DRAINAGE OF IRRIGATED LAND. 11 
in gravel to a number of feet in fine, silty sand, or clay, soils; the rise 
may even extend to many feet in special soils containing a high per- 
centage of gypsum or of calcium chlorid. Average soils show a 
range of from 1£ to 4 feet. 
Plants in the arid region are unusually deep rooted, and they can 
not thrive unless the air and moisture conditions are properly bal- 
anced. Therefore, the plane of supersaturation must be kept below 
the root zone, and since this plane is several feet above the free water 
level it is necessary to give drains a considerable depth. 
The presence of alkali salts complicates the problem of depth, for 
not only is the capillary rise of the water thus increased and expe- 
dited, but it is essential that the injurious salts themselves be kept 
down. It is highly important that a downward movement of the 
water in the root zone be maintained to offset the natural upward 
movement due to capillary attraction and evaporation. Owing to 
the presence of animal and worm burrows, cracks, root spaces, and 
other noncapillary openings, a great deal of water moves downward 
without coming into contact with salts; but the upward capillary 
movement is entirely through the capillary pores where the salts 
are confined. The natural tendency, therefore, is for the salts to 
move upward rather than downward. Drains should never be less 
than 5 feet deep, and experience has shown that depths of from 6 
feet to 8 feet are much more efficient. The optimum depth for 
drains is that which will prevent fluctuations of the ground-water 
level within the root zone, and yet will keep capillary water within 
reach of the plant roots. 
In determining proper depths the location of any stratum which 
is either more or less pervious than the adjacent soil is of great im- 
portance. This involves a careful study of the structure of the soil 
for considerable depths. It is quite possible to construct a well- 
arranged system of considerable depth which will be absolutely inef- 
fective but which would have been highly effective if the depth had 
been increased less than 1 foot. Figure 4, a, illustrates an actual case 
of this kind. This system would have been successful if the tile had 
been laid as is indicated in b. The porous stratum carries water 
from higher lands, and where the stratum pinches out the water is 
forced to the surface, forming a bog. In the system as constructed, 
the tile was laid above the water-bearing stratum, in dry material, 
and the water continued to pass under it. Had the system been laid 
a foot deeper the stratum would have been cut and the flow inter- 
cepted. 
Figure 5 shows a case where a tile line has been laid 5 feet deep in 
a soil underlain by a stiff, impervious clay at a depth of 6 feet. Here 
again the water passes under the drain and the system is a failure, 
though it could be made successful merely by deepening the drain. 
