14 



BULLETIN 512^ IT. S. DEPARTMENT OF AGRICULTUEE. 



It is assumed that the cross section of the stored water is triangular in 

 shape. 



Using the values, h=1.25 feet, and w=10 feet, then 



9.375+0.375S 



Hence, if the values of r and s are known, v, the vertical distance 

 between the terraces, can be computed from the above equation. 

 The value that should be assigned to r depends upon the absorptive 

 capacity of the soil and upon the amount of rainfall for the heaviest 

 single storm. From a general study of the rainfall records for the 

 United States it is found that rainfalls exceeding 8 inches per 48 

 hours do not occur frequently in a given locality, and it is believed 



\\\" qc 



^^^ ^.mmmmmy ^^^^^^^ 



95 . 



IIIIIIIIIIIIIIIINII 



Fig. 6. — Plan of hill protected by broad-base level-ridge terraces. 



that provision for 8 inches of rainfall in the design of a system 

 of terraces would give satisfactory results. 



By using values of r ranging from 2 inches to 8 inches, depending 

 upon whether a small portion or all of the rain runs off, and using 

 average slopes of land surface of 5, 10, 15, and 20 feet per hundred, 

 a curve for each slope was plotted. (See fig. 7.) The vertical scale 

 on the left of the axis indicates the percentage of an 8-inch rainfall 

 (in 48 hours) that runs off. This percentage depends upon the 

 amount of water absorbed by the soil. 



To determine the proper vertical spacing for a system of terraces 

 for any particular field it is necessary to know the average slope of 

 the land surface and the approximate percentage of the rainfall that 

 will percolate into the soil. The former can be measured readily 

 by some form of leveling instrument and the latter can be ascertained 



