Table I. --Results of mechaniaat analyses shewing the average percent 

 gravel, sand, silt, and clay for soils used in this study 



oOl i 



Percent 

 >2,000y* : 



Percent 

 2,000-61y : 



Percent 

 61y-2y '. 



Percent 

 <2y 



Low-elevation granitic 



23 



56 



15 



6 



High-elevation granitic 



36 



46 



12 



6 



Wasatch clay 







21 



28 



51 



*One micron equals 0.001 mm. 



flow varies as the square root of the slope gradient and the energy of overland flow 

 varies as the square of its velocity. Overland flow will move down a 40-percent slope 

 at twice the velocity of that on a 10-percent slope. By doubling the velocity, the 

 energy of flow will increase about four times; the size of particle that can be trans- 

 ported will be increased about 64 times (sixth power of velocity); and the quantity of 

 material of a given size that can be carried is increased about 32 times (fifth power 

 of velocity) . 



In this paper we siommarize the results of laboratory soil erosion tests performed 

 on bare soil plots under simulated rainfall. The effects of soil, precipitation, and 

 topographic factors were examined. Vegetation was not included because the influence 

 of nonvegetal factors on erosion is more difficult to isolate in the presence of vegetal 

 cover. Moreover, problem areas that exhibit high rates of soil erosion and stream 

 sedimentation are often nearly devoid of vegetal cover. Low cover densities may result 

 from a variety of causes; fire, logging, road construction, and grazing have been com- 

 mon ones in the past. 



This work was intended to develop information about the effects of soil, slope, 

 and rainfall variables on the erodibility of bare soil, to determine the magnitude of 

 these effects, and to identify relationships between these variables. 



Methods and Materials 



Three soils were used in this study. Two of these, formed on weathered granitic 

 rock, were collected in Idaho on the Boise National Forest, one at about 3,900 feet 

 m.s.l., the other at about 7,800 feet m.s.l. The third soil, formed over limestone 

 parent material, was collected in central Utah on the Wasatch Plateau, Manti-LaSal 

 National Forest, at about 10,100 feet m.s.l. Only the surface inch of soil was col- 

 lected. A mechanical analysis was performed on each of these soils (table 1). 



These soils were each sieved through a 6.3 mm. screen and loaded in a plot to a 

 depth of about 4 inches. Less than 2 percent of the soil would not pass through this 

 screen. The plot was 48 |j- by 18-| inches and contained no vegetation. Adequate 

 drainage was provided from the bottom of the plot. After loading the soil, the plot 

 was set to a specified slope {2\, 18, or 32 percent) so that the long axis of the 

 plot pointed downhill. Average bulk-density after loading the disturbed soil was 1.14 

 g. per cm. ^ The soil was wetted to saturation by a mist spray, covered with a 

 plastic sheet, and allowed to drain for 20 hours. Next, constant rainfall of approxi- 

 mately 3 or 7 inches per hour was applied to the soil plot for 30 minutes. The actual 

 rainfall intensity was measured during each run. The rainfall simulator used was the 

 same as that described by Packer (1957), except that the F-type nozzles were raised to 



2 



