Contents preface (VII) introduction 1—37

Sec. 5.3.2.)

165

166 IRRIGATION AND WATER RESOURCES ENGINEERING
An irrigation canal system consists of canals of different sizes and capacities (Fig. 5.1). Accordingly, the canals are also classified as: (i) main canal, (ii) branch canal, (iii) major distributary, (iv) minor distributary, and (v) watercourse.


Watershed

Boundaries of

G.C.A.

		(B)
Maj		(B)
Min		Maj
R		Min
	Min	Min
i
v
e
r
		Maj

B - Branches
Maj - Major distributaries
Min - Minor distributaries

Main	(B)
		Maj
Min
Maj	Min
		Maj
		B

M	Maj
a	Min
i
	Min
		n
B
Maj	Maj
	B
Min	Maj
	Min
Min
Maj	Min
	B
	Maj

R i v e r

Fig. 5.1 Layout of an irrigation canal network
The main canal takes its supplies directly from the river through the head regulator and acts as a feeder canal supplying water to branch canals and major distributaries. Usually, direct irrigation is not carried out from the main canal.
Branch canals (also called ‘branches’) take their supplies from the main canal. Branch canals generally carry a discharge higher than 5 m³/s and act as feeder canals for major and minor distributaries. Large branches are rarely used for direct irrigation. However, outlets are provided on smaller branches for direct irrigation.
Major distributaries (also called ‘distributaries’ or rajbaha) carry 0.25 to 5 m³ /s of discharge. These distributaries take their supplies generally from the branch canal and

CANAL IRRIGATION

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sometimes from the main canal. The distributaries feed either watercourses through outlets or minor distributaries.

Minor distributaries (also called ‘minors’) are small canals which carry a discharge less than 0.25 m³/s and feed the watercourses for irrigation. They generally take their supplies from major distributaries or branch canals and rarely from the main canals.
A watercourse is a small channel which takes its supplies from an irrigation channel (generally distributaries) through an outlet and carries water to the various parts of the area to be irrigated through the outlet.
5.2. COMMAND AREAS
Gross command area (or GCA) is the total area which can be economically irrigated from an irrigation system without considering the limitation on the quantity of available water. It includes the area which is, otherwise, uncultivable. For example, ponds and residential areas are uncultivable areas of gross command area. An irrigation canal system lies in a doab (i.e., the area between two drainages), and can economically irrigate the doab. It is, obviously, uneconomical to use the irrigation system to irrigate across the two drainages. Thus, the boundaries of the gross command of an irrigation canal system is fixed by the drainages on either side of the irrigation canal system.
The area of the cultivable land in the gross command of an irrigation system is called culturable command area (CCA) and includes all land of the gross command on which cultivation is possible. At any given time, however, all the cultivable land may not be actually under cultivation. Therefore, sometimes the CCA is divided into two categories: cultivated CCA and cultivable but not cultivated CCA.
Intensity of irrigation is defined as the percentage of CCA which is proposed to be annually irrigated. Till recently, no irrigation system was designed to irrigate all of its culturable command every year. This practice reduces the harmful effects of over-irrigation such as waterlogging and malaria. Also, due to the limitations on the quantity of available water, it is preferred to provide protection against famine in large areas rather than to provide intensive irrigation of a smaller area. The intensity of irrigation varied between 40 per cent to 60 per cent till recently. This needs to be raised to the range of 100 per cent to 180 per cent by cultivating parts of CCA for more than one crop in a year and through improved management of the existing system. Future projects should be planned for annual intensities of 100 per cent to 180 per cent depending on the availability of total water resources and land characteristics.
The culturable command area multiplied by the intensity of irrigation (in fraction) gives the actual area to be irrigated. The water requirements of the controlling crops of two crop seasons may be quite different. As such, the area to be irrigated should be calculated for each crop season separately to determine the water requirements.
5.3. PLANNING OF AN IRRIGATION CANAL SYSTEM
Planning of an irrigation canal project includes the determination of: (i) canal alignment, and (ii) the water demand. The first step in the planning of an irrigation canal project is to carry out a preliminary survey to establish the feasibility or otherwise of a proposal. Once the feasibility of the proposal has been established, a detailed survey of the area is carried out and,

168 IRRIGATION AND WATER RESOURCES ENGINEERING
thereafter, the alignment of the canal is fixed. The water demand of the canal is, then, worked out.