24 BULLETIN 190, U. S. DEPARTMENT OF AGRICULTURE. 
little so that the ground-water table rises very rapidly when irriga- 
tion is applied and soon creates a demand for drainage. The dam- 
aging water must be cut off as it leaves the rock formations and before 
it enters the soil, or it may be tapped by means of relief wells. 
Another situation that presents difficulty is that in which the irri- 
gation canals have been constructed in old watercourses and natural 
drainage channels. These channels are often higher than the sur- 
rounding lands, due to the fact that they have overflowed from time 
to time, and the soil adjacent to them is coarser than that at a dis- 
tance. Seepage from the canals waterlogs the adjacent soil and causes 
alkali salts to appear at the surface. Waste water and seepage from 
irrigation of the land find their way to the depressions and form 
ponds, or swamp the farm lands. To remedy these conditions, 
drains must be constructed through the lower portions to carry off 
surface water, waste water, and seepage, and to provide an outlet 
for tile drains on the higher portions. Intercepting drains must also 
be constructed parallel with the irrigation canals, to catch the direct 
seepage. It is usually feasible to construct these in the borrow pits 
adjacent to the irrigation canals. The main outlet drainage canals 
should not be placed in such locations, however, as they would not 
then afford an outlet for tile systems nor take care of the water that 
reaches the depressions. 
CONSTRUCTION OF DRAINS. 
The soils of the arid region are usually semi-fluid when wet, due 
largely to the absence of humus, and the construction of drainage 
systems therefore often requires the exercise of considerable patience 
and ingenuity. Special methods and devices have been called for 
and special machinery has been needed to overcome these difficulties. 
OPEN CANALS. 
In the construction of open canals in the irrigated section it has 
been found that the use of teams and scrapers is generally not feasible, 
owing to the considerable depth that must be obtained and to 
the miry condition of the soil. Hand labor is equally out of the ques- 
tion, owing to the excessive cost. The most satisfactory method of 
handling the work is by means of some efficient excavating machine. 
A number of these machines have been developed, but few of them 
are suitable for work in the irrigated section. A discussion of the 
comparative merits of the different machines is not within the scope 
of this paper. In general, the choice of the type of machine should 
be left to the contractor or other party doing the work. 
Construction work should always start at the outlet of the drain 
and proceed up the slope, so that the water developed will drain away. 
