22 



BULLETIN 190, U. S. DEPARTMENT OF AGRICULTURE. 



would apply to lands underlain by a sand stratum at from 4 to 6 feet 



below the surface. The 

 drains should cut through 

 this sand stratum, which 

 itself thus becomes a great 

 sheet drain. Under this 

 arrangement three lines of 

 tile to the 40-acre tract, 

 or less than 100 feet of 

 tile per acre, will drain 

 fairly tough clay soil. 



APPUCATION OF THE NATURAL 

 SYSTEM. 



The natural system of 



Fig. 16.— Plan and section showing method of draining where source of seepage is a buried shale knoll. 



drainage is also applicable 

 to irrigated lands. The 

 principle involved in the 

 application of this sys- 

 tem is to assist the natu- 

 ral drainage and expedite 

 its movement. To do this 

 the drains are laid in the 

 natural depressions and 

 the ground water lowered 

 in those locations, so that 

 the movement of water 

 toward the depressions is 

 made more rapid. The 

 system is especially ap- 

 plicable to lands under- ^^^ n.-Sketch map showing application of a uniform 

 lain by gravel which occurs system of drainage. 



inundulating strata or beds. It has been found, however, that in the 



