IMBIGATION AND DEY-FAEMmG 699 



the pipes to be placed several rods apart, thus reducing 

 the expense of installation. 



Sub-irrigation sometimes occurs naturally under condi- 

 tions similar to those just described, where water is sup- 

 plied from springs or by seepage. Where it can be em- 

 ployed, sub-irrigation is the ideal method of applying 

 water to the soil. 



601. Methods most used in arid regions. — The two 

 methods preeminently used to apply water to the soil 

 under arid conditions are by furrows and by flooding. 

 The land must generally be prepared to some extent for 

 either of these methods, by smoothing or leveling the 

 surface, throwing up levees, or constructing distribution 

 furrows. It is a fortunate fact that the subsoil in arid 

 regions is about as fertile as the soil, and therefore grad- 

 ing can be practiced with impunity. Both methods have 

 a large number of variations in detail to adapt them to 

 particular soils, topography, or crops. 



The chief factors determining the choice between flood- 

 ing and furrowing are (1) the nature of the crop, (2) the 

 character of the soil, (3) the contour of the land, and 

 (4) the quantity of water available. 



602. Flooding. — Flooding is especially employed 

 (1) where the crop occupies the entire area, such as in 

 grainfields and meadows ; (2) where the soil is of medium 

 porosity and does not bake seriously on drying ; (3) where 

 the surface is relatively flat ; and (4) where the supply of 

 water is relatively large. 



The advantages of this method are : — 



1. The handling of water is easy. 



2. There is economy in ditches. 



3. The necessity of tearing up the crop is avoided. 



4. The method is especially suited to certain crops 



