2, BULLETIN °339, U. S. DEPARTMENT OF AGRICULTURE. 
seem to prove that on an average about 14 feet of water per acre are 
required for the maximum yield of grain on the average soil of Idaho. 
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DQLPTH APPLIED 1N FEET. 
Fic. 3.—Irrigation requirement curve based on yields and amounts of water applied to 207 individual 
fields of grain on clay loam soils in Idaho during the seasons 1910 to 1913, inclusive. 
The curve shown in figure 4 is made up from the results secured on 
77 fields of alfalfa and clover on clay loam soil and shows marked 
tendency toward increase in yield as the application of water is in- 
