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<br />" <br /> <br />6 <br /> <br />t\) <br />~ <br />~ <br />o <br /> <br />, average slope for a geomorphic unit is obtained by mUltiplying the <br />percent of the area by its average slope percentfthis should be <br />done for all the individual slope-areas in the geomorphic unit. <br />TheSe values are then added together to give the weighted average <br />slope percent for the geomorphic unitf which in the case of this <br />example is 17.25% or 17%. A weighted slope average calculation <br />table and a slope percent rating chart is included on the revised <br />PSIAC Rating Chart~ ' <br /> <br />The rating' of this factor should be based on the watershed <br />considerations as documented in the PSIAC (1968, pages 5-6) as well <br />as the weighted slope average. Experienced judgement in ail PSIAC <br />ratings is critical to the obtaining the best results. <br /> <br />Modification 11: It is proposed that the factor (f) topic name be <br />revised to read EFFECTIVE GROUND COVER and it is proposed that the <br />Effective Ground Cover (f) rating format be modified to <br />specifically state that vegetation, litter and rock.fragments each <br />be rated by determining the percentage of effective ground cover <br />each item represents. <br /> <br />Ground cover should represent those factors which impede surface <br />flow, affect hydrologic time of' concentration and aide <br />infiltration. Canopy should be a peripheral to negligible factor. <br />The last sentence in the Ground Cover section (PSIAC, 1968; page 7) <br />states "For instance, in areas of pinyon-juniper forest having the <br />same percentages of ground cover as an area of grass, the absence <br />of understory in some of the pinyon-juniper stands would allow a <br />higher erosion rate then in the area of grass.". Field experience <br />in rating concentrated overland flow erosion in conjunction with <br />the last statement in the, Ground Cover section leads to the <br />conclusion that canopy is incidental in the concentrated overland <br />flow erosion process. Effectiye ground cover should be defined as <br />any material that slows concentration of overland flow, spreads <br />water flows, permits greater infiltration time and would lower peak <br />' flows. ' , <br /> <br />It should be emphasized that the PSIAC sediment yield procedure is <br />quite different from the Universal Soil Loss Equation, USLE, <br />(Wischmeier and Smith, 1978) because the USLE evaluates on-site <br />soil disturbance in relationship to agricultural cropland which is <br />the gross soil erosion in a individual soil and farm field setting <br />and the PSIAC sediment yield procedure rates sediment delivery from <br />rangeland and mountainland which is net soil loss'in a watershed <br />hydrologic unit setting. Also, the USLE was developed in a gentle <br />to moderate slope, deep soil regime in the midwest where <br />precipitation occurs dominantly as frontal storms of long duration <br />and the annual precipitation rates are around 30 inches. The PSIAC <br />procedure was developed for a low precipitation area of steep <br />slopes, relatively thin erosive soils, alluvial fan topography and <br />a predominantly infrequent high intensity thunderstorm <br />precipitation regime. Each procedure has its area of <br />