24 BULLETIN 190, U. S. DEPARTMENT OF AGEICULTUEE. 



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 drams 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 difiGiculties. 



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. 



