—Before a canal system can be properly designed, it is necessary to determine certain points in connection with the working of the distributaries. A distributary is intended to irrigate a certain kharif area. Its average kharif supply is determined from the assumed kharif duty. It generally runs full in the kharif but not always. In a very dry tract such as the Montgomery district of the Punjab, the demand is so great and so steady that a distributary practically runs full through the greater part of the kharif. In such a case the canal or branch must be so designed that it can keep all distributaries full at the same time. Its F.S. discharge will be the sum of all the F.S. discharges of the distributaries plus the losses of water by absorption.
But in other cases, especially if the rainfall is considerable, a distributary does not require its full supply, either all through the kharif or for long at a time. An estimate must then be made of what it will require. It may be estimated that its requirements will be met if, during the period of greatest demand, it is closed for two days out of a fortnight and receives full supply for the remaining twelve days. In this case, since the various distributaries need not all be closed on the same days, the canal or branch can be so designed that it will carry a full supply equal (after deducting losses) to ⁶⁄₇ths of the aggregate full supplies of the distributaries. In other cases the fraction may be ³⁄₄ths. It is likely to be lower the greater the rainfall of the district. Even in the case when the distributaries run full through nearly the whole of the kharif, there will be periods when they only run with about ³⁄₄ths full supply. If full supply were run at such times, many of the outlets would discharge more water than was required, the cultivators would partly close them, and breaches in the banks of the distributary might result. Thus the water level of a distributary must always be so arranged that it will have a good “command” when it is running with about three-fourths of the full supply discharge. The water level with ³⁄₄ths full supply is generally ·5 to ·75 feet below the full supply level but it should be calculated in each case. Generally it will be correct to make the water level, when ³⁄₄ full supply is run, about 1 foot above the high ground traversed by the distributary, excluding any exceptionally high portions of small area. A more exact method is given in [Art. 9]. The greater the proportion of the culturable area which is to be irrigated, the less should be the area of any high land which is excluded. The F.S. levels of the distributaries at their off-takes must be settled in accordance with the foregoing remarks, and these F.S. levels must be entered on the plan. Neglect to thus fix the F.S. levels of distributaries before designing the canals has frequently led to trouble.
The head needed at a bifurcation in order to get the supply into a branch or distributary is always small unless the velocity is high. For a velocity of 3 feet per second the head required is only about ·16 ft., for 2 ft. per second ·1 ft.
On an Inundation Canal which has no weir across the river, the mean supply downstream of the regulator (which is built a few miles down the canal lest it should be damaged by the shifting of the river) is, as has been mentioned, about half the full supply. The command in such canals is not generally very good. A distributary can often obtain only mean supply and it should be designed so as to command the country when it is carrying mean supply. A detailed description of Inundation Canals in Northern India, is given in Punjab Rivers and Works.
Let M, F, m, f, be the mean and full supply discharges at the heads of a canal and of an average distributary on it and let the number of distributaries be n. It has been seen ([Chap. I. Art. 6.]) that M = ·8F about. Let k be the proportion of the supply lost by absorption in canal and branches. Then n m = (1 - k) M = ·8 (1 - k) F. If the distributaries all run with full supplies—at the time of greatest demand—for 4 days out of 5, then,
n f = 1·25 (1 - k) F
fm = 1·25·8 = 1·56
Since k depends on the wetted area, it is not likely to be so great for F as for M, but the above gives a general idea of the ratio of the full kharif discharge to the mean kharif discharge. On a large canal the circumstances of the distributaries will not all be similar. Some will run full for a greater proportion of their time than others. They can be divided into groups and the ratio of the full to the mean supply calculated for each group. The mean supply is, as above stated, obtained from the area to be irrigated, and the duty as estimated at the distributary head.
At one time a system was introduced of making distributaries of large size with the idea of running them for short periods. One reason given for abandoning this arrangement, was that there was a tendency to run such a distributary for too long. This reason is not very intelligible. It would be applicable to any distributary which was not intended to be run without cessation. The result would be that some other distributary would be kept short of water and this would imply extremely bad management. The chief reason against such a distributary is the greater cost of its construction. It would effect a saving of water. The ratio of the discharge to the wetted area would be high, though this would be to some extent neutralized by the greater frequency of closures, since, when water is admitted to a dry channel, the absorption is at first great. There would also be some difficulty in the distribution of the water because of the short period for which it would remain open. It will be seen ([Chapter III. Art. 5]), that it is desirable to open and close always at the same hour of the day. An ordinary distributary might run for 11 days out of 14. One of double the size could not conveniently be run for 5¹⁄₂ days. A distributary can always be enlarged if necessary, but if made too large it is extremely difficult to make it smaller.
It was also, at one time, usual to make minors, when there were several on a distributary, of large capacities so that they ran in turns. The preceding remarks apply to this case. The system has been abandoned.