It has already been stated ([Art. 4]) that a distributary is so designed that its water level, when three-fourths of the full supply is run, shall be well above the level of most of the ground along its course. In other words it should have a good command. A good rule is to allow a fall of ·5 feet from the level of the water in the distributary to that in the watercourse, a slope of 1 in 4,000 for the water flowing along the watercourse, and a fall of ·3 feet for the water at the tail of the watercourse to the level of the ground. This last level is, like the other ground levels, taken from the contour map. This procedure, in short, consists in making the water level of the watercourse at its head govern that of the distributary, just as the water level in the distributary at its head was made to govern that in the canal.
The enlarged contour map of the distributary area shows, among other things, the boundaries of the lands belonging to each village. Generally a watercourse supplies water to only one village. When, however, a village is far from the distributary, its watercourse has to pass for a long distance through other villages and it would be wasteful of water to have two separate watercourses. In such cases one watercourse may serve two villages or more. When a village is near to the distributary and its land extends for a long distance parallel to the distributary, it may have several watercourses for itself alone. A watercourse can generally be most conveniently dug along the boundary line of two villages, or there may be some other line which the people particularly desire.[15] Subject to, or modified by, these considerations a watercourse is designed to run on high ground like a distributary.
[15] They also frequently wish the “chak”—the area irrigated by a watercourse—so arranged that two men who are “enemies” shall not be included in the same “chak.” This condition can be complied with only up to a certain point. Arrangements may be modified but not in such a way as to upset the proper rules.
Contour Map (part) and Line of Distributary.
The scale is 1 inch to 2 miles. The contour lines at 1 foot intervals are shown dotted, the roads by double lines. The line of the distributary, in order to follow the ridge of the country, would have gone more to the left of the plan near the village. The shifting of the line to the right brings it nearer to the centre of the irrigated tract—supposed to be the whole area shown—and enables a single bridge to be built at the bifurcation of the two roads. Suitable lines for main watercourses are shown in thin firm lines. It is assumed that the command is sufficient to enable the watercourses to run off at the considerable angles shown.
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The great object is to reduce the total length of channels, i.e., minors and watercourses. No watercourse can be allowed to run alongside of or near to another. It may run alongside a canal or distributary when really necessary to gain command but not otherwise. The longer the watercourse the larger the chak. The discharge of an outlet may be anything up to 4 or 5 c. feet per second. This limits the size of a chak. If a chak is too big it can be split up or a minor can be designed. Very small chaks are to be avoided, but it is difficult to fix a minimum size. The irrigation boundary of the distributary, as fixed in the project, is shown on the map but in practice it will not be exactly followed. For various reasons the boundaries of a chak may run somewhat outside it or stop short of it.
Where a distributary gives off a minor and there is a double regulator, watercourses should, as far as possible, be taken off from one or other of the branch channels and not from upstream of the double regulator. Otherwise, irregularities are likely to occur, both of the regulators being partially closed at the same time—a thing which is never necessary in legitimate distribution of the supply—in order to head up the water and increase the discharges of the outlets.
A watercourse nearly always gives off branches and generally a system of turns is arranged by the farmers among themselves, each branch in turn taking the whole discharge of the watercourse for a day or part of a day, the other branches being closed by small dams of earth. To irrigate a field alongside the watercourse a gap is cut in its bank. For fields further away, smaller channels run off from the watercourses at numerous points. Several gaps and several field channels may be in flow at one time, and there is a dam in the watercourse below the lowest one.