Irrigation 181 



IBBIGATIITO THE OECHAED 



In the discussion of, orchard irrigation, so many factors 

 enter into the problem, each of which is modified to a great 

 degree, depending on the soil, ages of trees and^yarious 

 other conditions, that no set of rules can be laid down. 

 The three problems to consider in actually irrigating an 

 orchard are : amount of water-to apply, when to apply it, 

 and the means of application. 



Quantity of water to supply. 



When water is abundant, there is always a tendency to 

 over-irrigate. It should be borne in mind that irrigation 

 is only supplemental to natural precipitation and every 

 effort should be made to conserve the natural moisture. 

 Excessive irrigation causes leaching of plant-food and is 

 injurious to the soil. The basis of water distributiqn is 

 variable. In general it may be said to vary from 35 to 

 100 acres to a second-foot. "(Expressed in acre-inches, the 

 average amount used in a year would cover each acre to a 

 depth of about 36 inches. This is augmented by the 

 normal precipitation which in most irrigated districts does 

 not exceed 10 to 12 inches per annum. The common unit, 

 for instance, for water distribution in the Wenatchee Val- 

 ley is the miner's inch (one-fiftieth of a cubic foot a second) . 

 The prevailing rate of distribution in this region is one 

 miner's inch to two acres. 



The water requirement of different soils is somewhat 

 variable. With a rich soil the water requirement is rela- 

 tively low, while with a leached soil it is high. The prac- 

 tice of growing leguminous shade or cover-crops in irrigated 

 orchards (usually alfalfa) has appreciably increased the 



