80 IRRIGATION FARMING. 



sand, or gravel the percentage of loss will be greater than 

 if the subsoil be of clay or hard-pan. Careful measure- 

 ments 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 filtration 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 may be almost 

 obviated by cementing the bottoms and sides of canals, 

 and in very sandy or gravelly soils this measure becomes 

 absolutely 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 

 interstices. This has been done with very many large 

 canals in the southern part of California, and, 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 effedtive 

 results obtained without the use of stone and with only 

 a thin crust of cement. The method followed is first 

 to completely saturate the bottom and sides of the canal, 

 which settles the earth thoroughly in place, and then 

 the coating of Portland or hydraulic cement is put on 

 with a thickness of three-quarters of an inch. It has 

 been found that this layer is durable and abundantly 



