This canal takes off from the river and derives water through a regulator. For a certain length in the beginning the canal may be in full cutting. Obviously direct flow irrigation from such a channel is not possible as its water surface is below the general ground surface. The main function of the canal is to take the required amount of irrigation water at the head and to deliver it in the canal system below.
After the main canal comes in the tract which is to be irrigated by the canal system, water is distributed over the entire commanded area. For this purpose the main canal is branched to cover the whole area. The bifurcated canals are called branch canals or simply branches.
From the branches also direct irrigation is very rarely done. The main function of the branches is to make the water available in different parts of the tract for further distribution. The section of a branch canal is smaller than the main canal.
They can be subdivided into:
i. Major distributaries, and
ii. Minor distributaries or minors.
Major Distributaries (D):
They are also named Rajbaha. Major distributaries are smaller in section than the branches. They are taken off mainly from the branches to distribute the water to various parts for direct irrigation purposes. Sometimes major distributaries may take off directly from the main canal. The distributaries provide water to field channels through outlets for irrigation.
Minor Distributaries (M):
They are smaller in section than the major distributaries. The main purpose of these distributaries is to reduce the length of the field channels. They are taken off from the major distributaries and sometimes from the branch canals. When the length of the field channels is more than 3 km the minor distributaries are introduced to reduce the length. They are also called minors.
They are also named as guls or field channels. They are the channels which take irrigation water to the fields. The water courses derive their supply from the distributaries through outlets. An outlet is a simple and a small irrigation structure constructed on the distributaries.
It may be a simple pipe outlet or any other type provided to allow flow of water into the water courses. The amount of water withdrawn through one outlet is in proportion to the area to be irrigated below that point. Table 8.3 gives the ranges of discharges carried by each type of canal.
Irrigation department is responsible for construction and maintenance of the canal system up to minor distributaries. The field channels are constructed by cultivators themselves as per the alignment sanctioned by the department. Nowadays the field channels are also constructed by irrigation department for cultivators. The job of maintenance of field channels is left entirely to the cultivators. The length of a water course should not exceed 3 km in any case. It generally serves an area of 40 to 50 hectares. The upper limit of discharge in a watercourse ranges between 0.08 to 0.06 m3/sec.