PREVENTION OF EROSION BY TERRACING. 15 
by a knowledge of the physical character, the humus content, and 
the tillage condition of the soil. The susceptibility of the subsoil 
to the percolation of water also is an important factor to be con- 
sidered in estimating the run-off. 
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Qy 
S 
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vel 
eS 
00d Le 
eon 
Wed above Er 
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Ww 
CS 720 
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AR 
12 
TIC. 
| 
| 
Percent of 8inch Raita! in 48 hours that runs off 
Uy 
S 
the Surkace and Collechs above Jerrace 
-Off in | 
Ay! 
un 
| 
| 
R. 
20 
Vertical Distance Between Terraces 17 -eer 
Fic. 7.—Curves showing vertical distances between broad-base level-ridge terraces, for 
different rates of run-off and land slopes. 
Tt is by no means an.easy matter to estimate the percentage of 
rainfall that will run off for the various types and conditions of 
soils. For instance, the difference in the rates of percolation for 
clay and sandy soils is very marked, the latter permitting a much 
