THE FLOW OF WATER IN IRRIGATION CHANNELS. 13 
point does not always correctly represent the maximum, since in 
in various verticals taken throughout the summer, in many kinds of 
channels, the maximum was found at points varying all the way from 
0.2 depth to 0.8 depth. These statements refer, of course, to veloci- 
ties found by holding the meter at these particular fractions of the 
depths. A close study of any one vertical might show that the maxi- 
mum velocity occurred at some place between these points, but that 
phase was immaterial to the studies covered in this publication. 
The necessary corrections were applied to the velocities indicated 
near the bottom and surface, on account of the fact that the cup 
current meter does not run true to standard rating curve when held 
nearer the surface than 0.3 of a foot and when held near the bottom. 1 
The particular points at which to hold the meter were chosen with 
a view to determining the variations from the discharge of water 
found in tins way that would be found if shorter and more generally 
used commercial methods of meter gauging had been employed. The 
various points at which the current meter was to be held were deter- 
mined as follows: After the tape had been stretched across the canal 
at the section where the measurement was to be made a bench mark 
was set at the surface of the water near one bank. The depths at the 
various verticals were determined by taking with the level the differ- 
ence in the rod readings between the rod held on the bench mark at 
the water surface and the rod held at the bottom of the channel. 
This method is better than using a graduated sounding rod, for the 
reason that all error due to water climbing the rod is eliminated. 
This method may be used in high velocities through the main portion 
of the channel for the reason that a nail can always be set near the 
bank, where the velocity is much lower than near the center. Where 
the conditions were such that the meter was to be used on a cable, 
then the desired points at which to operate it in any vertical were 
found by multiplying the depth of the vertical by 0.2, 0.4, 0.6, and 0.8 
and holding the meter the resulting distances below the surface. 
Where the meter was to be used on a rod, since the bottom of the rod 
rested on the bottom of the channel for all measurements and the 
meter is set on the rod while the rod and meter are out of the water, 
then the distances below the surface of 0.2, 0.4, 0.6, and 0.8 depth 
become the distances above the bottom of 0.8, 0.6, 0.4, and 0.2, 
respectively, of the depth, and are so set on the meter rod. 
Having taken the soundings and computed the depths in each 
vertical at which the meter was to be held, the actual meter runs were 
made. These were as a rule continued at each point for the nearest 
number of tens of revolutions in a period of about 30 seconds, the 
1 For a discussion of this phase of current meter use, see U. S. Dept. Agr., Jour, Agr. Research, 2 
(1914), No. 2, p. 77, 
