Introduction 



All soils erode to some degree. The rate and severity of erosional losses are 

 primarily controlled by four classes of variables--vegetal factors, soil factors, pre- 

 cipitation factors, and topographic factors. Smith and Wischmeier (1962) cite vegetal 

 cover as the greatest deterrent to soil erosion. Once the cover has been reduced below 

 some critical level, detachment and transport of soil function together to remove the 

 soil mantle, but at varying rates that depend upon both soil and nonsoil factors. 



A voluminous amount of literature exists on soil erosion from forest and range 

 lands. A large portion of this literature has been concerned with soil factors and 

 their effects on erosion. Bryan (1968) worked with 22 indices of soil erodibility 

 that have been reported in the literature. He concluded that none of the soil indices 

 was reliable in operation and capable of universal application. He expressed doubt 

 that such an index could be developed, but concluded that the percentage weight of 

 water-stable aggregates greater than 3 mm. in diameter was probably the most reliable 

 index of soil erodibility available. 



Many important questions concerning the influence of soil factors on erosion proc- 

 esses remain unanswered. This situation is partially due to the fact that soil factors 

 are difficult to isolate in the presence of vegetal factors. Another difficulty has 

 been the multiplicity of soil erosion criteria. Weight of eroded soil, numerous 

 measures of soil aggregation, stream turbidity, measurements from various types of 

 erosion gages, and several soil indices have all been used as erosion criteria. 



In addition to vegetal cover and soil variables, the dominant factors controlling 

 erosion are rainfall characteristics and topography. However, on forest and range 

 lands, the effect of these two factors is not well known. Most of our information 

 concerning rainfall factors and topographic factors has been obtained from studies on 

 farmlands . 



Wischmeier and Smith (1958) demonstrated the importance of rainfall energy to soil 

 loss from agricultural lands. However, information on the kinetic energy of rainfall 

 is totally absent for mountainous areas. Wischmeier (1959) and others have implied 

 that the relationship between rainfall intensity and kinetic energy is acceptably con- 

 stant in agricultural regions. No such relationship should be expected in mountainous 

 areas of the Intermountain Region because these areas are subject to rainstorms 

 that vary widely in those characteristics directly affecting rainfall energy; i.e., 

 distributions of drop sizes, rainfall intensity, and wind velocities. 



For most point measures of soil erosion at a given time, the precipitation factors 

 of greatest interest are rainfall intensity, raindrop- size distribution, and total 

 rainfall. If our interests in soil erosion expand in either time or space, the infor- 

 mation needed must include rainfall frequency-intensity-duration relations, and depth- 

 area-duration relations. In mountainous areas, such data are scanty. 



The topographic factors of major importance to soil erosion in mountainous terrain 

 are slope steepness and slope length. Sometimes aspect and elevation appear to be 

 related to soil losses; although these are probably indirect effects that reflect dif- 

 ferences in climate or vegetation, rather than topographic effects. Naturally, large 

 variations in both slope steepness and slope length occur in mountainous areas. There 

 is some evidence that the effects of slope length and steepness on soil erosion are 

 interrelated with other factors, such as soil texture, soil bulk density, vegetal type, 

 or rainfall intensity. However, most of our practical information on the effects of 

 slope steepness has come from basic kinematic theory; e.g., the velocity of overland 



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