Nov. 8, 1915 Use of Current Meters in Irrigation Canals 221 



depths, which is also true of the other methods. This is to be expected, 

 as the smaller velocities and depths usually occurred in canals of small 

 discharge, where the general conditions for the use of the current meter 

 are not so favorable. The accuracy does not appear to be affected by 

 the character of the channel or value of n. 



There is some indication that the correction to be used with the single- 

 point method should be greater than 5 per cent for low velocities and 

 less for the higher ones. This tendency is not marked, however, and it 

 is doubtful if it is sufficient in amount or that it is sufficiently proved 

 by these results to warrant the use of different corrections; also the 

 correction seems to vary with the depth in a similar way. 



The integration method seems to give the closest average results for 

 velocities from 2 to 3 feet. It also appears to be more accurate for the 

 greater depths. This latter result is to be expected. In the use of the 

 integration method the velocity in from 0.2 to 0.3 foot in depth must 

 be either missed entirely or imperfectly determined both at the bottom 

 and at the water surface. The velocity at the bottom is lower than 

 the average. Therefore the measurements in the remaining portions of 

 the depth would give results above the actual average velocity. As the 

 proportion of the depth for which velocities are undetermined is larger 

 in the shallow canals, the proportionate error would be greater. 



Another method sometimes used is that known as the three-point 

 method, in which the velocity is measured at 0.2, 0.6, and 0.8 of the 

 depth. This is more usually computed by giving the velocity at 0.6 

 depth equal weight with the mean of the 0.2 and 0.8 depth velocities. 

 As Table I shows the single-point method to be less accurate than the 

 two-point, there is no apparent advantage in the three-point method 

 over the two-point. In sections where the two-point method gave 

 results too low and the single-point too high, their combination might 

 increase the accuracy over that secured by the two-point method alone. 

 Where both were of the same sign, the use of the three-point method 

 would give less accurate results than the two-point alone. The two- 

 point and single-point methods gave results having opposing signs on 

 less than one-third of the total number of experiments, so that the three- 

 point would seem to have Uttle advantage over the two-point method. 



To definitely determine the relative accuracy of the three-point 

 method, the discharge of each experiment was computed, using both 

 the method by which the velocity at 0.6 depth is averaged with the mean 

 of the velocities at the 0.2 and 0.8 depths, and also the method by which 

 the velocities at the three points are. given equal weight. This latter 

 method would seem to be the more logical, as it has been shown that the 

 two-point, or 0.2 and 0.8 depth method, gives results more accurate than 

 the 0.6 point alone, so that in the use of the three points it would be 

 preferable to reduce the weight given to the velocity at 0.6 depth. 



