20 



MISC. PUBLICATION 656, U. S. DEPT. OF AGRICULTURE 



The difference in elevation between two terraces draining in the 

 same direction is practically the same throughout their length, but 

 the width of area between the terraces may vary widely, depending 

 on the variation in the slope of the land. Thus terraces converge 

 or close in when draining toward a steeper slope and conversely they 

 diverge or spread out when draining toward a more gradual slope. 

 This change in slope and in width between terraces affects the grade 

 and direction of drainage in crop rows which parallel either terrace 

 as illustrated in figure 12. This diagram represents two terraces with 

 a vertical interval (a difference in elevation) of 3 feet and shows the 

 grades and direction of drainage for every fifth row beginning with 

 the guide terrace. 



Rows parallel upper terrace 



Terrace: Direct 



amount in inches per ioo feet 



9" 



Elev. 

 18-8" 



- Demonstration row: Direction of slope > 

 and amount in inches per IOO feet. 



z- -E E; Crop rows. 



Figure 12. — Diagram of two terraces with assumed elevations showing how 

 slope changes affect row grades. In the upper diagram, all rows parallel the 

 upper terrace, while the lower diagram shows this same area with all rows 

 paralleling the bottom terrace. 



