ZONE 1 ZONE 2 



JFMAMJJASON JFMAMJJASO 



ZONE 3 ZONE 4 



JFMAMJJASO JFMAMJJASO 



MONTH 



Figure 9— The monthly values as a percentage 

 of R for zones I, 2, 3, and 4 of Davis 

 County experimental watershed, Utah. 



the balance of the United States. Briefly, the method 

 the mean annual erosion per acre, A, is equal to the 

 product of R; an erodibility factor, K; a slope length 

 factor, L; a slope steepness factor, S; and a cover 

 practice parameter, CP. 



The latter group also substituted VM for CP as more 

 appropriate to nonfarm areas. The VM symbolizes vege- 

 tation management. All other treatments could be re- 

 duced to changing the values of slope or slope length, 

 for example contour ditches, terraces, etc. 



In the universal soil loss equation the values of L are 

 in terms of the ratio of soil lost from a slope length of 

 72.6 feet (22 m) and the values of S are in terms of the 

 ratio of the soil lost from a slope steepness of 9 percent 

 gradient. 



Example. A small homogeneous (soil, cover, and 

 slope) area rectangular in shape has a slope length of 

 73 feet (22.3 m) and a gradient of 9 percent. There is 1 

 ton per acre (2.25 t/ha) of litter on the surface of the soil 

 whose erodibility is 0.50. Is this sufficient protection to 

 have less than 4 tons per acre (9 t/ha) of erosion if a 1 0- 

 year event should occur? The R value is 31 and the 

 R 10 /R2 is 8. Figure 10 shows the tons per acre (t/ha) of 

 litter as related to the RKLS value that the mulch can 

 withstand without failure or appreciable erosion. 



Solution. Enter figure 1 with 1 ton/acre (2.25 t/ha) 

 and read RKLS = 74. This is the RKLS the litter can 

 safeguard if the VM value is 1.0. 



LITTER (TONS/ ACRE) 



Figure 10— The relationship between 

 RKLS and the tons per acre of litter. 



ZONE 2 



DURATION (HRSI 



Figure 1 1—The mass frequency distribution of storm dura- 

 tion in hours lor zones 1. 2, and 3 of Straight Canyon 

 barometer watershed, Utah. 



8 



