n 



Distance Between Drains 437 



limit not only lessens the amount of underflow which 

 becomes available to the crop, but it also diminishes 

 the amount of the heavy summer rains which the 

 crop may use, because when the ground water is 

 carried too low much of the water, in times of pro- 

 longed heavy rains, may pass below the limit of root 

 feeding before the crop has time to avail itself of it. 



DISTANCE BETWEEN DRAINS 



There are three chief factors which determine the 

 proper distance between underdrains : (1) the freedom 

 with which water may flow through the subsoil 

 toward the drains, (2) the depth at which the drains 

 are placed, and (3) the interval of time between 

 rainfalls sufficiently heavy to produce considerable 

 percolation. 



It should be clearly understood that it is the 

 character of the subsoil, rather than that of the 

 soil, which determines the rate at which water moves 

 toward and into the drains, and it should be further 

 understood that the subsoil which takes part in the 

 lateral flow of the water may be several feet, even 10 

 or more, below the level at which the drains are 

 laid. 



If, for example, the field to be drained has a 

 rather close clay surface soil underlaid with two, three 

 or four feet of heavy clay, which in turn is underlaid 

 by a stratum of sand, then the movement of water 

 from the surface toward and into the drains will 

 be such as is represented by the arrows in Fig. 



