Ch. 22] POTENTIALITIES FOR SEDIMENT CONTROL 399 



where E = rate of erosion in tons per acre of dry soil. 



P 30 = maximum 30-minute rainfall of 2-year frequency 



For degree of slope, 



E oc S 1 - 35 



where S — slope in feet per hundred. For length of slope, 



E oc L 035 



where L = length of slope in feet. 



By equating measured erosion rates for 19 different soils on which 

 extensive data are available to a common 1.25-inch, 30-minute rain- 

 fall on a 10 percent slope 72 feet long with minimum vegetal cover, 

 the inherent relative soil erodibility was found to vary from 0.03 to 

 0.96 inch per year. In other words, because of characteristic textural 



10 20 30 40 50 60 70 80 90 100 

 Relative erosion rates 



Fig. 10. Relative erosion rates under different vegetal covers in Pacific Northwest. 



and structural differences of soils, an extreme variation of 33-fold in 

 their erosion potential may be expected. 



By far the greatest relative difference in rate of erosion was found 

 to result, however, from the character and density of vegetal cover. 

 If the same soil, slope, and rainfall are assumed, the rate of erosion 

 on land used for continuous row crops (such as uncontoured cotton, 

 corn, or tobacco) was found to be more than 100 times as great as 

 on land used for hay, pasture, woodland, and forest. Figure 10 shows 

 relative erosion values for different vegetal covers in the Pacific 

 Northwest. 



