436 Irrigation and Drainage 



represent conditions which are very common in locali- 

 ties where underdrainage is needed, and, therefore, 

 furnish a good basis upon which to form a judgment 

 regarding the distance apart tile should be laid. 



DEPTH AT WHICH DRAINS SHOULD BE LAID 



The depth to which water should be lowered by 

 drainage need seldom exceed 4 feet for ordinary farm 

 crops, and often- the lowering of the water surface 

 may be less. 



It should be kept in mind that the level of the 

 ground water changes with the season, and that many 

 lands benefited by underdrainage are only too wet 

 early in the spring, and if such lands are to be used 

 for ordinary farm crops, it may only be needful to 

 draw the water down so far as to make the surface 

 dry enough to give good working conditions for the 

 soil. In such cases, tiles placed 2% to 3 feet deep, 

 rather than 3% to 4 feet, will usually be found suffi- 

 cient. If the tiles are placed deeper than this, not 

 only will there be a permanent lowering of the ground 

 water, but the low stage will be reached so much 

 earlier in the season that a smaller amount of the 

 water flowing under the field may .be used by the 

 crop. 



Where fields are underlaid by sandy subsoils, it 

 is quite important not to draw the water down far 

 into the sand, because the height to which the water 

 can be lifted rapidly in these by capillarity is quite 

 short. To carry the ground -water surface below this 



