440 



Irrigation and Drainage 



4 feet below the surface. The conditions there shown 

 had developed 48 hours after a rainfall of .87 inches, 

 and the facts were obtained by sinking lines of wells 

 at right angles to the drains, there being 3 wells 

 between each pair. It will be seen that the height of 

 the water on the crest between the drains varies, 

 being much greater at 1 and 2 than elsewhere, and 

 this is where 'the soil is more clayey, and so closer in 

 texture. 



In Fig. 138 .is represented the heights of the 

 ground -water surface midway between the drains as 

 they occurred 2 days, 2% days and 5% days after the 

 same rain, and the differences in the steepness of the 

 slopes in the several cases should be understood as due 

 chiefly to differences in the size of the soil grains. It 

 will be seen that after a period of nearly 6 days the 

 surface of the ground water in the upper portion of 



Fig. 138. Changes in the level of the ground-water surface in tile-drained field. 



the field has become quite flat, having fallen below the 

 level of the drains, and the gradient being reduced 

 to 1 foot in 175 feet, while at the lower end, where 

 the soil is heavier, the slope is still 1 in 27. 



Taking these two cases, let it be assumed that it 



