CHAPTER XVII 



THE DUTY, MEASUREMENT AND DIVISION 

 OF WATER 



IN the foregoing chapters have been elucidated the 

 known laws governing the relationship that exists between 

 soils, plants and water. Results obtained under well- 

 controlled laboratory or experimental field conditions may 

 often differ from those obtained in general field practice. 

 This chapter, therefore, discusses the practical duty of 

 water and the methods of measuring and distributing 

 irrigation water, so that ideal conditions may be ap- 

 proached. 



197. The duty of water. The duty of water, a term 

 long since coined, means the quantity of water needed to 

 mature crops. It may be expressed in various ways. 

 Sometimes the duty of water is expressed as the number of 

 pounds of water required to produce one pound of the 

 dry matter of the crop; under other conditions, as the 

 depth of water over the field, required during the growing 

 season, to produce the crop. More commonly, however, 

 the duty of water is expressed as the number of acres 

 that may be irrigated by a definite quantity of water, 

 say a second-foot, flowing continuously throughout the 

 growing season. In Canada, the United States, India, 

 Australia and other irrigated countries, this is by far 

 the most common method of expressing the duty of 

 water. The reason for this popularity seems to be that 

 irrigation canals are generally taken directly from 



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