DRAINAGE OF IRRIGATED LAND. 
23 
irrigated region the seepage moves down the ridges more readily 
than down the depressions, so it is generally necessary to modify 
the drainage system by means of intercepting Laterals cut through 
the ridges near the upper 
edge of the tract. These 
are indicated at a, 5, c, d, e, 
and/, in figure 18. 
APPLICATION OF DOUBLE 
LINES. 
If the depressions are 
wide and the pervious ma- 
terial does not extend un 
der the soil in the depres- 
sions, it will be necessary 
to employ two lines, one 
on either side of the de- 
pression, cutting into the 
pervious material. How- 
ever, if it is possible to 
completely intercept the 
seepage by means of later- 
als at the upper edge of 
the tract, it will be cheaper 
to construct a single line 
down the center of the de- 
pression. Figure 19 shows 
how a double-line system would be located 
Fig. 18.— Plan and section illustrating application of natural 
system modified by intercepting laterals. 
OTHER SPECIAL CASES. 
In some localities the soil is underlain at shallow depths by lava or 
other rock formations, and the problem of drainage is exceedingly 
j g.h ,dei 
Fig. 19.— Cross section showing modification of natural system in which two lines of tile are employed, 
one on either side of the wide depression and cutting through the pervious layer at the line of pinching 
out. A single drain in the center of this depression would accomplish very little. 
difficult. Often the formation is so stratified and shattered that the 
natural drainage seems to be very good. Water enters the spaces 
very readily, however, and the rock masses themselves absorb but 
