CANAL CONSTRUCTION. 59 



ing mouths the average will be about one inch, making 

 for the year from three to five feet of loss by evaporation. 

 To the loss in this must bo added the loss by seepage or 

 filtration either into the earth or through the banks. 

 The amount of seepage through the banks will depend 

 not only upon the character of the soil of which they are 

 made but also upon the solidity with which they are 

 thrown up. So with seepage into the earth. If the soil 

 is of soft loam, sand or gravel the percentage of loss will 

 be greater than if the subsoil be of clay or hardpan. 

 Careful measurements made in a number of cases show 

 that with canals having a good grade and not more than 

 ten to fifteen miles in length, nearly fifty per cent of the 

 water diverted into them at the head is lost before the 

 point of distribution is reached. The matter of filtra- 

 tion or seepage will be dwelt upon more fully later 

 on in this work, as it bears upon irrigation systems 

 other than that of canals. 



Cementing Canals. Seepage loss maybe almost 

 obviated by cementing the bottoms and sides of canals,and 

 in very sandy or gravelly soils this measure becomes ab- 

 solutely imperative. At first most of this work was done 

 by lining the surface with stones, usually cobbles or 

 small bowlders with faces roughly smoothed, and then 

 plastering cement over them and filling all the in- 

 terstices. This has been done with very many large 

 canals in the southern part of California, arid as may be 

 imagined, it is a very expensive process, especially when 

 the canals are very long and remote from the sources of 

 supply of the stone needed. In California, however, where 

 some of the most expensive stone and cement lining has 

 been done in the past, it has been found that just as good 

 work can be done and effective results obtained without 

 the use of stone and with only a thin crust of cement. The 

 method followed is first to completely saturate the bot- 

 tom and sides of the canal, which settles the earth thor- 



