228 



an open ditch is always in the way. It interferes very seriously in 

 many cases with tillage of laud, but one of the most serious ob- 

 jections is the lack of physical benefit to the soil from open ditches 

 in comparison with tile drains. ' This is principally due to the 

 fact that small open drains are never as deep as the corresponding 

 tile drain and do not remove the water as completely. The growth 

 of weeds and grass clogs the ditch and renders it less effective. 



(b) Tile Drains. Since the object of drainage is to lower the 

 water table, the tile should be amply large and the lines sufficiently 

 close together and at such depth that the water may be removed 

 before the crop suffers serious injury. If the tile is laid deep 

 enough to lower the water table to only two feet beneath the sur- 

 face on an average, a rain of two or three inches will raise it in- 

 juriously near the surface, and if frequent rains follow the crop 

 will be damaged in spite of the fact that the land is tiled. If 

 the tile is too small this slow removal may permit very serious 



of Soil 



/ 2 



FIG. 101. Showing the water table at a, with lines of tile at 1 and 3, and at bb, soon 

 after the insertion of another line at 2 and later at b'b'. The slope of the water table between 

 the lines of tile varies with the perviousness of the soil. 



injury. If the water table is three feet beneath the surface and 

 the foot of soil above it is two-thirds saturated, a rainfall of two 

 inches will raise the water table a foot at least and damage to the 

 crop may result. 



The topography of the water table in tile-drained land consists 

 of a series of ridges, with the crests about midway between the 

 lines of tile. The height of these crests above the tile depends upon 

 the texture and character of the soil strata, the distance between 

 the lines of tile and the amount of rainfall (Fig. 101). In laying 

 tile the character of the soil should be taken into account and the 

 lines placed close enough together so that the water table will be 

 lowered to at least 30 inches beneath the surface at its highest 

 point. It must be remembered that the most of the water does not 

 simply pass downward into the tile, but it must move laterally 

 from two to five rods, depending upon the distance between the 

 lines. The lateral movement is comparatively slow, so much so in 



