CHAPTER V.
Proposed Improvements in Irrigation Canals.
1. Preliminary Remarks.
—The chief improvements which have been under consideration during recent years are three in number. The first is increased economy of water in its actual use in the fields; the second is reduction of the losses by absorption in the channels; and the third is distribution by means of modules.
Regarding the first, it has long been known that the ordinary methods of laying on the water are more or less wasteful. In California, when the water instead of being applied to the surface of the ground, is brought in a pipe and delivered below the ground level, the duty is increased from 250 to 500 acres. In India a field is divided, by means of small ridges of earth, into large compartments. The water is let into a compartment and gradually covers it. By the time the further side is soaked the nearer side has received far too much water. Frequently the water for a compartment, instead of being carried up to it by a small watercourse, is passed through another compartment and this adds to the waste. Also the number of waterings given to a crop is often 5 or 6, when 4 would suffice. Experiments made on the Upper Bari Doab Canal, by Kennedy, showed that the water used in the fields was nearly double what it might have been. The 53 c. ft. shown in [Chapter 1, Art. 4], as reaching the fields, were used up when 28 c. ft. would have sufficed. It is not certain that the waste is generally quite as much as the above. It is possible that the restricted supplies might have given smaller yields of crops. More recent experiments made by Kanthack on the same canal give the needless waste as about 25 per cent. The field compartments ought, according to Kennedy, to be 70ft. square, the small branch watercourses being 140ft. apart. It would be better to have still smaller compartments, but this would be rather hard on the people.
At one time Government issued orders, in Northern India, that compartments of 1296 square feet were to be used, and that, otherwise, increased water rates would be charged, but the orders were never enforced. They were thought to press too hardly on the people. Extreme measures for enforcing economy in the use of water in any country are likely to be introduced only when they become absolutely necessary owing to the supplies of water being otherwise insufficient.
2. Reduction of Losses in the Channels.
—For several years experiments have been going on in the Punjab as to the effect of lining watercourses with various materials. The following conclusions have been arrived at[51]:—
[51] Punjab Irrigation Paper No. 11 C. “Lining of Watercourses to reduce absorption losses. Experiments of 1908-1911.”
I. Ordinary Unlined Trenches.
(a) The rate of absorption varies greatly, and this is due probably to unequal fissuring of the upper layers of the soil.