372 IRRIGATION PRACTICE 



year. There is always a possibility, also, of seepage from 

 reservoirs for the storage of water. If the reservoirs are 

 well silted large losses will not occur. A reservoir from 

 which a depth of water of not more than 2 feet is lost 

 annually is said to be practically impervious. The smaller 

 the reservoir, the better silted it is, and, therefore, the 

 smaller the loss of water from it. From canals, also, there 

 are large losses of water by seepage. For instance, the 

 United States Irrigation Investigations have found that, 

 as an average for all canals investigated, the loss by seepage 

 was 5.77 per cent of the total water carried, for every 

 mile of canal. The aggregate loss of water by seepage 

 from canals must, therefore, be tremendous. Individual 

 canals differ, however, greatly in this respect. In the 

 government investigations, the losses from canals varied 

 from none to 60 per cent of the total quantity of water 

 carried, for each mile of canal. If a canal passes over a 

 rocky ledge, much water may be lost through cracks and 

 crevices. Canals passing over gravelly soils lose most; over 

 sandy soils, nearly as much, and over heavy clay soils, 

 least water. New canals always lose more water by 

 seepage than do old ones, because silting and settling 

 has not been fully established. Small canals lose propor- 

 tionately more than large ones, and canals not used in the 

 winter lose less than those filled with water throughout 

 the year, although at the time of opening the canals in 

 the spring, before silting has gone on, the loss is greater. 

 On the average, 30 per cent of the water taken in at the 

 head-gates of irrigation canals is lost by seepage from 

 the canals themselves. In comparison with this great loss 

 all other losses are small. The loss of water by evaporation, 

 for instance, is ordinarily less than 15 per cent of the loss 

 due to seepage. Carpenter concluded that the water lost 



