CHAPTER VI. 



CANAL CONSTRUCTION. 



Water is king, and the most important adjunct to 

 the greater requirements of irrigation is a good canal 

 system. The gravity supply of water is by all odds the 

 best that can be employed, and the farmer who has a 

 good ditch in perfect working order may consider that 

 he has a fortune lying at his threshold. In laying out a 

 system of ditches for a farm, use care and time. Think 

 it over well, and it may be economy to employ a hydrau- 

 lic engineer to run levels and determine grades. No 

 large canal system should be undertaken without con- 

 sulting an expert engineer. Each farm to a certain ex- 

 tent requires a ditch system adapted to its peculiar to- 

 pography, soil and crops. See to it that the water can 

 get oil the land as well as on it. Remember at all times 

 that drainage is quite as necessary to successful irrigat ion 

 as the water supply itself. The matter of grade for a 

 ditch is one which depends so much upon circumstances 

 as almost to preclude rules. It is safe, however, to make 

 the grades as light as possible to avoid "silting up" or 

 settling. Cutting may be called perpetual motion, for 

 if once begun it seems never to stop. The ditch gradu- 

 ally irets lower and lower until the water cannot be got 

 out of it at all and it must either be abandoned or 

 have falls built in it to keep the flow near the surface. 

 As far as possible keep the grade uniform, as changing 

 the grade tends to cause both cutting and silting. A 

 ditch for irrigation on a farm should alwa\- 1>- much 

 larirer than the actual demands require. In Spain their 



42 



