <br />studies, geologic formations, rock weathering, climate, sediment <br />trapping efficiency and measured sediment yield for 15 reservoirs <br />and debris basins over a period of 8-12 years beginning ,in 1936. <br />The data presented in Table 5 (King and Mace, 1953) relates the <br />average annual sedimentation rates in acre-feet per square mile to <br />rock weathering categories'. These data form an excellent basis for <br />evaluating PSIAC procedure sediment yield rates in the same or <br />comparable rock units throughout the upper Colorado Plateau. None <br />of the SCS modelled rangeland sediment yield rates using the PSIAC <br />procedure exceeded the rating's given in King and Mace (1953). It <br />should be noted that some of the SCS fire related ratings did <br />exceed the King and Mace (1953) rates. These high ratings occurred <br />on areas where fire had been extremely hot, burned almost all <br />vegetation to the ground, burned the root mass to a depth of a few <br />inches and located on the very steep Wasatch Mountain Front, and <br />therefore, warranted the high sediment yield ratings. <br /> <br />., <br /> <br />r <br />. <br />