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.