| 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.
P = 3·5 √d WL 1,000,000
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.
There is an obvious connection between the duty of water and the total depth of the water, known in India as “delta,” given to the fields. Calculations are much simplified, while still being accurate enough for all practical purposes, by assuming that the number of seconds, (86,400) in a day is twice the number of square feet, (43,560) in an acre. Assuming this to be the case a discharge of 1 c. ft. per second for a day gives 2 acre-feet, i.e., it will cover an acre of ground to a depth of 2 feet in a day; and in six months it will cover 100 acres to depth of 3·65 feet. In Northern India the year is divided into two halves in each of which a crop is grown and the duty is calculated for each crop. In this case, if the flow of a canal has been continuous, a duty of 100 acres per cubic foot of its mean discharge per second, corresponds to a total depth of 3·65 feet over the area irrigated. Generally the flow in the half-year has not been continuous. In other countries, and in India on canals other than the perennial canals, the periods of flow vary a great deal. The duty cannot be calculated from delta or vice versa until the period of flow is stated.
The daily gauge-readings and daily discharges corresponding to them, having been booked, the discharges are added up. The total, divided by the number of days on which the canal has been running, gives the average daily discharge. Suppose that during the “kharif” or summer crop which is considered to last from 1st April to 30th September or 183 days, the canal was closed for 13 days and that the total of the daily discharges on the remaining 170 days comes to 850,000 c. ft. per second. The average daily discharge is 5,000 c. ft. per second. Suppose the kharif area irrigated to be 500,000 acres, the kharif duty is 100 acres. To find delta the total of the daily discharges has to be multiplied by the number of seconds in a day and divided by the number of square feet in an acre (these figures are, as already stated, very nearly in the ratio of two to one) and divided again by the number of acres irrigated. Thus, in the above case, delta is very nearly 850,000 × 2500,000 or 3·4 feet. For comparing the results of one canal or one year with another, delta is the more convenient figure to take. As soon as the areas irrigated by the canals are known for any crop, the Chief Engineer of the province issues a statement of the value of delta for each perennial canal and compares them with those for previous years. The value of delta for the Punjab canals ranges from 3 to 4 feet for the kharif and from 1·8 to 2·1 feet for the “rabi” or winter crop. Individual canals vary greatly, the worst having nearly twice as high a figure as the best. The differences are due to the causes already mentioned.