TRRIGATION EXPERIMENTS IN IDAHO. 41 
the reason that the water is absorbed so rapidly that it can not be 
forced over the field. Where water can be conducted in pipes or 
flumes to within 100 or 200 feet of the farthest part of the field to 
which it is to be applied, a method common among southern Cali- 
fornia orange growers, large heads are not necessary, but where sur- 
face irrigation is practised on large field units of porous or semiporous 
soil, the use of large heads is imperative if economy of water is to be 
secured. The lack of efficiency of small irrigation heads was shown 
by several of the experiments which were included in the investiga- 
ale 
O 
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Eee eess 
| fe 
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fee 
X pi 
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et | 
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Death (Whsorbed per trrigation weet. 
@=Soil more moist than normal prior to these irrigations. 
Fia. 11.—Effect of size of irrigation head upon amount required per application, based upon results of 
an experiment upon 4.3 acres of porous gravelly loam soil in Boise Valley planted to pasture. 
tion. The average size of the irrigation head which is used for nearly 
all purposes in the larger portion of Idaho seldom exceeds 1 to 2 
second-feet, yet the use of heads three or four times this size would 
give a much higher efficiency, particularly if the soil is inclined to be 
at all porous. With rather impervious soils there is not such a loss 
in efficiency. The smaller the size of the head and the greater the 
distance water is run the larger the irrigation requirement. The 
accompanying curve (fig. 11) based on an experiment conducted by 
W.G. Steward, of the Boise project, United States Reclamation Sery- 
ice, clearly indicates the saving that may be made by using large 
heads for the irrigation of porous soils. 
