296 Ground Water, Wells and Farm Drainage. 



367. Distance Eetween Tile . Drains. There are three 

 prime factors which determine the distance between tile 

 drains. 1. The effective size of soil grains and pore space 

 of the subsoil through which the water must move to reach 

 the drain. If the subsoil has a close fine texture the re- 

 sistance to the flow will be great, and hence the water sur- 

 face will rise faster back from the drain, bringing it near 

 the surface sooner and making it necessary to place the 

 lines closer together. 



2. The depth at which the drains are placed. It is clear, 

 that when it is desired to hold the water midway between 

 a line of tile- a certain distance below the surface, that the 

 deeper the tile are placed the further they may be apart, 

 and Fig. 115 illustrates both this point and the first. 



3. The interval between rainfalls sufficiently heavy to 

 produce percolation. In regions where the rainfall is both 

 heavy and frequent "tiles need to be placed nearer together 

 than where the reverse conditions exist. 



FIG. 115. Showing 1 ho influence of distance between tile drains on the 

 relation of the ground water to the surface of the ground. 



In general practice for field crops it is usually sufficient 

 to place the lines of tile from 50 to 100 feet apart. In 

 favorable cases they may be placed even further apart than 

 this and in special cases they may be required as close as 

 30 feet. 



368. Observed Ground Water Surface in a Tile Drained 

 Field. In Fig. 116 is represented the observed ground 



