332 IRRIGATION PRACTICE 



streams having a continuous flow during the irrigation 

 season. 



Various units for measuring water are used in different 

 parts of the country, such as the miner's inch, but the 

 only one in modern general use is the cusec or second-foot, 

 which means 1 cubic foot of water passing a given point 



FIG. 91. Canal crossing river in an inverted syphon. 



each second of time. The duty of a given stream would 

 then be the number of acres irrigated per second-foot, 

 flowing continuously during the season. This duty may 

 easily be converted into acre-inches or acre-feet of water. 

 One second-foot, flowing for twenty-four hours, will 

 cover 1 acre to a depth of nearly 2 feet. If the time is 

 known during which a second-foot of water has been 

 flowing over a given area, it is but a moment's calcula- 



