TABLE VI.
| Day. | A | B | C |
|---|---|---|---|
| 1 | 2,500 | 200 | |
| 2 | 2,500 | 200 | |
| 3 | 2,500 | 200 | |
| 4 | 2,500 | 200 | |
| 5 | 2,500 | 200 | |
| 6 | 2,000 | 700 | |
| 7 | 2,000 | 700 | |
| 8 | 2,000 | 700 | |
| 9 | 2,000 | 700 | |
| 10 | 2,000 | 700 | |
| 11 | 1,200 | 1,500 | |
| 12 | 1,200 | 1,500 | |
| 13 | 1,200 | 1,500 | |
| 14 | 1,200 | 1,500 | |
| Total | 17,300 | 11,000 | 9,500 |
| Correct Discharge. | 15,700 | 12,600 | 9,500 |
Considering both the above tables, A always receives more water than its share, while B and C on the whole receive too little. Considering table V. by itself, matters might, perhaps, be set right by altering the total number of days from 14 to 13 or 12, but this, besides being somewhat objectionable for the reason already given, might not improve matters when [table VI.] came into operation. It is desirable to avoid frequent changes or complicated rules. It is objectionable to make any turn consist of other than a whole number of days. The shifting of the regulator gates is begun at sunrise, a time when officials are about and can see what is happening. All gauges are read early in the morning, and those at regulators are read after the regulation has been done and the flow has become steady. If any regulation were done in the evening, the entry in the gauge register of that day would convey a wrong impression, and the discharge would be incorrectly booked. Moreover, any system of regularly booking evening as well as morning gauges leads to swelling of the already voluminous gauge register.
The best method of adjusting matters is to make slight alterations in the full supply gauges. Suppose the normal full supplies in all three branches to be 6 feet. When [table VI.] is in operation the full supply of A can be reduced to about 5·8 feet. This would give, during the first 5 days, less water to A and more to B, and there is the further advantage that a very small supply, 200 c. ft. per second, is not run in any branch. As regards [table V.], branch A never receives full supply. This is a rare case.[31] If it were safe, as it might be, to run slightly more than full supply in C, this could be done, and it would increase the supply in C during the last four days and reduce that in A. Otherwise a certain gauge would have to be fixed for A which would give it less than 2,200 c. ft. per second during the first 5 days, and the balance would go to branch B. Similarly, the gauge of B could be slightly reduced, and this would increase the balance going to C. The orders given to the gauge reader are, as before, to send the full supply down one channel, and the balance to the next. The only additional procedure necessary is to inform the gauge reader from time to time what the full supply gauges are. In any case such information has probably to be conveyed to him at times because the channels undergo changes, and the discharge corresponding to a given gauge also changes.
[31] The total discharge, 2,200 c. ft. per second, assumed, is very low compared with the full supply of 6,000 c. ft. per second.
When the discharge of the canal exceeds 3,500 c. ft. per second there is, when B and C are receiving water, a second residuum, which goes to A. Tables can be worked out for several discharges of the main canal, but it is the minimum discharge which is the most important factor in the case. The minimum discharge, or something very near it, generally lasts through about half the crop, and it is when the supply is at a minimum that care and justice in the distribution are most needed.
The chief objection to the arrangements above described is that the surplus to be sent down one channel or another is sometimes so small that it must be to a great extent wasted. The best means of preventing this is to have the discharge tables, including one for the main canal at some point higher up than the trifurcation, constantly corrected. In that case, it is known under what circumstances a small surplus will occur, and the orders can be modified so as to prevent its occurrence. The orders will of course be more complicated, and will have to be dealt with by an engineer and not a gauge reader.
The turns, once satisfactorily arranged, may go on for years without alteration. They may require altering if any branch is found, in the course of time, to be doing worse than or better than the others, though the correction can probably be made by altering the full supply gauge.
The turns of the branches having been arranged, it remains to settle those of the distributaries. The total available discharge being, as before, assumed to be rather more than one-third of the full supply discharge, each distributary taking off from the main canal, where it is not possible or not desirable to regulate the height of the water level in the canal, can be run with full supply for four or five days out of each fortnight, and then closed. Whether it be four days or five may often depend on special circumstances such as whether the distributary is doing well or otherwise. If necessary the full supply can be adjusted. When the canal supply increases the four or five days can be increased.
The same principle can be adopted for any distributary whose off-take is in the upper part of a branch, i.e., where the branch is many times larger than the distributary, and where it is not possible or not desirable to regulate the water level of the branch. For a distributary further down the branch, the turns of branch and distributary can be arranged as explained above for a canal bifurcation. The orders given to the gauge reader are, as before, to give the channel whose turn it is, full supply and to send the balance down the other channel. When the turn of distributary is over it becomes the turn of the branch. The distributary would not be closed if this would cause the full supply in the branch to be exceeded. Care must be taken that every distributary receives full supply during part of the time when the branch is receiving full supply. If its turn came only when the branch was receiving a residuum supply, or rather when the residuum supply was reaching the distributary off-take—for in the case of a distributary whose off-take is far down a long branch the two things are not the same—it might, in the event of the supply in the main canal falling exceptionally low, receive no water at all.