80 AGRICULTURAL ENGINEERING 



tends to increase the velocity of the flow in the mains. 

 These modifications are not governed by any law and they 

 require judgment for their use. 



Run-off from Underdrained Land. In addition to know- 

 ing the capacity of a tile drain, the engineer must know 

 something about the amount of water which must be taken 

 care of from the given area. This is usually spoken of as the 

 "run-off, "and is measured by the depth of the water received 

 if spread over the entire area. Thus a run-off of J^ inch for 

 an acre is the water received from that area in 24 hours, and 

 is sufficient to cover the acre to the depth of % inch. Many 

 experiments have been conducted to determine the run-off 

 from given areas. Sometimes this quantity is spoken of as 

 the " Standard Drainage Coefficient," or " Standard." 

 The common standard used for small areas in which tile 

 drainage is practiced is the J^-inch standard. For larger 

 areas the standard is larger. In this connection, due con- 

 sideration should be made for surface water which may flow 

 to the underdrained land from adjoining land. This may 

 necessitate the doubling of the capacity of the tile otherwise 

 required. 



Application of Formula. In order to use the formula for 

 the capacity of the drain tile, it is necessary to know the 

 quantity of water discharged per second. This is a simple 

 matter, as the quantity of water is equal to the area of the 

 drain, times the velocity. Thus, 



Q = av 



where Q is equal to the quantity of water discharged per second, a is 

 equal to the area in square feet of the cross section of tile, and v 

 equals the velocity in feet per second. 



In addition to this it is necessary to know the number of 

 cubic feet per second that is equivalent to the standard used. 



