RAINFALL 179 



up the soil so as to increase its ability to absorb and retain mois- 

 ture, and at the same time decrease its stickiness, helps to lessen 

 its tendency to erode. In general, the greater the tenacity of a 

 soil the less likely is erosion to occur. 



Rainfall. — The amount and distribution of the rainfall 

 constitute an important factor in determining the occurrence of 

 erosion. In order that no erosion may occur, the greater por- 

 tion of the water faUing on the land must be absorbed or held 

 back by the soil, temporarily at least. As pointed out, the 

 movement of the unabsorbed water over the soil's surface is 

 largely controlled by the vegetative cover, and, of course, on 

 plowed fields by the character of the tillage and sometimes by 

 terraces. 



In the study of the effect of rainstorms of varying intensity 

 on the specially selected areas in the Wasatch Mountains of 

 Utah,^ twenty-six rain storms occurred in a single season dis- 

 tributed over four months of the main growing period. Of 

 these only one storm produced run-off. During this storm .70 

 of an inch of rain fell on each of the areas. The run-off from 

 area B, 10 acres in size, was 335 cubic feet, and it carried 74 

 cubic feet of air-dry sediment. As compared with this, the 

 run-off from area A, also 10 acres in size, was 3,019 cubic feet, 

 and this carried with it 717 cubic feet of air-dry sediment. 

 Not only do these figures show the effect of rainfall on 

 erosion; but, if it is kept in mind that area A has a steeper slope 

 and a less dense plant cover than has area B, the effects of the 

 slope and vegetation are also evidenced. 



The rate also at which rain falls is an important factor. For 

 instance, a fall of .36 of an inch of rain, received in 3 hours and 

 25 minutes, resulted in no run-off on either erosion area. An- 

 other storm of .65 of an inch, which was received in 8 hours and 

 45 minutes, likewise produced no run-oflf. In contrast to these 

 storms, a fall of .85 of an inch of rain, which was received in 40 

 minutes, resulted in the deposit from area A of 50,000 pounds 



1 Sampson, Arthur W., and Weyl, Leon H., "Range Preservation and its Re- 

 lation to Erosion Control on Western Grazing Lands." U. S. Dept. of Agr. Bui. 

 675, pp. 11-15, 1918. 



