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<br />2 <br /> <br />c <br /> <br />~, at the top of the page or at best a cu~ersome phrasing on the <br />~ whole title page that leaves the user w~th a reference to a <br />~ committee rather than to a subject. It is proposed that the <br />~v title be changed to: PACIFIC SOUTHWEST INTER-AGENCY COMMITTEE <br />SEDIMENT YIELD PROCEDURE - REVISED 1991. The acronym PSIAC would <br />still be valid for easy reference and clear purposeful title with <br />a date would be available for formal reference. <br /> <br />Modification 2: It is proposed that the PSIAC procedure be <br />considered applicable for land units as small as 100 acres as <br />opposed to the current lower applicability limit of 10 square <br />miles. Renard (1980) favorably compared PSIAC results with <br />measured field data in watersheds 100 acres or greater. SCS <br />experience in PSIAC ratings on wildfire impacted watersheds that <br />drain into urbanized areas along the wasatch Front has found the <br />PSIAC procedure to be applicable to units of 100 acres or greater <br />(Nelson and Rasely, 1989; Nelson and Rasely, 1990; Rasely and <br />Robison, 1987; Robison and Rasely, 1988). <br /> <br />Modification 3: It is proposed that the PSIAC procedure does not <br />necessarily have to be applied to complete hydrologic units. The <br />PSIAC procedure may be applied to geomorphic units of 100 acres <br />or greater. The geomorphic units can be defined on any <br />combination of the following criteria: land use, climate, <br />precipitation, elevation, geology, soils, erosion condition, <br />hydrology or other criteria that are used to determined that a <br />landform is unique. This modification should be used with <br />discretion to avoid excessive lumping or splitting of a watershed <br />into too gross a division or too many divisions. In other words, <br />PSIAC can be used for land use planning on major hydrologic units <br />by dividing the hydrologic unit into appropriate geomorphologies. <br /> <br />Modification 4: Modification 3 presents the problem that a PSIAC <br />rating in a geomorphic unit will be a rating on a land form and <br />not on a discrete hydrologic unit with a defined outlet or yield <br />point. It is proposed that such a geomorphic unit PSIAC rating <br />be used in conjunction with a sediment delivery ratio procedure. <br />A sediment delivery ratio (SDR) is generally defined as a <br />percentage of the total gross estimated sediment yield that is <br />transported to a watershed outlet or major stream channel. The <br />SCS National Engineering Handbook (USDA, SCS, NEH) defines <br />sediment delivery ratio theory and describes techniques for <br />estimating a sediment delivery ratio. The PSIAC sediment yield <br />should be routed to a perennial flowing stream channel or <br />appropriate delivery point. <br /> <br />Modification 5: It is proposed that the PSIAC sediment yield <br />procedure may be used to model salt yield from rangeland caused <br />by accelerated erosion. The salt is a natural constituent of <br />some soils and is associated with rock fragments as a cementation <br />agent within sedimentary rock. Salt is also derived directly <br />from the weathering of limestone, halite, evaporite and gypsum <br />rock formations. The salt content is defined as the percentage <br />by weight of salt contained within a soil and associated rock <br />