3. Gauge Readings and Discharges.
For the head gauge of each distributary and for certain gauges in the canals, discharge tables, based on actual observations, are prepared. If changes occur in the upper part of a channel, the discharge corresponding to a given gauge reading is altered. One remedy for this is to have a second gauge downstream of the “silt wedge” or scoured or narrowed reach. The indents are then made out with reference to the second gauge, but any slight adjustments due to fluctuation in the water level of the canal, are effected by means of the head gauge. Unless the zilladar and Subdivisional Officer are on the alert, the gauge reader is likely to evade going to the lower gauge every morning, and to enter fictitious readings for it, inferring them from the readings of the head gauge. If there are any outlets between the two gauges, their discharge has to be observed or estimated and added to the discharge of the distributary as entered in the table corresponding to the readings on the second gauge. The above system can be worked with advantage in cases where the distributary bifurcates two or three miles from its off-take. The men in charge of the two regulators can work together, one of them or an assistant, going daily from one regulator to the other and back.
Usually, however, the vitiating of the discharge table at the head gauge has to be faced, and the table to be constantly corrected. It is impossible to frame beforehand any rule or formula which would give a certain correction for a certain depth of silt deposit. Moreover, there might or might not be a contraction of the channel due to deposit on the sides. The usual plan is to observe a discharge some time during each month. If the result is in excess of the tabular discharge, all the discharges for that month are increased in the same proportion. They can be booked according to the table and totalled, and the correction applied to the total.
Discharges of canals and branches at their heads or at the boundaries of divisions, are observed by the Subdivisional Officer about once a month. Discharges of distributaries are observed about once a month, usually by zilladars. They are also to some extent observed by the Subdivisional Officer, but much is left to his discretion. Delta is worked out for each distributary month by month, and also, of course, for each crop. Thus a general duty “at distributary heads” can be obtained, and may be used in new projects[26] instead of the duty at the canal head, allowance being made for the water lost by absorption in the canal and branches.
[26] See [Chap. IV., Art. 2].
It cannot be said that these important figures are obtained as carefully as they could be. If the Subdivisional Officer personally observed the discharge at each distributary head, even every other month, the reliability of the results would be much increased. In addition to this the discharges of canals and branches at the boundaries of subdivisions should be observed and the results compared with the distributary discharges, so as to show the loss by absorption. At first grave discrepancies among the results would be found, but they would be reduced as the causes of error became known. For the method of investigating the causes of discrepant discharges see River and Canal Engineering, Chap. III., Art. 5.
A specimen of a Subdivisional Officer’s gauge register is given in [table I.] The zilladar keeps a similar register. The columns headed G contain the gauge readings, those headed D the discharges. Until some years ago there were no columns for discharges. The daily discharges of the canal and of the branches at their heads—and at intermediate points if they were at the boundaries of divisions—were entered in the Executive Engineer’s office and the duty was worked out at the end of each crop. The zilladar merely indented for a certain gauge reading at the distributary head, and the Subdivisional Officer could tell pretty nearly what gauge reading he required in the canal at the beginning of his subdivision. Since the year 1900 or thereabouts, the zilladars have been required to learn a good deal about discharges. They have to know how to observe the discharge of a distributary, and to learn how the discharge of an outlet varies with the head or difference between the upstream and downstream water levels. They are supposed to indent for certain discharges, and not merely for certain gauge readings. All this knowledge is useful to the zilladars and tends to increase their efficiency, but a practice of constantly thinking in discharges instead of in gauge readings is unnecessary. If the channels were of all sorts of sizes matters would be different. Actually the size of a channel is apportioned to its work, and the proportion of its full supply which it is carrying at any moment is easily grasped by means of gauge readings alone.
TABLE I—GAUGE AND DISCHARGE REGISTER.
| October, 1912. | D a t e . | Main Line, Upper Bari Doab Canal. | ||||||||||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| Tibri Regulator | Dhariwal | Kunjar | Aliwal Regulator | |||||||||||||||
| A- bove | Main Canal | Kasur Branch | Nangal Distributary | Kaler Distributary | A- bove | Amritsar Branch | Lahore Branch | Escape | ||||||||||
| G. | G. | D. | G. | D. | G. | D. | G. | D. | G. | G. | D. | G. | D. | G. | D. | |||
| 1 | 4 | ·0 | 100 | |||||||||||||||
| 2 | 4 | ·0 | 100 | |||||||||||||||
| 3 | 4 | ·0 | 100 | |||||||||||||||
| * | * | * | * | * | * | * | * | * | * | * | * | * | * | * | * | * | ||
| 29 | 4 | ·2 | 110 | |||||||||||||||
| 30 | 4 | ·2 | 110 | |||||||||||||||
| 31 | 4 | ·2 | 110 | |||||||||||||||
| Total | 127 | ·1 | 3255 | |||||||||||||||
| No. of days in flow | 31 | 31 | ||||||||||||||||
| Average | 4 | ·1 | 105 | |||||||||||||||
As regards the weekly indents, the dealing with discharges instead of gauge readings is of little practical value. The zilladar merely knows that on some outlets the demand is great, on others moderate, and he judges that the distributary needs say, 4 feet of water, its full supply gauge being 5 feet. He cannot tell how many cubic feet each outlet requires. If he is required to indent in cubic feet per second (he is not always required to do this) he probably gets at the discharge from the gauge reading, and not the gauge reading from the discharge. As regards the general indent made by the Subdivisional Officer, the same remarks apply. He can probably tell what gauge he requires without going into discharges.