Depth of Drains. 



293 



age are only too wet during the spring it may be best 

 to lay the drains only so deep as is needful to bring 

 the field into condition for working in due season, 

 and in such cases tile placed 2.5 to 3 feet, rather than 3.5 

 to 4 feet, will usually be found sufficient for general farm 

 crops. 



When tile are placed needlessly deep not only is the cost 

 greater but, in all of those cases where there is an under- 

 flow of water from the higher land, the level of the ground 

 water is drawn down earlier in the season to such a depth 

 that the crop will get less advantage by the subirrigation 

 resulting from the capillary rise of the underflowing water 

 into the root zone. 



FIG. 112. Representing an apparatus for demonstrating the slope of the 

 ground water surface back from a tile drain and the changes in 

 pressure when discharge is taking place. A, front elevation of tank, 



with a, b, c, d, faucets from drain tile, and 1, 2, 3, 15, pressure 



gauges; B, B, vertical sections lengthwise, with 1, 2, 3, 4, tile and 

 faucets, and 5, supply tile at end; C, cross-section, with 1, 2, tile; 

 D, section at 1 in B, showing connection of faucet with tile. 



Rise of Ground Water Away from Drainage Outlet 



If reference is made to the contour map of the ground 

 water surface, Fig. 89, p. 257, it will be easy to compute 



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