IRRIGATION AND DRY-FARMING 



707 



terms because of their definiteness and relation to the 

 common method of stating rainfall. Usually an inch or a 

 foot of water refers to that depth over an acre. 



Various mechanisms are employed for measuring l 

 water in irrigation practice. The commonest of these 

 are the weir and the flume. (See Fig. 81.) The weir is 



FIG. 81. Plank measuring box and a Cippoletti weir used in deterniin- 

 ing the flow of irrigation water. 



a simple device to give the stream a definite cross section 

 and to aid in the measurement of the depth, and there- 

 fore the volume of flow. It is usually a knife-edged notch, 

 of a standard shape calibrated to a grade stake a short 

 distance up stream from which the depth of water and 

 its velocity are rated. The measuring box, frequently 

 termed a module, is a box for measuring the flow of water 

 from an orifice under fixed conditions. The Staldate 

 module, developed in Italy, is most generally adopted for 

 the purpose. Small streams are divided by a knife-edge 



1 Carpenter, L. C. The Measurement and Division of Water, 

 Colorado Agr. Exp. Sta., Bui. No. 150, 4th ed. 1911. 



