32 BULLETIN 190;, U. S. DEPARTMENT OF AGRICULTUEE. 



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 

 power lines, as well as telegraph and telephone hues. 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 du'ectly, and this, of 

 course, reduces the cost of farm dramage 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 niore 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, wiU 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 wiU run up to $50 per acre and even more if 

 much sheeting is required. In a few special cases, drainage of small 



