IRRIGATION AND DRY-FARMING 693 



sentially an agricultural problem. It is important to 

 maintain this clear distinction in dealing with the prac- 

 tice of irrigation, especially in its larger aspects. The 

 two functions are largely exercised by different groups 

 of men, and they involve widely different types of knowl- 

 edge and skill. The supreme test of an irrigation system 

 is efficient use of the water on the land in the production 

 of crops. 



595. Sources of water for irrigation. The practice 

 of irrigation is dependent on some adjacent supply of 

 water that may be diverted on to the land. It may be 

 derived by (1) the diversion of streams flowing from 

 well-watered regions; (2) the melting of snow on moun- 

 tain areas; (3) the regulation of the flow of streams by 

 storage reservoirs ; and (4) the utilization of underground 

 water by means of wells. All these sources may require 

 the construction of large and costly works, which are well 

 exemplified in the structures built by the United States 

 Reclamation Service and by the Egyptian government 

 in the Nile valley. Dams hundreds of feet high and 

 thousands of feet long, containing millions of cubic yards 

 of masonry and concrete, have been constructed for these 

 purposes. 



596. Canals. The conveyance of the water from the 

 point of supply to the place where it is to be used necessi- 

 tates further difficult engineering problems, which in some 

 cases have entailed the construction of large tunnels 

 under mountains and the development of large pumping 

 and power plants as well as the construction of thousands 

 of miles of main and lateral canals. In 1909 the length 

 of main irrigation ditches in the United States was 875,911 

 miles, and of laterals 38,062 miles. As a rule the water 

 is conveyed by gravity flow without pressure. Important 



