2 BULLETIN 126, U. S. DEPARTMENT OF AGRICULTURE. 



UNLINED CANALS. 



The census of 1910 showed for that year 81,837 main and lateral 

 ditches, aggregating 125,591 miles in length and having a maximum 

 capacity of 618,097 second-feet. Assuming that not over 4 per cent 

 of the total volume of water used in irrigation was carried in pipes, 

 flumes, and lined canals and deducting all channels having imper- 

 vious linings, there remained over 120,000 miles of unlined irrigation 

 canals in the West. 



Transmission losses in the channels considered herein may be 

 grouped under the headings leakage, evaporation, and seepage. 

 Cheap and faulty structures are a common cause of leakage, but such 

 losses are only a small percentage of the total loss. 



A large percentage of the water used to moisten the top layer of 

 soils is evaporated. 1 Water flowing in an open conduit evaporates 

 from the surface an amount dependent upon the temperature of 

 both air and water, the velocity of the wind, and upon other factors. 

 This loss, as in the case of leakage from faulty construction, is so 

 small that it may be neglected without causing appreciable error. 



Evaporation data obtained from 37 different stations throughout 

 the various arid States and covering the months of June, July, August, 

 and September give average daily rates in inches for this period a-> , 

 follows: Maximum, 0.34 ; minimum, 0.18; and mean, 0.26. For prac- 

 tical purposes the loss of water from a canal through evaporation 

 is a negligible quantity. 



SEEPAGE LOSSES. 



The results of measurements of seepage show that this is the most 

 important source of loss from canals. 



As Table I indicates, in many cases full data are not available, 

 particularly as to the character of the materials through which the 

 canals are excavated. The canals measured vary widely in capacity 

 and with one exception are situated in various parts of the arid regions 

 of the United States. About 1 ,500 miles of separate canal sections are 

 represented in the data collected and considering the character of mate- 

 rials, erosion, age, etc., they cover a wide range of conditions. It must 

 of course be recognized that with measurements taken under such 

 diverse conditions and by the different methods used in collecting these 

 data allowance must be made for probable inaccuracies in the results. 

 It may, and probably does, happen that in cases where the amount 

 of loss or gain is small, the variation may really amount to a gain 

 where it is given as a loss and vice versa depending on the accuracy of 

 the methods used and the care taken in making the measurements 

 for a given canal. However, in collecting these data an effort was 



i U. S. Dept. Agr., Office Expt. Stas. Bui. 248. 



