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<br />6 <br /> <br />~, cover each item represents. <br />CD <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 <br />factor. The last sentence in the Ground Cover section. (PSIAC, <br />1968, page 7) states "For instance, in areas of pinyon-juniper <br />forest having the same percentages of ground cover as an area of <br />grass, the, absence of understory in some of the pinyqn-juniper <br />stands would allow a higher erosion rate then in the area of <br />grass.". Field experience in rating concentrated overland flow <br />erosion in conjunction with the last statement in the Ground <br />Cover section leads to the conclusion that canopy is incidental <br />in the concentrated overland flow erosion process. Effective <br />ground cover should be defined as any material that slows. <br />concentration of overland flow, spreads water flows, permits. <br />greater infiltration time and would lower peak flows. . <br /> <br />It should be emphasized that the PSIAC sediment yield procedure <br />is 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 <br />is the gross soil erosion in a individual soil and farm field <br />setting and the PSIACsediment yield procedure rates sediment <br />delivery from rangeland and mountainland which is. net .soil loss <br />in a w'atershed hydrologic .unit setting. Also, the USLE was' <br />developed in a gentle to moderate slope, deep soil regime in the <br />midwest where precipitation occurs dominantly as frontal storms <br />of long duration and the annual precipitation rates are around 30 . <br />inches. The PSIAC procedure was developed for a low <br />precipitation area of steeP slopes, relatively thin erosive <br />soils,. alluvial fan topography and a predominantly infrequent <br />high intensity thunderstorm precipitation regime. Each procedure <br />qas.its area of applicability. The concepts of .gross soil <br />erosion.(USLE) and .net sediment yield are separate. processes and <br />should not be equated. Also, the concept of ground cover canopy <br />(USLE) and ground cover in a concentrated overland flow sense <br />(PSIAC) should not be equated. <br /> <br />In terms of litter, it should be noted that not all litter is <br />effective in controlling erosion or overland flow. An area with <br />15% litter on the surface may have most of the litter oriented <br />parallel to flow paths. Also, sticks may have only the ends in <br />contact with the ground, thereby forming an arc with flow <br />possible under the middle of the stick. This' is ineffective <br />litter. Litter should be considered in an "effective" sense. <br />Litter oriented perpendicular to flow paths and in contact with <br />the ground is effective litter. Abundant small pieces of litter <br />may actually be floated by overland flows and not act as a <br />sediment yield control agent. The rating of litter should be <br />carefully considered in terms of effectiveness. <br /> <br />The ground cover rating section on field sheets should include <br />the items VEGETATION, LITTER and ROCK with some space behind each <br />