226 Journal of Agricultural Research voi. v, no.6 



Values of n 



O.OI2. Straight wood flume in good condition; clean concrete lining having very- 

 smooth finish; no moss or gravel. 

 0.014. Ordinary' straight wood flumes, little rock or sand; unplastered concrete lining; 



no moss or gravel. 

 0.016. Worn wood flumes containing gro^^^s or sand and gravel; average concrete 



linings, irregular finish, moss growths or gravels; best earth canals, uniform 



silted and clean sections. 

 0.018. Very poor wood flumes; rough concrete with covering of moss or gravel; very 



good earth canals; uniform section, silted, free from gravel and moss. 

 0.020. Concrete in poor condition, much moss and gravel; better than average earth 



sections without gro\\lhs and fairly regular sections. 

 0.022. Earth sections, generally free from moss or gravel. 



0.024. Average earth canals, fairly clean and regular, some gravel and vegetation. 

 0.026. Earth canal; gravel and some cobbles, some moss, irregularities in cross section; 



masonry-lined canals. 

 0.028. Canals with some cobbles; moss and other unfavorable conditions. 

 0.030. Earth canals, much moss or weeds, irregular section, gravel or cobbles; fairly 

 smooth rock cuts. 



It is preferable to make float measurements on straight portions of 

 canals. If it is necessary to use a length containing curves, a coefficient 

 should be selected for a value of n about 0.002 higher than would other- 

 wise be used. 



These experiments give data both on the most probable coefficients 

 to be used in float measurements and also on the limitations of accuracy 

 to be expected. Such measurements are often desirable for quick 

 approximate determinations. The most rapid of several floats should 

 be used and the proper coefficient selected to fit the conditions. The 

 error from the float determinations should not often exceed 10 per cent, 

 although error in estimating the cross-sectional area may result in much 

 larger errors in the resulting discharge for earth canals. In flumes or 

 section of regular forms the error in determining the water area should 

 not be large. 



EFFECT ON ACCURACY OF CURRENT-METER GAGINGS FROM THE USE 

 OF DIFFERENT NUMBERS OF OBSERVATIONS ACROSS THE V/IDTH 

 OF CANALS 



The number of verticals across a gaging station at which velocity 

 measurements should be made is a question on which there has been 

 much difference of opinion. 



In the sections of irrigation canals at which current-meter gagings are 

 generallv made, the cross section is more regular than in the usual stream 

 gaging station, so that usually fewer measurements should be required. 

 In the exiieriments discussed, measurements were made in from 13 to 20 

 verticals with a minimum distance apart of the verticals of 0.5 foot on 

 the smaller canals. These measurements are more than are usual in 

 general field practice. The results obtained were compared with the 



