IRRIGATION 



253 



the water to a lower level, the main ditch can go through 

 land in which a canal can be cheaply constructed. 



Often it is necessary to drop the level of water in a 

 ditch to avoid long ditches around hills or to avoid too 

 much fall and thus prevent erosion by too rapid flow of 

 water. The general plan of structure with boards, 

 shown in Figure 147, can be followed, or a permanent 

 waterfall may be constructed of stones or cement. 



Water gates. In all complicated systems of irrigating 

 canals, gates and weirs must be employed to be used in 

 restraining the water and in directing it into the desired 

 channels and fields, also to measure out the proper 

 amount of 

 water to each 

 party entitled 

 to water. In 

 Figure 148 is 

 shown a sys- 

 tem of three 

 gates. The 

 gate in the 

 main ditch is 

 raised so as to 

 hold the water 

 above it at an 

 even height. 

 The side gates 

 are raised to 

 .discharge a 

 given amount 

 of water, as 

 determined by a measuring weir below each gate. 



Figure 149. Simple head gate. 



In 



Figure 149 is shown a simple head gate used by a farmer 

 in regulating the water which he desires to flow upon a 

 given field. In Figure 150 is shown a side or head gate 

 used by a company to regulate the water which its ditch 



