SIZE OF DRAINS 



181 



"As a rule, tile larger than 12 inches in diameter is not 

 used in individual farm drainage. 



"The size of tile required depends on the amount of 

 water to be carried and on the slope of the drain. The 

 latter can be decided upon when the survey of the land 

 is made and the fall to the outlet is ^measured. The 

 former is not so easy to determine. It depends on the 

 location of the tract, the nature of the soil, the slope of 

 the ground both on the tract and above it; on the quantity 

 of water used in irrigation and on the method of irrigating, 

 both on the tract and on higher land; on the rainfall and 

 evaporation; on the seepage from reservoirs, canals, and 

 ditches; and on many other factors. Indeed, the de- 

 termination of the required capacity of a drainage system 

 is the most difficult problem confronting drainage engi- 

 neers, and demands their best efforts. Intricate measure- 

 ments and calculations must be made in each instance. It 

 is therefore impossible to give definite instructions in re- 

 gard to this important matter. It is possible, however, 

 to give a general idea of required sizes based on a wide 

 experience under a great variety of conditions. The fol- 

 lowing table is intended to apply to fairly uniform land 



TABLE XX. SIZE OF TILE REQUIRED TO DRAIN GIVEN AREAS 

 HAVING DIFFERENT TYPES OF SOIL 



