8 BULLETIISr 190^ U. S. DEPARTMENT OF AGRICULTTJEE. 



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 f aU, 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 DRAIN. 



Both the open canal and the covered conduit are appHcable 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 



