COVER AND SLOPE 



As found in other studies of a similar nature, 

 erosion on the seven study areas is influenced 

 more by cover than by any other site variable. 

 The magnitude of the effect of cover on erosion 

 depends on slope gradient. The curves in figure 

 3 are derived from the above regression equa- 

 tion; sand, clay, and organic matter contents of 

 the surface inch of soil are fixed at their respec- 

 tive averages of 30, 24, and 8 percent. Unfor- 

 tunately, it was necessary to ignore the litter 

 weight term in calculating these curves, but the 



20 30 40 50 60 70 80 90 100 



PLANT AND LITTER COVER (%) 



Figure 3. — Erosion calculated as a function 

 of slope gradient and percentage of the 

 soil surface protected by plants and lit- 

 ter. Sand, clay, and organic matter are 

 fixed at their respective averages of 30, 

 24, and 8 percent. 



effects of litter weight will be quantified 

 shortly. 



The effects of cover are greater on steeper 

 slopes. Similarly, slope becomes increasingly 

 important as cover decreases. At less than 50- 

 percent cover, erosion rates double for each 

 10-percent increase in slope. The amount of 

 cover needed to hold erosion within some spec- 

 ified limit varies widely with variation in slope. 

 For example, 40-percent cover on a 5-percent 

 slope apparently is as effective as 80-percent 

 cover on a 35-percent slope because calculated 

 erosion is about 1 pound per milacre in both 

 cases. 



The regression term for litter weight 

 (— 0.058A 3 L) indicates that weight of litter is 

 not important unless cover is virtually com- 

 plete (table 1). At 60-percent cover, the pres- 

 ence of 1 pound of litter per milacre reduces 

 estimated erosion to 97 percent of the value 

 shown in figure 3. At 100-percent cover, the 

 same amount of litter reduces erosion to 87 

 percent. Twelve pounds of litter per milacre 

 with 100-percent coverage reduces estimated 

 erosion to one-fifth that shown in figure 3. 

 Thus, litter weight serves mainly to explain var- 

 iations in erosion when cover is complete or 

 nearly so. So far as soil stabilization is con- 

 cerned, however, ground coverage provided by 

 litter is much more important than its weight. 



Table 1. — Correction factors for litter weight 



Air-dry weight of litter 



Cover 



(percent) Pounds per milacre 



1 2 4 8 12 



— Correction factors — 



60 



0.97 



0.94 









70 



.96 



.91 



0.83 







80 



.94 



.87 



.76 



0.58 





90 



.91 



.82 



.68 



.46 



0.31 



100 



.87 



.76 



.58 



.34 



.20 



6 



