Use and W-ASTt: oi* \\'ati:r 211 



LOSSES FROM CANALS AND FIELD LATERALS 



Earth canals arc more or less porous; new canals in sandy soils 

 are sometimes little better than sieves. Fortunately, most of the 

 irrigating supplies taken from streams in Arizona carry consider- 

 able line silt ; and this silt, settling in the canals, forms a blanket 

 and reduces to some extent the excessive losses which occur in new 

 canals. Since the construction of the Roosevelt Reservoir the pro- 

 portion of clear water carried in the canals of Maricopa County 

 has increased greatly and the loss by seepage from the canals has 

 increased correspondingly. 



Measurements of seepage losses on scores of ditches and canals 

 in the Western states have been compiled and published.* The 

 results are startling. Losses of over 10 percent per mile are not 

 infrequent, and it is concluded that "a large percentage of the water, 

 estimated at 40 percent of the amount taken in at the heads of the 

 main canals, is lost by absorption and percolation along the routes." 

 The records of the U. S. Reclamation Service in the Salt River 

 Valleyf state that during the past six years the canal losses between 

 the Granite Reef and Joint Head diversion dams and the points 

 where water is delivered to the water users have been from 40 to 

 45 percent of the total amount diverted. While the losses as given 

 in the records may be overstated somewhat, it is certain that at least 

 one-third of the water diverted is lost in the canals. The loss from 

 the Avondale Canal is 40 percent in the first four miles. 



Practically all of the loss in canals is by seepage, for the loss 

 by evaporation is small. The evaporation from a free water sur- 

 face during the summer months at Tucson averages 10 inches per 

 month in depth. On that basis a canal with a water surface 10 

 feet wide and carrying 25 second-feet of water would lose just one 

 acre-foot per month per mile by evaporation w^hile the total dis- 

 charge in the same time would be 1500 acre-feet. 



An excessive seepage loss can be reduced somewhat by puddling 

 the canal with clay. This method, however, is not recommended 

 for general practice, for it is a temporary half-way measure, and 

 the puddling must be repeatedly injured by ditch cleaning. Oil 

 lining has been tried to some extent in California and is said to 

 reduce the seepage about 50 percent, but it does not prevent the 

 growth of weeds and ditch cleaning becomes more difficult. The 

 best method is to line the canal with concrete, or, in the case of 



*U. S. Dept. Agri.. Bull. No. 126. 1914. This bulletin, designed for irrigation 

 engineers and superintendents, contains descriptions of many concrete linings. 

 tReclamation Record, 9, 11, Nov., 1918, p. 532. 



