10 BULLETIN" 190, U. S. DEPARTMENT OF AGBICULTUBE. 
LOCATION OF OPEN CANALS. 
In considering the question of location, the specific purpose of the 
drain must be kept in mind. Outlet systems require treatment 
different from that given small canals or ditches intended to accom- 
plish farm drainage directly. The former must usually follow the 
natural depressions and watercourses, while the latter must be 
located with strict regard to the source of the damaging water, as 
is the case with tile drains. For this reason the discussion of location 
of covered drains (pp. 18-24) may be understood to refer to the 
location of open farm drains as well. 
It is generally a feasible and satisfactory practice to have small, 
open ditch outlet drains and laterals extend along the highways, as 
the roadway is thus drained and less right of way is required, since 
the spoil may be thrown into the roadway and crowned, making an 
excellent thoroughfare where roads may ordinarily be impassable 
during wet seasons. Figure 3 shows how this arrangement may be 
""* 58 Roadway 
-S-t3 
— 66' OHginat Road— \ ** jUgStiSff" > 
99 
Fig. 3. — Illustrating how a canal may be built along a 4-rod road by purchasing a 2-rod right of way and 
placing all the spoil on the road. 
effected, in the case of a 4-rod road, by the acquisition of a right of 
way 33 feet in width. A walk 5 feet in width is provided on either 
side, the drainage canal has the necessary depth and a desirable 
cross section, a small surface drain is provided at the left side of the 
road, and a roadway is afforded which is 58 feet in width and has a 
crowned surface rising Z\ feet above the general ground surface. 
DEPTH AND LOCATION OF COVERED DRAINS. 
One of the most important questions in drainage practice in the 
irrigated section is the proper depth at which to lay drains. Water 
often rises in soils, by capillary attraction, to a height of several feet 
above the free water level, and the presence of salts in solution in- 
creases the height of the rise and the rapidity of the movement. 
Evaporation takes place, and as a result the salt solution is concen- 
trated at the upper limit of saturation. The height to which capil- 
lary water will rise depends upon the type of the soil, the wetness, 
the amount of foreign material in the soil, the amount of salts in the 
water, and the temperature. The rise may vary from a few inches 
