In Italy the duty is 55 to 70 acres, in Spain from 45 to 205 acres, in the Western States of America generally 60 to 150 acres. In South California the duty is 150 to 300 acres, when, as is usual, surface irrigation is employed, but 300 to 500 acres with subsoil irrigation, the water being delivered in a pipe below ground level ([Chapter V.])
In basin irrigation in Egypt the duty is 20 to 25 acres, but the period of flow is only 40 days. The basins are flooded to about 3 feet in depth.
6. Sketch of a Project.
—The tract of country to be dealt with in an irrigation project may be limited either by the natural features of the country, by its levels, by the quantity of water available or by financial considerations. If the tract is small or narrow, and particularly if it is not very flat, it may be obvious that there is only one line on which the irrigation channel can conveniently be constructed but in any considerable scheme a contour plan of the whole tract is absolutely necessary. The surveys for such a plan are expensive and take time and it is desirable, as far as possible, to settle beforehand the area over which they are to extend. This may be done to some extent by the examination of any existing levels and of the tract itself. Very high, sandy or swampy ground, whether occurring at the edge of the tract or in the middle of it, may have to be left out. The remainder, as already mentioned, is called the commanded area. When land occupied by houses or roads or which is very much broken, or which for any reason cannot be irrigated, has also been deducted, the balance is the “culturable commanded area.”
Either before or after the culturable commanded area has been approximately ascertained, the proportion of it which is to be irrigated must be settled. This depends on local circumstances. In India the supply of water is calculated on the supposition that a fraction, generally from ¹⁄₃ to ³⁄₄, of the culturable commanded area will be irrigated each year. The rest will be lying fallow or be temporarily out of use or be used for crops which do not require canal irrigation. The restriction of the area is necessary either because the supply of water is limited or in the interests of the people. Too liberal a supply of water tends, as already stated, to over cultivation, and exhaustion and water-logging of the soil.
The next step is to estimate the duty and the discharge of the canal and then to fix its main dimensions. In Northern India the duty in the rabi is higher than in the kharif. It may be 200 acres in the rabi and 100 acres in the kharif. Local circumstances determine which crop has the greater area. Suppose that it is estimated that both will be equal. Then the total annual area for which water is to be provided must be divided by two and this gives the kharif area. During the kharif there is usually an ample supply of water and the kharif mean supply of the canal is based on the kharif area and the kharif duty. The full supply is not run all through the crop because the demand fluctuates, the demand being greatest when all the crops have been sown and when there is no rain, but from experience of other canals the ratio of the kharif full supply to the kharif mean supply can be estimated. The ratio is generally about 1·25. On the kharif full supply depends the size of the channel, every channel being constructed so as to carry a certain “full supply” or maximum discharge and the top of the bank being made at such a height that there shall be a sufficient margin or “free-board” above the “full supply level.” The canal runs full provided that there is a sufficient supply in the river or that the water level of the river is high enough—this last condition referring to canals which have no weir in the river—and provided also that there is a sufficient “demand” for the water. At other times a canal runs with less than full supply. This generally occurs throughout most of the rabi, the supply of water in the river being then restricted. The distributaries are generally run full or ³⁄₄ths full, some being closed, in turn, to give water to the others. In the case of a country where there is only one crop in the year, the average discharge of the canal can be found by dividing the area by the estimated duty. The F.S. discharge can be assumed to bear such a relation to the average discharge as may be found by experience to be suitable. On some Indian inundation canals the F.S. discharge is taken as twice the average discharge.
The F.S. discharge of the canal having been arrived at, the alignments of the canal and branches are next sketched out on the contour plan and certain tracts and discharges are assigned to each branch. The gradients can be ascertained from the levels of the country and the cross-section of the channel can then be sketched out. If the velocity is too great for the soil “falls” can be introduced. The above procedure will enable a rough idea to be formed of the cost of the earthwork of the scheme. The cost of the headworks and masonry works and distributaries can be best estimated by obtaining actual figures for existing works of similar character, the distributaries being reckoned at so much per mile. The probable revenue which the canal will bring in will depend upon the rate charged for the water and the cost and maintenance, matters which can only be determined by local considerations based on the figures for existing canals.
The masonry works on a canal consist of the headworks and of bridges, regulators and drainage crossings. The principles of design for such works have been dealt with in River and Canal Engineering. It is of course economical to make a bridge and fall in one. If the off-take of a distributary is anywhere in the neighbourhood the fall should of course be downstream of it. The positions of the falls should be fixed in accordance with these considerations. If the longitudinal section is such that the position of the fall cannot be much altered, it may be feasible to divert a road so that the bridge may be at the best site for the fall. In the case of a railway crossing, a skew bridge is often necessary. In the case of a road crossing it may be feasible to introduce curves in the road but here also a skew bridge is often necessary.