33 



canals in general lose less in proportion than small ones. The measure- 

 ments show that the loss per mile averages about 1 per cent, for canals 

 carrying 100 cubic feet per second or more, about 2y 2 per cent, for canals 

 carrying 50 to 100 cubic feet per second, 4% per cent, for canals carrying 

 25 to 50 cubic feet per second, and i'l % per cent, for canals carrying less 

 than 25 cubic feet per second. 



For some purposes it is preferable to know the extent of seepage ex- 

 pressed in cubic feet of water per day per square foot of wetted area of the 

 canal. This is equivalent to stating the depth of water in feet lost each day. 

 A number of measurements have been made in various parts of the country 

 and some of these have been assembled by P. W. Hanna, Project Engineer 

 of Boise U. S. Reclamation Service Project, in Idaho, who states that from 

 careful consideration of the data assembled, it would appear that a seepage 

 loss of 0.5, 1 and 1.5 cubic feet per square foot wetted surface per day might 

 be assumed for canal losses respectively for rather impervious, mediumly 

 pervious and rather pervious soils. Based on the above figures and as- 

 suming a common form of cross section, he obtains the following results as 

 the seepage loss per mile expressed as per cent, of flow. 



Capacity of Loss in per cent of flow per mile 



canal. 



'Cubic feet For rather imper- Mediumly pervious Rather pervious 



per sec. vious soil. soil. soil. 



1C or less 4 8 12 



11 to 25 2.5 4.5 7 



26 to 50 1.5 3. 4.5 



51 to 75 1. 2. 3. 



76 to 100 .75 1.5 2.5 . 



The above table gives results which agree with those obtained by the 

 Irrigation Investigations Office as closely as can be expected because of the 

 numerous factors involved. 

 2. Evaporation Loss From Water Surface of Canals. 



The losses above stated include seepage and evaporation, but contrary to 

 the general belief, the losses of evaporation from flowing water in a canal 

 are insignificant when compared with those of seepage. It has been shown 

 that the losses of seepage and evaporation per day might be assumed at 

 0.5, 1 and 1.5 cubic feet of water per square foot of wetted surface, respec- 

 tively for rather impervious, mediumly pervious and rather pervious soil. 

 These are equivalent to losses of water 6, 12 and 18 inches deep. As com- 

 pared to these figures, the evaporation from water surface for the irrigation 

 season will generally be about ^4 of an inch per day, which is from 25 to 

 75 times less than the above seepage losses. Seepage and evaporation 

 measurements made at Twin Falls, Idaho, and reported by Elias Nelson 

 (Bulletin 58, University of Idaho) show that the evaporation ranged from 

 less than 1 per cent, to less than 2 per cent, of the total loss in the canals. 

 On one of the largest systems in the San Joaquin Valley, California, the 

 total length of canals is 165 miles and the total seepage loss was 28 per 

 cent, and 30 times greater than the evaporation loss. These and other 

 numerous experiments show that the evaporation losses in the conveyance 

 of water are so small as compared with the seepage losses that they are of 

 no importance. 



PREVENTION OF SEEPAGE LOSSES IN CANALS. 



To prevent the loss of water in conveyance lining the canals with differ- 

 ent materials has been tried. Those used or experimented with are con- 



