Contents preface (VII) introduction 1—37

8.6.1. Layout and Discharge Requirements of Irrigation Channels
The layout of irrigation channels should ensure equitable distribution of water with minimum expenditure. It is, obviously, advantageous to align all irrigation channels on the watershed. This will ensure least expense on cross-drainage structures and cause gravity flow on either side of the channel. In hilly regions, the channels are aligned along the contours. Alignment of irrigation channels should be such that there are very few curves. However, if the channel curves are unavoidable, the radius of curvature of these curves should be as large as possible. For large channels, the minimum radius of curvature should be about 60 times the bed width of the channel. In case of smaller channels, the minimum radius of curvature of the curves should be about 45 times the bed width of the channel.

304 IRRIGATION AND WATER RESOURCES ENGINEERING
The maximum discharge to be carried by an irrigation channel at its head to satisfy the irrigation requirements for its command area under the worst conditions during any part of a year is said to be the designed full supply discharge (or capacity) of the channel. The water level in the channel when it is carrying its full supply discharge is termed full supply level (FSL) and the corresponding depth of flow is called full supply depth (FSD).
After fixing the layout of all irrigation channels (i.e., canal system) on a map, the total command area proposed to be irrigated by each of the channels of the canal system is estimated. Knowing the crop requirements and intensity of irrigation, and allowing for seepage and evaporation loses, the amount of the discharge requirement at various points of the canal system are determined. Obviously, this requirement will progressively increase from the tail end of a channel to its head reach. Accordingly, the size (i.e., width and depth) of a channel will progressively decrease form its head reach to its tail end. The size of a watercourse is, however, kept uniform.