INTRODUCTION 



Early detection of incipient erosion is necessary for efficient management of range- 

 watersheds. The early stages of sheet erosion are much more difficult to recognize than 

 the pedestals, rills, and gullies typical of advanced erosion. Yet, sheet erosion 

 profoundly affects the range; productivity declines as fertile topsoil and humus are 

 gradually lost. This loss can proceed undetected for years until its adverse effects 

 on plant growth and infiltration capacity lead to the more obvious stages of erosion. 

 Once the advanced stages are reached, regaining control of erosion is much more diffi- 

 cult than preventing excessive sheet erosion at the incipient stage. 



Sheet erosion is usually caused by convectional rainstorms. These storms are char- 

 acterized by many large raindrops falling at velocities ranging up to more than 20 miles 

 per hour (Laws 1941). Upon striking bare soil these large drops detach particles from 

 the soil mass and the resulting splash carries them as far as 2 or 3 feet from their 

 original site. Since the rainsplash tends to move farther downhill than uphill, the net 

 effect of rainsplash is downhill soil movement even in the absence of overland flow. If 

 rivulets of overland flow are present, soil particles splashed into such rivulets are 

 carried even farther downhill. 



Soils vary in their susceptibility to erosion. Clays, particularly those that are 

 tightly bound into large aggregates, tend to be difficult to detach. However, once de- 

 tached, clays are easily transported and can be suspended and carried in overland flow 

 for great distances. Sands are less cohesive and are easily detached but because of 

 larger size are less easily transported and are not carried as far by overland flow un- 

 less it is rapid and turbulent. 



Vegetative cover is the best practical protection against excessive sheet erosion 

 because it breaks raindrop impact and favorably influences infiltration capacity. How- 

 ever, the amount of vegetative cover needed to achieve a given level of control of sheet 

 erosion will vary with slope and soil properties because susceptibility to detachment 

 and transportation vary with these factors. 



To obtain maximum use of forage without risking excessive erosion, and to recognize 

 potential erosion hazard, the range manager needs to know quantitative relations between 

 vegetative cover and potential sheet erosion under diverse climatic, soil, and topograph- 

 ical conditions. At present, quantitative information on this subject is limited to a 

 few geographical areas. 



Osborn (1956) studied the effects of vegetative cover and soil on splash erosion 

 on rangeland in Texas and Oklahoma and developed vegetative cover requirements to con- 

 trol splash erosion on various soil textures and plant species compositions in that area. 



On the basis of simulated rain experiments on granitic soils in southern Idaho, 

 Packer (1951) concluded that adequate control of summer storm runoff and erosion on 

 wheatgrass (Agropyron inerme) range requires at least 70 percent ground cover of plants 

 and litter and that bare openings should be no larger than 4 inches. Ground cover con- 

 sists of plant basal area plus surface litter. On cheatgrass {Bromus tectorum) range, 

 70 percent ground cover is required also, but bare openings should be no larger than 2 

 inches. The effects of slope gradient, soil depth, soil porosity, and root abundance 

 in the soil were also investigated but these effects were not great enough at this lo- 

 cation to warrant their inclusion in the protection requirements. 



On an aspen site in northern Utah, Marston (1952) found that ground cover of 65 

 percent or more was required for effective control of overland flow and erosion caused 

 by storms having rainfall intensities in excess of 5 inches per hour. 



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