[Illustration: BIFURCATION AT TAIL OF CANAL.
The Distributaries have Gates and Winches.
It has been stated (CHAPTER I, Art. 2) that it is not desirable to let one channel tail into another. In old canals a distributary used sometimes, after running parallel to a canal, to be brought back towards it and tail into it. The advantage of this was that the distributary had not to be made very small towards the tail and that, if the demand abruptly ceased, the distributary was not likely to breach. The principle was, however, essentially bad. The lower part of the distributary was obviously too near the canal and not centrally situated as regards the irrigated strip. The portion at the extreme tail was superfluous. Again, whatever volume of water was carried through the distributary and back into the canal, was needlessly detached instead of being kept in bulk. Moreover the duty of water on such a distributary cannot be ascertained without a tail gauge and the observation of discharges at the tail. There are similar objections to one distributary tailing into another. Each should be separate and distinct.
A major distributary is one whose discharge is more than 40 c. ft. per second. It may be as much as 250 c. ft. per second. A branch, as soon as it reaches a point where its discharge becomes only 250 c. ft. per second should be considered as a major distributary. A minor distributary is one whose discharge is from 8 to 40 c. ft. per second. A minor distributary is nearly always a branch of a major distributary. There are instances of “direct minors,” i.e., minors taking off from canals or branches. Such a minor, unless its discharge is a large fraction of that of the canal which supplies it--and this can seldom be the case--is objectionable because the petty native official who has to see to the regulation of supplies can manipulate the supply easily and without detection, and the number of persons irrigating from it being small, he can make private arrangements with them. On the Sidhnai Canal there are some half-dozen distributaries each of which had one or two minors which took off close to the head of the distributary. The people who irrigated from the minors managed to get the heads shifted and taken off direct from the canal, on the ground that, the water level in the canal being higher than in the distributary, there would be better command and less silt deposit. The irrigation on all these minors ran up to a figure far in excess of what had been intended, to the detriment of lands further down the canal. The minor heads have all been retransferred to the distributaries, the difficulty as to command being got over, as it should have been at first, by constructing weirs in the distributaries. The fall in the water surface at the distributary head, i.e., the difference between the water level in the canal and that in the distributary downstream of its head but upstream of the weir, is quite trifling or even inappreciable.