Nov. 8, 1915 
Use of Current Meters in Irrigation Canals 
227 
discharge which would have been obtained had a less number of verticals 
been measured. The different types of canal sections were grouped 
into general classes. For each gaging, discharges using only every 
other vertical measured were computed and also using only every fourth 
vertical. Two computations of each gaging using the two sets of alternate 
verticals were made, and also two sets for every fourth vertical. These 
results were then compared with the discharge obtained by the use of 
all the verticals measured, in order to determine the probable errors to be 
expected when fewer verticals were used. The average number of 
verticals observed in the experiments was 16; the number in the com¬ 
parisons averages 8 and 4. In general field current-meter work, if only 
8 or 4 verticals had been measured, the ones used might have been located 
in the cross section differently from the arbitrary method used in this 
computation, so that the selection of alternate verticals as used should 
give errors larger rather than smaller than are to be expected. The 
results of this comparison are given in Table V. 
Table V ,—Effect on the accuracy of current-meter gagings of varying numbers of verticals 
Type of canal. 
Num¬ 
ber of 
detail 
gag¬ 
ings 
made. 
Aver¬ 
age 
num¬ 
ber of 
verti¬ 
cals in 
detail 
gag¬ 
ings. 
Comparisons using one-half 
of observed verticals. Va¬ 
riation (per cent). 
Comparisons using one-fourth 
of observed verticals. Varia¬ 
tion (per cent). 
Aver¬ 
age. 
Minimum and 
maximum. 
Average. 
Minimum and 
maximum. 
Flumes, vertical 
sides. 
23 
15 
0.9 
+0. 05 to-3.82 
2.9 
0 to —7. 50 
Concrete-lined ca¬ 
nals; steeply slop¬ 
ing sides. 
II 
14 
•9 
- .04to -2.95 
2.9 
-1. 08 to -5. 85 
Concrete-lined ca¬ 
nals; wide and 
flatly sloping 
sides.. 
6 
17 
1.4 
- . 37 tO -3. 22 
3-8 
— . 70 to —6. 52 
Average earth ca¬ 
nals, sloping sides. 
18 
16 
2.9 
+ . i to -8. 3 
9. 2 
— I. 5 to —17. 6 
Average earth ca¬ 
nals, steep sides.. 
21 
16 
2 - 5 
+ . i to -7. 3 
9. 0 
— . 4 to — 21. 1 
Earth canals, rela¬ 
tively deep sec¬ 
tions . 
10 
16 
2.7 
- . 5 to -5. 5 
7 * 7 
— . 6 to —19. 4 
Mean of all... 
89 
16 
I. 0 
6. 2 
y 
Table V gives both the average difference in percentage and the range 
of variations in single gagings. Occasionally the use of a less number of 
verticals may give a greater discharge than that obtained from a more 
detailed gaging, owing to irregularities in the cross section or velocity. 
Where an average of 4 verticals were used, less than, 2 per cent of the 
observations gave larger discharges than the use of all verticals, so that 
the average difference is practically equal to the mean error. Where 8 
verticals were used for all observations, one in each seven measurements 
