40 



BULLETIN 339, U. S. DEPARTMENT OF AGRICULTURE. 



irrigated with not over 6 acre-inches at one application if economy of 

 water is desired. It is hardly considered that it will ever be practical 

 for the farmer to predetermine just how much should be applied at 

 each irrigation and then apply this quantity and no more, but it is 

 believed that intelligent and economical practice demands at least an 

 approximate knowledge of the volume that is being applied. The 

 fact that a head of 1 cubic foot per second delivers almost exactly 1 

 acre-inch per hour should make it comparatively easy for an irrigator 

 to determine at least approximately how much water he applies to 

 his land. 



LENGTH OF IRRIGATION SEASON. 



The length of the season during which crops require water depends 

 upon both the climate and the crops grown. Grains require water 

 for only a comparatively short season, winter grains frequently re- 

 quiring but one irrigation, while alfalfa and pasture grasses require 

 irrigation from early spring until late fall. The climatic conditions, 

 with particular reference to the temperature, which determines the 

 length of the growing season, have a very direct influence upon the 

 length of the irrigation season, since the grass crops, provided an 

 abnormal amount of natural precipitation does not occur, will require 

 water for as long as the climate will permit them to grow. The 

 length of the irrigation season in Idaho has been carefully determined 

 by the investigation and is shown for the different crops by Table 

 XX. 



Table XX. — Average length of irrigation season of plots included in the 4 years' 



investigation. 



Crop. 



Year. 



Total 

 number 

 of plots 

 consid- 

 ered. 



Average 

 number 

 of irriga- 

 tions. 



Number 



having 1 



irrigation 



only. 



Average 



length of 



irrigation 



season. 1 



Maxi- 

 mum 

 length of 

 irrigation 

 season. 



Average dates of 

 irrigation. 



First ir- 

 rigation. 



Last ir- 

 rigation. 





1910 

 1910 

 1911 

 1911 

 1912 

 1912 

 1913 

 1913 



76 

 27 

 96 

 34 

 60 

 25 

 66 

 15 



3.6 

 4.7 

 2.1 

 6.1 

 3.7 

 5.6 

 3.1 

 5.1 



3 

 

 17 

 1 



10 

 



8 

 



Days. 

 46.0 

 95.4 

 35.5 

 111.4 

 39.7 

 87.3 

 48.9 

 96.5 



Days. 



87 

 144 



64 

 142 



61 

 123 

 110 

 119 



May 27 

 May 12 

 June 13 

 May 14 

 June 9 

 May 23 

 June 5 

 May 16 



July 12 



Alfalia and clover 



Aug. 12 

 July 19 



Altalfa and clover 



Sept. 2 

 July 18 

 Aug. 18 

 Aug. 20 



Alfalfa and clover 



Alfalfa and clover 



Do. 



1 Exclusive of plots having one irrigation only. 



Average length of irrigation season for alfalfa for 4 years was 97.6 days. 

 Average length of irrigation season for grain for 4 years was 42.5 days. 



SIZE OF IRRIGATION HEAD. 



The size of the irrigation head used, particularly with the porous 

 soils, has a great effect upon the irrigation requirements. It is a fact 

 well known to irrigators who are accustomed to handling porous soils 

 that very little can be accomplished with small heads of water for 



