334 IRRIGATION PRACTICE 



unless it is carefully specified that the water was used 

 during a certain number of days. 



Canals taken directly out of the river usually carry 

 water from early spring until late fall, but the water so 

 delivered before and after the irrigation season should 

 not be charged to the duty of water. When the waters 

 are held back in reservoirs, water is of course allowed to 

 flow through the canal only during the irrigation season. 



The relationships existing among the quantity of flow- 

 ing water, the number of acres to be irrigated and the 

 depth to which the land will be covered may be shown in 

 simple formulas: 



A = Area to be irrigated. 



D = Duty of water, that is the acres matured by 1 



second-foot flowing continuously for a definite 



period. 

 B = The time in days 1 second-foot flows to mature 



crop. 

 S = The depth in inches of the given volume of water 



over the area irrigated. 

 F = The discharge of second-feet necessary to irrigate 



the given area A, with the duty D. 

 The following relationship may then be established: 



S = X 23.8. 



D BX23.8 



198. Classes of duty. The theoretical duty of water 

 is never quite realized in practice. The term "duty of 

 water" does not refer to the theoretical deductions in 

 laboratory experiments, but refers invariably to the water- 

 cost of crops under practical conditions. It is, therefore, 



