Measurement -of Water 243 



be made to include a fraction of a day, say 8% days instead of 

 8, as in the one cited; or, after a certain number of rotations, 

 the water may be given first to a different member in the 

 circuit, and thus change the time of day at which each gets 

 his turn. 



In those cases where the supply of water in the ditch is 

 always the same, this is the most accurate and best method of 

 dividing water which has been devised, and where the amount 

 of water which the ditch carries is known, it gives every one a 

 definite knowledge of the amount of water he is using. 



It often happens, however, that the volume of water changes 

 from time to time, and when this is true those who chance to 

 be using water when the supply is high will receive most. 

 But if the period of rotation is short, the injustice will seldom 

 be very great, and where the periods of rotation are short, the 

 service is usually more convenient and better for other reasons 

 than that of a more equitable division of the water, because it 

 permits a user to apply his .water to certain fields one date and 

 to another on his next turn, thus permitting him to do his fit- 

 ting and cultivation between irrigations to a greater advantage. 



The Subdivision of Laterals 



Where the lateral carries too much water to be used to 

 advantage by single indi- 

 viduals, this may be sub- 

 divided readily into two 

 exactly equal portions, and 

 these two divisions may 

 be again subdivided into 

 two precisely equal streams. 



But in order that the di- 



Fig. 43. Branching of canal to divide water 



Vision may be exact, it equally (A and B) and unequally ( (7). 



must be done in certain 



ways, as represented in Fig. 43. If the two branches of the lateral 

 form equal angles with the main, have the same fall, and their 

 bottoms at the same level where they start, they will carry equal 



