CEREALS UNDER IRRIGATION 427 



irrigation water plus the existing soil moisture and the 

 precipitation. The gross duty of water is the crop acre- 

 age served by the water at the intake of the canal or large 

 lateral. The net duty of water is the crop acreage served 

 by the water at the farm headgate (see Widtsoe, 1914, 

 p. 335). 



459. Duty of water and water requirement must be 

 carefully distinguished. Duty of water measures the 

 efficiency of water in producing a crop, while water re- 

 quirement measures the efficiency of the crop in maturing 

 with a certain quantity of water (280, 281). The water 

 required is the net quantity used by a crop unit; the 

 duty of water is the acreage served by a gross unit of 

 water. 



460. Duty of water in Idaho. Welch (1914, pp. 17- 

 21) gives average results of 4 years' work at Gooding, 

 Idaho, on the duty of water for spring wheat, oats, and 

 barley. In the spring wheat experiments 90 plats in all 

 were tested, and four varieties employed, Sonora, 

 Dicklow, Little Club, and Palouse Bluestem. All first 

 irrigations of spring wheat were in the first week of June. 

 There was no difference in the dates of heading, but the 

 maximum application of water delayed ripening 5 days. 

 There was an increase in yield with the increase in water 

 applied up to about 2\ feet, after which any further 

 increase of water was not only unnecessary, but became 

 positively harmful. The conclusion from these experi- 

 ments is that If to If acre-feet of water, given in 4 or 

 5 applications, is the proper irrigation for spring wheat 

 on new sagebrush land. 



The results of the oats experiments justified the decision 

 that If acre-feet of water, in 4 to 5 applications, is the 

 best irrigation for oats. It was also concluded, however, 



