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 
Kk 
