Journal of Agricultural Research 



the discharge by other methods than by the use of the current meter would 

 not have been practicable. Greater detail regarding the methods used 

 and the experiments in general will be found in a recent publication/ 

 which discusses the results of the determinations of the value of n in 

 Kutter's formula. The field work was carried on by various members 

 of the Division of Irrigation Investigations, as stated in the bulletin 

 referred to. 



COMPARISONS OF DIFFERENT METHODS OF MEASUREMENT OF 

 VELOCITIES IN THE VERTICALS 



There are four principal methods by which the velocities in different 

 verticals are determined with the current meter: The multiple-point 

 method; the mean of the velocities at the 0.2- and o.8-depth points, 

 called the "two-point method"; the velocity at 0.6 depth, called the 

 "single-point method"; and the vertical-integration method. 



As the main purpose of these experiments was the determination of 

 the value of n, it was desired to make the discharge determinations with 

 as great accuracy as possible. The multiple-point method was used, 

 readings being taken usually at six points in each vertical. This was 

 assumed to give the correct discharge and is the discharge used as the 

 basis of the following comparisons. 



The multiple-point readings were usually taken at o.i, 0.2, 0.4, 0.6, 

 0.8, and 0.9 of the depth. The meter was held from 30 to 60 seconds 

 at each point. From these measurements the discharge by the two- 

 point or the single-point method was computed and compared with the 

 results of the multiple-point method. 



When the field measurements were made, in most of the experiments 

 gagings were also made by the vertical-integration method. Generally 

 one or two complete round trips were made with the meter at each ver- 

 tical, the vertical movement being from 3 to 16 feet per minute. Much 

 care was used to give the meter a uniform vertical velocity so that each 

 portion of the section would be equally represented in the integrated 

 mean. Two complete round trips were usually made, consuming from 

 40 to 150 seconds, depending on the depth. The meter was generally 

 moved more slowly in the shallower sections in order to give a sufficiently 

 long time for the reading. 



In Table I are given the general results for all experiments. These 

 are divided in five different classes of canal sections, although there is 

 no marked variation for the different groups. These include nearly 100 

 experiments for the two-point and the single-point methods on canals 

 having discharges of from 2 to 2,600 second-feet. Only 55 experiments 



' Scx)bey, F. C. The flow of water in irrigation channels. U. S. Dept. Agr. Bui. 194. 68 p., 9 fig., 20 pi. 

 1913. See also Scobey, F. C. Behavior of cup current meters under conditions not covered by standard 

 ratings. In Jour. Agr. Research, v. 2, no. 2, p. 77-83- 1914- 



