32 BULLETIN 190, U. S. DEPARTMENT OF AGRICULTURE. 
the root depth is increased so that both the available plant-food 
supply and the available moisture are increased by drainage. 
The downward movement of water through the soil leaches out 
the excess of harmful salts, and this is one of the most important 
functions of drainage in the irrigated section. The movement of 
water also develops the pores of the soil, so that the physical char- 
acter of the latter is improved. 
Drained lands may be plowed earlier than undrained lands; thus 
the season is made both earlier and longer. An indirect result of this, 
which is of great importance in some localities, is that most of the 
irrigation may be done earlier than is usual and before the water sup- 
ply becomes reduced. Moreover, drained lands require less irrigation 
water than undrained lands, and the water discharged by the drains 
may be employed for the irrigation of other areas, so that drainage 
practically increases the available irrigation supply. 
Drainage performs an educational function in that it causes men 
to consider the use of water, to realize the vast difference between 
scientific irrigation and mere " watering," and to improve their 
methods of irrigation, resulting in a reduction in the amount of water 
used and in the subjugation of additional arid lands. 
Drainage improves the public highways, railway roadbeds, and I 
power lines, as well as telegraph and telephone lines. It increases 
the stability of buildings and structures and serves directly and 
indirectly to promote health and economic conditions in many ways. 
COST OF DRAINING. 
A distinction must first be made between farm drainage and outlet 
systems. The latter are intended only to afford outlet facilities to 
farm drainage systems and are rarely designed to accomplish drain- 
age directly. Farm drainage systems are designed upon the suppo- 
sition that natural drainage outlets exist or that artificial outlets 
will be available. Manifestly, it is impossible to build an outlet 
system that will not accomplish some drainage directly, and this, of ' 
course, reduces the cost of farm drainage in the vicinity. 
The cost of outlet drainage systems varies from about $3 per acre 
to as much as $15 per acre. In the latter case very little farm 
drainage will be necessary, and the system may prove more econom- 
ical than one costing much less but requiring more farm drainage. 
The cost of draining ordinary-sized farms having an average soil 
that is neither so hard as to require picking nor so soft that extreme 
trenching difficulties will be encountered, will range from $10 per 
acre to $20 per acre with the average between $14 and $15 per acre. 
If hardpan be present or if the soil is so finely divided and so wet as 
to be fluxible, the cost will run up to $50 per acre and even more if 
much sheeting is required. In a few special cases, drainage of small 
