IRRIGATION 251 



is raised into the main canal by means of a dam in the 

 stream supplying it. The main canal is shown as pass- 

 ing through a tunnel under a rocky ridge, through a flume 

 across a ravine, and into a reservoir where a small flow 

 of water is accumulated for use when needed by the 

 crops. 



The map shows the manner of carrying water to the 

 respective fields and of distributing it by flooding on 

 sloping land, as on alfalfa in Field E; by flooding be- 



Figure 147. Ditch drop to carry the water of a ditch to a lower level. 



tween dikes on land not quite level, as for alfalfa on Field 

 A ; by large furrows, as in the orchard in Field O ; by 

 small furrows, as for vegetables in Field V; by flooding 

 in a check system, as the orchard in Field D. An alkali 

 swamp is also shown, and beside it a swamp with no 

 alkali, drained by open ditches. In many cases alkali 

 swamps located like the upper one are benefited by 

 underdrainage. Since irrigation structures are usually 

 permanent there is especial reason for the use of great 

 intelligence in making plans which will best serve the 



