Irrigation and Drainage 129 



The two principal devices for measuring flowing water 

 are the weir and the current meter. With the former, a 

 measuring gate of known size is placed in the stream and 

 the height of water flowing over it determined. From 

 standard tables the discharge is found. When the current 

 meter is used, the velocity of the stream-flow is obtained, 

 together with its cross-section ; from these the amount of 

 water is calculated. 



Of the many methods of expressing quantities of water, 

 the ones in most common use are the second-foot and the 

 acre-foot. A second-foot represents one cubic foot of 

 water flowing each second. An acre-foot is the amount 

 of water required to cover an acre of land one foot deep, 

 that is, 43,560 cubic feet. A second-foot flowing for 

 twelve hours will flow almost exactly an acre-foot. If 

 a weir is placed in the ditch, it is very easy to compute 

 the depth of water applied at each irrigation. 



Preparing land for irrigation. 



Considerable care should be taken in preparing land 

 for irrigation. This often calls for a great expenditure 

 of money to make smooth a surface that is rough and to 

 give a uniform slope to the land ; but since a single level- 

 ing will serve for many years, the expenditure is usually 

 justified. Too often farmers, not wishing to spend so 

 much money during any one season, leave the land uneven 

 year after year, and as a result each crop is diminished. 

 It may be that the loss each year would not be sufficient 

 to pay for grading the land, but many years would not 

 be required to do so. 



Losses result from an uneven soaking of the land in 



