CHAPTER IX 

 THE TIME OF IRRIGATION 



UNDER ideal conditions of irrigation, a plentiful supply 

 of water would always be at the disposition of the farmer. 

 In practice, such a condition seldom exists. The flow of 

 water in the rivers from which the canals are taken, varies 

 from season to season, and, unless the water is stored in 

 reservoirs, there is not, throughout the season, a uniform- 

 supply of water. In the spring, the flow is beyond the 

 capacities of the canals; in midsummer and later it is 

 often insufficient for the needs of the system. Different 

 crops have different water requirements, both as to total 

 quantity and periodic application. Young plants use less 

 water than do the larger and stronger plants some weeks 

 older; and the mature plant, the life activities of which 

 have ceased, has very little need of water. The life his- 

 tory of the plant determines, largely, the best time of 

 irrigation. It seldom happens, however, that the periodic 

 natural flow of water coincides with the periodic crop 

 requirements. The problem of applying the best quantity 

 of water at the proper time, which will determine the 

 principles of canal management, is one of the most com- 

 plex in irrigation practice. 



110. The ideal principle. It may be laid down as 

 an ideal principle, that, so far as possible, the same 

 percentage of moisture should be maintained in the soil 

 throughout the growing season, irrespective of the age 

 of the plant. That is, the soil-water film should be kept 



(173) 



