[4] When the channel was in continuous flow.
[5] Maximum value when flow was intermittent.
Some information as to losses of water is also given in CHAPTER IV. Art. 2.
The relative losses of water in the channels of the Upper Bari Doab Canal were as follows:--
Relative Loss. In main line and branches 20 In distributaries 6 In watercourses 21 Used in the fields 53 --- Total 100
The reasons for the great variation in the value of _c_ are not properly known. The depth of water is not likely to have much influence on it. It is well known that the fine silt carried by the water tends to render the channels watertight when it deposits. The canals and branches receive either no deposits or deposits consisting chiefly of sand. The distributaries, especially in their lower reaches, receive deposits of fine silt which is only occasionally cleared away. The watercourses receive similar deposits but they are very frequently cleared out by the cultivators. This is perhaps the reason why the rate of loss of water in the watercourses is nearly three times as great as the rate of loss in the distributaries of the same canal. On the Sirhind canal the distributaries have more branches than on the Bari Doab canal and the watercourses are smaller. This accounts for the different relative losses in the two cases. The sandy nature of the soil on the Sirhind canal accounts for the general higher value of _c_ on that canal.
The following formula has been deduced as giving the loss by absorption on a Punjab Canal.
Where P is the loss by absorption in c. ft. per second in a reach whose length is L, width (at water level) W and depth d. According to the formula the loss per million square feet is 10·5 c. ft. per second when d is 4 ft. and 7 c. ft. per second when d is 2 feet, These figures do not agree with those in the preceding table and it is clear that there are not yet sufficient data from which to construct a formula.
The first steps taken on the Bari Doab Canal, and subsequently on other canals, to reduce the losses of water, consisted in the reduction in the number of watercourses. This will be referred to again (CHAPTER II. Art. 9). Further steps will be considered in CHAPTER V.
5. =Duty of Water.=--The number of acres irrigated annually by a constant discharge of 1 c. ft. of water per second is called the “duty” of water. In India on perennial canals the duty may be as much as 250 or even 300 acres. On inundation canals which flow for only five months in the year and are situated in tracts of scanty rainfall and light or sandy soil, the duty may be only 70 acres. The duties of most existing canals whether in India or elsewhere, are known only approximately. The duty is calculated on the average discharge entering the canal at its head less the water which is passed out at escapes. It thus includes all losses of water. The duty varies not only as between one canal and another but on the same canal from year to year. It depends on the character of the soil, a sandy soil requiring more water than a clayey soil. It also depends on the rainfall. A moderate amount of rain causes the canal water to go further, but heavy rain may enable some crops to do without canal water or may permit of the concealment of canal irrigation. The duty also depends on the kind of crops grown, on the losses in the channels by absorption and on the quantity of water available. A liberal supply of water leads to carelessness in the use, but a very restricted supply is largely wasted owing to the shortness of the “turns” or rotational periods of flow in the different channels.