DRAINAGE OF IRRIGATED LAND. 9 
little maintenance. There is little danger of irrigation water getting 
into the drains, and the only way in which vegetation may do damage 
is by the entrance of water roots from certain trees and plants. This 
trouble may be avoided by keeping such trees as willows, cotton- 
woods, tamaracks, etc., well away from the drain lines, and by cutting 
away a narrow swath of such plants as the sugar beet from directly 
over the tile line. Black willows will send out roots to a distance of 
200 feet and choke a drain, while sugar beets planted directly over a 
line will reach and obstruct a drain 5 feet deep. These roots penetrate 
only the disturbed soil of the trench, however, so it is necessary to 
remove but a narrow swath. 
In deciding whether a large covered drain or an open canal shall 
be employed, it is necessary to calculate the original cost of each, 
taking account of all auxiliary and protective devices required, and 
then to add to each sum an amount large enough to give an annual 
return, at current rates, sufficient to cover the cost of maintenance. 
The consideration of the first cost alone gives very misleading results, 
as it has often been found that the difference in the cost of a very 
few years' maintenance would have more than paid the difference in 
first cost between the two types of drains. Thus, if a covered drain- 
age system costs $1 per foot to construct, and the annual mainte- 
nance is 1 per cent of the first cost, 20 cents per foot must be added 
to yield an annual income of 1 cent per foot at 5 per cent interest, 
which makes the total cost $1.20 per foot. An open drain having 
the same capacity will cost about 30 cents per foot for excavation, 
25 cents per foot for right of way, and 10 cents per foot for inlets, 
flumes, bridges, culverts, fences, etc. If the annual maintenance be 
taken at 10 per cent of the excavation cost, which is reasonable, 
60 cents per foot must be added to yield an income of 3 cents per 
foot at 5 per cent, which gives a total of $1.25 per foot. The covered 
drain is to be preferred, therefore, even from the standpoint of actual 
cost ; and when the other factors are considered there is no room for 
comparison between the two types. 
Practice in the humid section is leaning more and more toward the 
covered drain, and tile having an inside diameter of 3 feet are not 
uncommon, while still larger sizes are sometimes employed. Until 
quite recently, very little tile over 12 inches in diameter had been 
used in the arid section. However, conservative estimates based on 
present prices and conditions show that it would be economical to 
use 20-inch tile, rather than the open canal of the same capacity; 
while improvement of methods, increase of land and crop values, 
and the decrease in the cost of materials that are now being wit- 
nessed, make it seem reasonable to predict that very shortly the 
eastern standards of practice will be adopted. 
77733°— Bull. 190—15 2 
