694 SOILS: PBOPERTIES AND MANAGEMENT 



problems presented relate to the prevention of seepage, 

 erosion, and evaporation. The loss ^ of water in transit 

 from its source to the field has been found to average 60 

 per cent, and to range from 0.25 per cent to as much as 

 64 per cent a mile with an average of about 6 per cent. 

 The seepage water from canals may result in further loss 

 by accumulating in low lands, where the evaporation, 

 coupled with the solution of the soluble salts in the soil, 

 causes injurious accumulation of alkali in the surface soil, 

 and in extreme cases a swampy condition which destroys 

 the value of the soil for agricultural purposes. In order 

 to prevent seepage many kinds of lining and treatment 

 of the walls of canals have been employed. Cement 

 lining in different forms, wooden flumes, clay puddling, 

 oiling, applications of tar, and silting have been used. 

 The need of a lining depends much on the nature of the 

 formation through which the ditch passes. Silt is an 

 excellent means of checking seepage. Where clear water 

 is carried, the ditch must usually be lined, and the prac- 

 tice of lining canals in order to reduce seepage is increas- 

 ing rapidly. Sand and gravel permit much seepage and 

 are easily eroded. Clay permits little seepage and is not 

 easily eroded. The velocity of flow of water in canals 

 should not exceed three feet a second. In large canals 

 this will not permit a grade of more than six inches in a 

 mile ; in very small ditches a grade of from forty to fifty 

 feet in a mile may be necessary to cause the same velocity 

 of flow. A lining that is not subject to erosion, together 



1 Teel, R. P. Irrigation and Drainage Investigations. 

 U. S. D^ A., Office of Exp. Sta., Ann. Rept. 1904, p, 36. Also, 

 Mead, E., and Eteheverry, B. A. Lining of Ditches and Reser- 

 voirs to Prevent Seepage Losses. Calif. Agr. Exp. Sta., Bui. 

 No. 188. 1907. 



