248 THE PRINCIPLES OF SOIL MANAGEMENT 



is a large improvement in the general sanitary condi- 

 tions of the region, which was recognized long before 

 the economic advantages of the practice, and has gener- 

 ally been sufficient reason for public interest in the prac- 

 tice. It is only within recent years that the economic 

 benefits of drainage have been recognized as of sufficient 

 public concern to warrant regulative legislation. 



122. Principles of drainage. There are two general 

 types of drains: (1) Open, or "surface drains." (2) Cov- 

 ered, or "under drains." Each of these types has a partic- 

 ular range of usefulness and, while they may be substi- 

 tuted one for the other under some conditions, their 

 respective spheres of usefulness are fairly distinct. 



123. Open, or surface drains. Open or surface 

 drains remove water from both the surface and from 

 the depths of the soil. Their efficiency in removing water 

 from the subsoil depends upon their depth and fall, 

 and upon the level of water in the channel. There are 

 certain conditions to which open surface drains alone 

 are adapted. These are: (1) Where the volume of water 

 to be moved is very large. (2) Where the water table 

 is so near the surface, and the fall so slight, that it is 

 not possible to place a drain below the surface. (3) 

 Where the drainage is designed to be for only a short 

 time. 



As open ditches their efficiency depends on the sur- 

 face flow of water into their channel. They usually 

 tap the low areas where the water accumulates. Some- 

 times, as in river bottoms, they may be arranged regu- 

 larly at intervals, and be of such size as to hold the water 

 which may fall upon the surface during any ordinary 



