8 BULLETIN" 190, U. S. DEPARTMENT OF AGRICULTURE. 
silting, and the higher the velocity the smaller the cross-sectional area 
necessary to provide for a given discharge. The velocity is usually 
limited by the available fall, however, and it is quite the general thing, 
in the irrigated section, to have the drains run across the greatest 
slope, rather than with it. The smaller-sized tile should have a fall 
of at least 1 foot per thousand feet, and the larger sizes at least 
one-half foot per thousand feet. 
Tile having an inside diameter of less than 4 inches should not be 
used, and even 4-inch tile should be used sparingly, usually at the 
extremities of small branches. Experience has shown that the use 
of tile less than 5 inches in diameter is not warranted by the com- 
parative results and cost. The cost of trenching and laying is about 
the same for 4-inch as for 6-inch tile, while the latter has about three 
times the carrying capacity of the former. The 6-inch tile also 
presents a much larger surface to the surrounding soil and has more 
than double the area of bore, so that a given amount of silting repre- 
sents a much less obstruction to the flow; also it is much easier to 
insert devices for clearing out the 6-inch line than is the case with 
the 4-inch one. 
CHOICE OF TYPE OF DRArN. 
Both the open canal and the covered conduit are applicable to the 
drainage of irrigated lands. Each serves a purpose and under certain 
circumstances there is no question as to which to employ. There is, 
however, a zone in the scale of varying conditions in which the choice 
is not easily made. These conditions are worthy of special con- 
sideration. 
The primary purpose of open canals is for main outlet systems or 
large laterals in which provision must be made for a considerable 
flow. Covered drains are for farm drainage proper. There are few 
reasons, save that of economy, why covered drains should not be used 
throughout, as they are more desirable in most respects; and when 
the problem as to which type to select arises, the question of desira- 
bility should be considered with that of economy. Open drains are 
unsightly and harbor obnoxious weeds; they occupy valuable space 
and often cut the land into inconvenient shapes, increasing the 
difficulties of cultivation and irrigation. Bridges, culverts, and 
flumes must be provided, and a constant watch must be kept lest 
irrigation streams find their way into the canals and do great damage 
to both the canals and adjacent lands, as well as waste the water. 
The maintenance cost of open canals is usually high in the irrigated 
sections, owing to the nature of the soil and to other causes. 
The covered conduit usually requires no right of way and occupies 
no valuable land. If properly designed and laid it requires very 
