THE FLOW OF WATER IN IRRIGATION CHANNELS. 7 



of course, remains the same througliout the reach tested, but as a rule 

 the mean area. A, is not identical with the area at the place where the 

 discharge was measured, so V will not necessarily be the same as the 

 mean velocity at the section where the discharge was measured. 



The other field data to be taken in order to make the resulting 

 value of n fully comprehensible is a careful description of the mate- 

 rial forming the containing channel, including such growths as affect 

 the flow of the water and a description of the influence of all struc- 

 tures in the canal and all changes in alignment throughout the reach 

 tested. This general description should not only cover just the reach 

 tested, but should extend up and down stream for sufficient distances 

 to cover anything influencuig the flow within the reach. 



Temperatures of the air and water may be taken, but it is doubtful 

 if any deductions may be made as to the direct influence of the vari- 

 ous temperatures on the flow of water in the usual more or less 



irregular channel. 



SCOPE OF EXPERIMENTS. 



Tests were made on channels in Nebraska, Colorado, Utah, Idaho, 

 Oregon, Montana, California, Arizona, Texas, and Louisiana. These 

 channels ranged m size from small ditches carrying less than 1 second- 

 foot up to canals carrying over 2,600 second-feet. The containing 

 materials of these channels comprise wood, concrete, earth, rubble 

 masonry, cobblestones, and a few special combinations. The veloci- 

 ties encountered extend up to about 10 feet per second. From other 

 sources the writer has obtained the data for additional tests, where in 

 his opinion there was not sufficient evidence m our own experiments 

 frpm which to draw conclusions. This is particularly true of steel 

 flumes, as none of these visited by members of this force was carry- 

 ing water at the time. The data covering very high velocities, such 

 as are found in chutes, also came from outside sources. In several 

 cases it was possible to get data covering several tests on exactly the 

 same reach of channel, with varying discharges of water, with a view 

 to proving or disproving the theories of some writers who have con- 

 tended that the value of n diminishes as the discharge increases, in 

 proportion to some function of the latter, such as the velocity, the 

 hydraulic radius, or the square root thereof. 



EQUIPMENT AND METHODS EMPLOYED FOR COLLECTING FIELD DATA. 



In order to weigh correctly any new data advanced wliicli tend 

 either to corroborate or to ciiange existing elements in a standard for- 

 mula it is necessary to know in detail the instruments employed and 

 tlie mothorls of taking field meiisiiremenls. Consequently, the equip- 

 ment usr;d and tiie steps j)ijrsiied in {|<!vel()j)ing the values of n for 

 irrigation canals for various conditions arc discussed in some detail 

 in th(; following y)aper. Tiu^ ef|ni[)ment jmd tlie methods described 



