Nov. 8, 1915 Use of Current Meters in Irrigation Canals 231 
velocity alone. The lined sections were subdivided into two classes: 
Those with relatively steep sides and those following the flatter slopes 
more usual to earth canals. There is no marked difference in the results 
of the two types. These measurements indicate, as was to be expected, 
that the points of mean velocity are farther from the water edges in 
sections with sloping sides than in the vertical-sided flumes and occur 
between one-fifth and one-fourth of the width of the water surface from 
the edges. The average and maximum errors are not large. 
For earth canals results are given for both velocities and for total dis¬ 
charges. The results for such sections are more variable. The velocity 
at from one-fifth to one-fourth of the width from the edges will average 
to give results close to the actual velocity for the whole section, but indi¬ 
vidual gagings may vary from the mean by over 15 per cent. The 
results for the total discharge are more consistent than those for velocity 
alone. The error in the cross-sectional area, due to using the two measure¬ 
ments of depth to give the mean depths, tends to balance some of the 
errors in velocity. For all measurements the determination of the 
depth and velocity at points one-fifth of the width of the water surface 
from the sides gives average results from 1 to 3 per cent too high. Any 
single gaging will average to give errors of 5 to 7, and they may be as 
high as 15 per cent. 
These results indicate that under favorable conditions two index 
verticals can be found in canals, the velocity at which will agree with 
the average for the whole cross section. These points are from one-fifth 
to one-sixth of the width of the canal from the sides in sections with 
vertical sides and from one-fourth to one-fifth for other types. In sec¬ 
tions with vertical sides, such as flumes, and in earth sections the depths 
at these index verticals will also be quite close to the average depth in 
the whole section, so that the index points can be used also to determine 
the total discharge. In definite sections with sloping sides, such as con¬ 
crete-lined canals, it is preferable to use known relations of depth and 
area and use the index points for the determination of velocities only. 
Such short-cut methods would not generally be desirable at permanent 
rating stations. They might be useful for approximate measurements 
where time was an important factor, or as checks on the division of water 
in canal at large turnouts. Such gagings could be made of the canal 
above and below, and also of the turnout. Where other means of meas¬ 
uring or controlling the device are not available, such rapid methods 
might be of value. 
SUMMARY 
Comparisons of various methods of current-meter gaging of irrigation 
canals are made with measurements in which the velocities at from 70 
to 120 points were taken. Canals of various types of cross section having 
discharges of from 2 to 2,600 second-feet and velocities of from 0.5 to 
8.0 feet per second were included. 
