IRRIGATION 255 



co-operative association of farmers can determine the 

 amount of water supplied to each farmer; and the farmer 

 can determine the amount of water allowed to run upon 

 a given field. Weirs are so constructed in relation to 

 the opening in the water gate in the ditch above that the 

 water in the weir stands at a uniform height, that it may 

 flow out of the weir with a stream of uniform size and 

 velocity. The standard or unit used in the measurement 

 of flowing water is usually the cubic foot per second of 

 time. Thus a head gate so adjusted that it allows a 

 cubic foot of water to pass through the weir each second 



Figure 151. The measuring weir. 



is said to be adjusted to one cubic foot or one foot of 

 flow per second. The head gate can be adjusted to 20 

 cubic feet per second or to any other amount. And after 

 adjusting the head gate to a given flow the superinten- 

 dent of the canal can leave it one or more days with the 

 assurance that the flow will be practically uniform for 

 the full time required to give the farmer his allotted 

 share of water. 



The measuring weir is simply a notch of a certain 

 shape and size in a dam placed across a stream, so ar- 



