OPEN DRAINS. 159 



must be answered at the beginning of every drainage 

 enterprise. There are three distinct elements that 

 must be taken into consideration in deciding this matter. 



1. Area of the drainage-basin^ and also its shape; 

 that is, the number of acres that will be drained by the 

 ditch when made, and whether this area lies in a broad 

 tract on either side of the proposed ditch, or is in the 

 form of a narrow strip along its course. 



2. Slope of the land; that is, whether it is broken up 

 by alternate steep slopes and flats or is a plane having 

 but little variation in level. 



3 . The fall that can be obtained for the ditch along 

 the various parts of its course. 



The bearing which these conditions have upon the 

 size of ditches may be stated in a general way as fol- 

 lows: If the tract is level and broad, drainage-water 

 will be held back a longer time and be distributed to 

 the main ditch much more slowly than if the tract were 

 narrow and has steep slopes which will shed water 

 rapidly toward the main. If the surface is broken up 

 and slopes are steep the soil will absorb less, and the 

 greater surface slope will impel the surplus water with 

 greater velocity over the surface toward the main ditch, 

 and thereby tax its capacity more than if the land were 

 more nearly level. 



Quantity of Water per Acre to be Removed, 



It is a common opinion that if a ditch of certain 

 capacity will drain a given area that it will require a 

 ditch of double that capacity to drain twice the area. 

 An examination of natural streams and their water- 



