14 BULLETIN 512, U. S. DEPARTMENT OF AGRICULTURE. 
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 
ped 3/9-10.3758 
r 
Hence, if the values of 7 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 7 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 
Wy 
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yy My 
itt 
RR ULL LLL RO 
i! 95 
—_— —_ _ 
RB 
6 
< STINE @ ih us "TERRACE 
(BEFORE 
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On snqyr eT 
SCALE OF FEET Cortours 
TRS ae) GRRE aT RES Terraces. _—~—— \Wiiiiiiiniti 
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 7 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 
