THE FLOW OF WATEE IIST IREIGATIOlSr CHANNELS. 13 



pomt does not always correctly represent the maximum, since in 

 in various verticals taken throughout the summer, in many kinds of 

 channels, the maximum was found at points varying all the way from 

 0.2 depth to 0.8 depth. These statements refer, of course, to veloci- 

 ties found by holding the meter at these particular fractions of the 

 depths. A close study of any one vertical might show that the maxi- 

 mum velocity occuiTed at some place between these points, but that 

 phase was immaterial to the studies covered in this publication. 

 The necessary corrections were applied to the velocities indicated 

 near the bottom and surface, on account of the fact that the cup 

 current meter does not run true to standard rating curve when held 

 nearer the surface than 0.3 bf a foot and when held near the bottom.^ 



The particular points at which to hold the meter were chosen with 

 a view to determining the variations from the discharge of water 

 found in this way that would be found if shorter and more generally 

 used commercial methods of meter gauging had been employed. The 

 various points at wliich the current meter was to be held were deter- 

 mined as follows : After the tape had been stretched across the canal 

 at the section where the measurement was to be made a bench mark 

 was set at the surface of the water near one bank. The depths at the 

 various verticals were determined by taking with the level the differ- 

 ence in the rod readings between the rod held on the bench mark at 

 the water surface and the rod held at the bottom of the channel. 

 This method is better than using a graduated sounding rod, for the 

 reason that all error due to water cUmbing the rod is eliminated. 

 This method may be used in high velocities through the main portion 

 of the channel for the reason that a nail can always be set near the 

 bank, where the velocity is much lower than near the center. Where 

 the conditions were such that the meter was to be used on a cable, 

 then the desired points at wliich to operate it in any vertical were 

 found by multiplying the depth of the vertical by 0.2, 0.4, 0.6, and 0.8 

 and holding the meter the resulting distances below the surface. 

 Where the meter was to be used on a rod, since the bottom of the rod 

 rested on the bottom of the channel for all measurements and the 

 meter Is set on the rod while the rod and meter are out of the water, 

 then the distancas below the surface of 0.2, 0.4, 0.6, and 0.8 depth 

 become the distances above the bottom of 0.8, 0.6, 0.4, and 0.2, 

 respectively, of the depth, and are so set on the meter rod. 



Huvnng taken the soundings and computed the depths in each 

 vertical at which the meter was to be held, the actual meter runs were 

 made. Th(rse were as a rule continued at each point for the nearest 

 iiumb<T of tens of revolutions in a period of about 30 seconds, the 



■ For a dtocoaBlon of this phase of current meter use, see U. B. Dept. Agr., Jour. Agr. Roseorcb, 2 

 (1914), No. 2, p. 77. 



