FARM IRRIGATION 263 



volume of water is to be halved this is not 

 necessary. 



Modules. When a certain amount of water is 

 to be taken out of a ditch or flume, rather than a 

 certain proportion, "modules" are used. There 

 are many forms of these, from the simple inch- 

 square hole cut in a plank to the complex weirs and 

 patented measuring boxes. No measuring device 

 now known is entirely satisfactory, because of the 

 rapid fluctuation in the height and velocity of water 

 in the ditch. King concludes "the most exact and 

 generally satisfactory way of apportioning water 

 among users that has yet been devised is that of 

 bisecting the stream until its volume has become 

 suitable for individual use, and then subdividing 

 by time under some system of rotation." 



Units in Measuring Water. The amount of 

 water used in irrigation is commonly stated in one 

 of two ways ; either the depth of standing water on 

 the surface, or the amount of water flowing through 

 an opening of a certain size during the irrigating 

 season. An "acre inch" is enough water to cover 

 one acre of land one inch deep, which is 27,150 

 gallons. It is gradually becoming the standard of 

 measurement in this country. A "miner's" inch 

 is the quantity of water that will flow through an 

 opening one inch square with a certain head, usual- 

 ly six inches, from the upper side of the opening. 

 This is about twelve gallons per minute. But 

 the amount of head varies by law in different states. 

 In California fifty miners' inches are equal to one 

 second foot, but in Colorado 38.4 miners' inches 

 eaual a second foot. The " second foot" is the unit 

 when one cubic foot of water is discharged each 

 second. It will cover an acre about two feet deep 

 in 24 hours, or 23.8 acre inches, and is sufficient to 



