148 IRRIGATION. 



It is not the purpose here to treat of drainage with 

 reference to itself alone, but only so far as it may be used 

 in connection with, or as an adjunct to, irrigation. 

 Drainage may be superficial or subterranean. Superficial, 

 or surface drainage, is the simplest. Nothing is needed 

 for its practice but to provide open channels into which 

 the surplus surface water may find its way. As a matter of 

 necessity, these to be perfect must be placed at the lowest 

 levels of the ground to be drained. Besides, they need 

 to be placed in such relation to the distributing furrows 

 of the irrigating system, as to catch the water as soon as 

 it has completely accomplished its purpose, and remove 

 it in the most effective manner. Sufficient description 

 of needed methods has already been given, to make clear 

 the means of doing this. For subterranean, or subsoil 

 drainage, much more elaborate and costly methods are 

 necessary. Not only must expensive ditches be made, 

 and earthen tiles be used, but the arrangement of the 

 drains, with reference to the irrigating ditches or furrows, 

 must be carefully made. 



No drain should exist immediately beneath an irrigafc 

 ing ditch, canal or furrow, for the reason that excavated 

 earth cannot be so returned as to be as compact as it laid 

 before. If then a water channel passes across, or along 

 a line of earth, that has been disturbed, a rapid infiltra- 

 tion occurs, the water makes itself a channel, which is 

 rapidly enlarged ; sand or earth is carried into the drains, 

 and the water not only escapes without doing its work, 

 but chokes the drains in a short time. Thus no drain 

 should be made nearer to an irrigating furrow, or canal, 

 than six feet, and no irrigating furrow should terminate 

 at a less distance from the line of a drain, than six feet. 

 The usual arrangement of drains and furrows is shown at 

 figure 75. Here A, A, is the main canal ; A, , A, C, 

 the feeders, with the lateral distributing canals or fur- 

 rows ; 0, the main drain, which discharges into the 



