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<br /><, <br /> <br />I'\) <br />ClO <br />~ <br />(Q <br /> <br />5 <br /> <br />five points. This modification proposal is based on research that <br />,has occurred since 1968. Hereford (1976) documents the sediment <br />yield from the shale and sandstone units in the Chinle Formation as <br />having a 1.8 year recurrence interval. This is an almost two year <br />time gap between sediment yield within drainages of the Colorado ' <br />Plateau that have deeply weathered sandstone and shale bedrock on <br />the surface. These badlands areas are subject to intense and <br />infrequent thunderstorm events. After a major sediment yielding' <br />storm event, the next storms would result in minimal to no sediment <br />yield due to the upland having been stripped ,of weathered soil and <br />sediment. It would take an average of almost two years for the <br />bedrock to weatherby freeze and thaw, temperature differentials <br />and chemical weathering before the area would yield sediment again. <br /> <br />The PSIAC sediment yield procedure results in an annual figure of <br />sediment yield from an area. In light of the above work by <br />Hereford (1976) it is proposed that the Geology (a) rating factor <br />should not be given equal weight as the Soil (b) 'rating factor. <br />The maximum rating of five is considered appropriate in view of <br />recent research. <br /> <br />Modification 9: It is proposed that the Soil (b) rating <br />, description in the ,text be modified to emphasize that the entire 60 <br />inch soil profile is not involved in the sediment yield and erosion <br />process. The top two to three inches is generally all of the soil <br />profile that is involved in concentrated OVerland flow sediment <br />yield. The sediment yield character of these few inches of soil is <br />critical to rating this factor. ' <br /> <br />Should gully erosion be dominant within an area, then the entire <br />soil and sediment profile would be involved in the sediment yield <br />and erosion process but the PSIAC sediment yield procedure would, <br />not ,be the appropriate procedure to characterize an ,area' of ' <br />dominant gully erosion. The PsrAC procedure should be' used to <br />characterize upland and channel erosion. A direct volume sediment <br />yield procedure (USDA, SCS, 1983) should be used to characterize an <br />area dominated by' gully sediment yield~ Together the two <br />procedures would serve to characterize the sediment yield of the <br />area. <br /> <br />Modification 10: It is proposed that the Topography (e) be rated <br />by a weighted average for slope-area in conjunction with the <br />characterization in PSIAC (1968, pages 5-6). The weighted slope- <br />area average can be based on field observation, planimeter work or <br />GIS data. The geomorphic unit should be divided into general <br />slope-related subunits. Then the area of the subunit and the <br />characteristic slope should be determined. The geomorphic unit <br />average slope may then be calculated by the following example: A <br />butte, mesa, bench and escarpment topography characteristic of the <br />Colorado Plateau may have 15% of the area with a characteristic <br />slope of 70%, 35% of the area with a characteristic slope of 5% and <br />50% of the area with a characteristic slope of 10%. The overall <br />