148 IRRIGATION FARMING. 



cut a very important figure. As we have already indi- 

 cated, where flooding takes thousands of gallons the 

 furrow system only requires hundreds, and subirriga- 

 tion tens of gallons for a similar area. 



Measurement of Water. — There are also many 

 different standards of measurement of water for irrigat- 

 ing, and so many different conditions under which it is 

 applied, that one is apt to become confused and will 

 decide that there is a good deal of technicality about it 

 that is perplexing and intricate. As before stated, the 

 units of measurement are the miner's or statutory inch, 

 cubic and acre feet, or by the gallon. With engineers 

 the cubic foot per second is the standard unit, and the 

 quantity is determined in large volumes by the rate of 

 flow in the sedlional area of the channel and in the 

 smaller volumes by the flow over a measuring weir. 

 The theoretical capacity of a channel as determined by 

 formulae is almost always in excess of the adlual 

 capacity as determined by experiment, by a varying 

 percentage dependent upon the following conditions : 

 First^ — Sinuosity or aggregate degree of curvature. 

 Second — Sharpness of bends or degree of curvature. 

 Third — The uniformity and symmetry of cross-secftion. 

 Fourth — The chara(5ler of the fridlional perimeter of the 

 sides and bottom. The simple theory of flowing water 

 in channels is not a difficult matter of understanding, 

 but it is the modification of this theory by the various 

 coefficients of fri<5tion that leads to misunderstanding. 



To properly estimate the flow of water in canals 

 and its distribution through headgates a number of de- 

 vices have been invented, and these include such things 

 as nilometers, current meters, hydrometric sluices, 



