224 



Journal of Agricultural Research 



Vol. V, No. 6 



eddies. The floats were generally timed over the 500 to 1,000 feet of 

 canal used in the value-of-w experiment. 



The value of the coefficient for each experiment was compared with 

 the coefficient given in Kutter's table for the same value of R and n. 

 For all measurements the coefficients differed by an average of 0.06. 

 The mean of all observations was 0.013 lower than Kutter's. This is not 

 an unreasonable variation when it is remembered that at best the method 

 is only an approximate one. 



The selection of the coefficient based on the value of R and n is not, 

 however, a convenient one for field use. The determination of the canal 

 cross section, except for flumes and lined sections, will be approximate 

 and the determination of the value of R even more uncertain. A varia- 

 tion of the coefflcient with the water area would be the most convenient 

 for field use. A field measurement involves the determination of the 

 mean cross section of the canal and the velocity of the float. If 

 the selection of the proper coefficient is based on the cross section and 

 an estimated value of n no additional measurements or computations 

 are required in order to select the proper coefficients. The experiments 

 give evidence that the coefficient varies with the character of the wetted 

 surface, so that some knowledge of the value of n is required. 



In order to determine the value of the coefficients for different condi- 

 tions, the results of each measurement were plotted with the value of the 

 coefficient and the cross-section area as coordinates. A series of curves 

 for the different values of n were fitted to these plotted observations and 

 adjusted until they gave results equaling, on the average, the results of 

 the actual field determinations. From these curves the values of the 

 coefficients given in Table IV were secured. No attempt was made to 

 derive an equation for the variation in the value of the coefficient, 

 graphical methods being used throughout. 



Table IV. — Coefficients to be applied to velocities of floats to obtain mean velocity in 



canals 



Area of water 

 cross section. 



Value of n in Kutter's formula. 



Square fed 



2 



4 



6 



8 



10 



15 



20 



25 



30 



35 



40 



50 



Over 50 . ... 



0.80 

 81 



83 



0.76 



•77 

 .78 



•79 

 •79 

 .80 

 .81 

 .82 

 .82 

 .82 

 .82 

 .82 

 .82 



0-73 



•74 

 •74 

 •75 

 .76 



•77 

 .78 

 .78 

 •79 

 •79 

 •79 

 •79 

 •79 



D.63 

 .64 

 .64 

 .65 

 •65 

 .66 

 .67 

 .68 

 .68 

 .69 

 .69 

 .69 

 .69 



o. 61 

 .62 

 •63 

 .63 

 .64 

 .65 

 .66 

 .66 

 .67 

 .67 

 .67 

 .67 

 .67 



o. 60 

 .61 

 .62 

 .62 

 •63 

 .64 

 •65 

 •65 

 •65 

 .66 

 .66 

 .66 

 .66 



