Nov. 8, 1915 
Use of Current Meters in Irrigation Canals % 
229 
accurate results as the more detailed observations. The use of the 
o.6-point method gave results averaging 4.8 per cent too high, and the 
use of from 3 to 5 verticals in flumes and lined sections gives an average 
of 3 per cent too small. Apparently where few verticals are to be 
observed, the use of the o. 6-point method may be preferable, as the 
errors will tend to balance. This may be expressed by saying that about 
the same relative detail should be used in measuring the velocities in the 
verticals that is used in the number of verticals observed. 
The results are obtained , by using the verticals taken in the detailed 
measurements and selecting every alternate or every fourth vertical and 
computing the discharge that would have been obtained had only these 
verticals been observed. It is possible that gagings where the lower 
numbers of verticals were to be observed could be made to give closer 
results by using some means for the selection of the location in the canal 
section at which the verticals should be taken. It has been previously 
shown that the use of velocity measurements at the 0.2- and o.8-depth 
points will give very nearly the same results as measurements at 6 or 
more points in the vertical and that a single observation at 0.6 gives 
results within 5 per cent of being correct. 
If one or two points can be found in the vertical velocity curves the 
velocities of which can be used to determine the average velocity of the 
whole vertical, it would seem probable that perhaps 2 verticals on the 
horizontal velocity curve could be found which could be used to give 
the average v.elodty in the whole cross section. Such points, or index 
verticals, as they may be called, would be useful in the rougher meas¬ 
urements often needed in canal operation, and information as to the 
relative accuracy of such methods should be of value. 
Two such selected verticals may be used to determine the discharge 
in two ways. In one the velocities only might be used and the cross- 
section area more carefully determined, if not known from previous 
observation. In the other the observed verticals may be used to obtain 
not only the mean velocity but also the depths at these verticals, and 
the width of the section may be used to determine the cross-sectional 
area. 
The use of such index-vertical methods is, of course, most applicable 
to canal sections such as flumes which have practically uniform depths, 
as the error in determining the cross section is largely eliminated. 
The measurements were examined to see whether such index verticals 
could be found. The horizontal velocities and cross sections were 
plotted on a sufficiently large scale so that the velocity and depth at any 
point could be read from the curves. Such index verticals would be 
most easily used if their distance from the sides is some definite propor¬ 
tion of the water-surface width. Verticals located at different points 
were tried. The different types of canal cross sections are discussed 
separately. The general results are given in Table VI. 
