Canal Irrigation System

By

Dr. Dipankar Roy

Alignment of Canals

Irrigation canals can be aligned in any of the following three ways:

• as watershed canal or ridge canal
• as contour canal; and
• as side-slope canal.

Watershed Canal or Ridge Canal:

The dividing ridge line between the catchment areas of two streams (drains) is called the watershed or the ridge. Thus, between two major streams, there is the main watershed (ridge line), which divides the drainage area of the two streams, as shown in Fig. Similarly, between a main stream and any of its tributaries, there are subsidiary watersheds (ridge lines), dividing the drainage between the two streams on either side.

Contour Canals

The method of building a canal along the ridge line is not practical in hilly regions due to significant differences in terrain compared to plains.
In hills, rivers flow in valleys, significantly lower than the watershed (ridge line).
The ridge line can be hundreds of meters above the river, making it impossible to construct a canal on top of it. In such cases, contour canals are built.

Side slope canal

A side slope canal is that which is aligned at right angles to the contours; i.e. along the side slopes, as shown in Fig. Since such a canal runs parallel to the natural drainage flow, it usually does not intercept drainage channels, thus, avoiding the construction of cross-drainage structures

Distribution System for Canal Irrigation

It has been emphasized earlier that the direct irrigation scheme using a weir or a barrage, as well as the storage irrigation scheme using a dam or a reservoir, require a network of irrigation canals of different sizes and capacities. The entire network of irrigation channels is called the Canal System. The canal system consists of:

(i) Main canal;
(ii) Branch canals;
(iii) Distributaries, also called major distributaries;
(iv) Minors, also called minor distributaries;
(v) Watercourses.

Main Canal (Head reach)

The canal headworks are generally situated on the river flowing in a valley, and the canal should reach the ridge line in the shortest possible distance. The canal, in this reach, must, therefore, be aligned very carefully, and has to be generally excavated deep cuttings below N.S.L. Sometimes, it has to cross various drainage lines. Many a times, straight alignment has to be sacrificed and detours need to be accepted, in order to achieve a good site for cross drainage works.

(b) Main Canal:

(Portion below head reach.) Attempts are made to align the canal along the ridge and somewhat central to the command area. Sometimes, ridge line has to be sacrificed, to bypass towns and villages, etc. Main canal is not required to do any irrigation.

(ii) Branch Canals:

Branch canals are taken off from the main canal on either side to take irrigation water to the whole tract required to be irrigated. Very little irrigation is in fact, done from the branch canals themselves, as they serve to supply water primarily the distributaries. Attempts are made to align them along subsidiary ridges. Discharge in a branch channel, is generally, more than 30 cumec.

(iii) Distributaries:

Smaller channels which take off from the branch canals and distribute the supply water through-outlets into minors-or water courses, are called disributaries. They are aligned either as ridge canals or as contour canals. Discharge in a distributary is generally less than 30 cumec.

(iv) Minors:

Sometimes, the country is such that the distance between the distributary outlet and the farmer's field is very long ; say more than 3 km or .so. In such a case, small channels called minors, are taken off from the distributaries, so as to supply water to the cultivators at the point nearer to their fields. Discharge in a minor, is generally, less than 2.5 cumec.

(v) Watercourses:

These are not the government channels and belong to the cultivators. They are small channels, which are excavated and maintained by the cultivators at their own costs, to take water from the government-owned outlet points, provided in the distributary or the minor.

Curves in Channels

Attempts are made to align the channels Straight as far as possible. But many a times, the curves become inescapable. Whenever, a curve is proposed,
while aligning unlined channels, it should be as gentle as possible. A curve causes disturbance of flow and results in silting on the inside (i.e. convex side) and scouring on the outside (i.e. concave side).

Curves in Channels

Pitching is, therefore, sometimes proposed on the concave side, so as to avoid scouring. If the discharge is more, the curve should be more gentle and should, therefore, have more radius.

Gross Command Area (G.C.A.):

It is the total area, bounded within the irrigation boundary of a project, which can be economically irrigated without considering the limitation of the quantity of available water.
It includes the cultivable as well as the un-cultivable area. For example, ponds, residential areas, roads, reserved forests, etc. are the uncultivable areas of the gross command area.

Culturable or Cultivable Command Area (CCA):

Culturable area is the cultivable part of the gross command area, and includes all land of GCA on which cultivation is possible. It will, thus, include pastures and fallow lands, which can be made cultivable.

Obviously, it does not include uncultivable part of the gross command, like.populated areas, ponds, roads, reserved forests, ushar land, etc.
In the absence of detailed data, CCA may be assumed to be equal to 80% of GCA

Intensity of Irrigation (Seasonal and Annual):

The entire cultivated portion of the culturable. command area (CCA) is not proposed to be irrigated at one time (in one season) to avoid intensive irrigation of a particular area, which may cause harmful effects like water logging, salinity and malaria, etc.

Some land of a particular sub area is thus, either allowed to take rest; or is shown with crops which do not require irrigation water.

only a small percentage of CCA is brought under irrigation over a given season. This percentage of CCA proposed to be irrigated in a given season is called the intensity of irrigation of that season, or seasonal intensity of irrigation.

Say for example, the sanctioned intensity of irrigation under Bhakra canal system is only 27.6% for Kharif season, and 34.4% for Rabi season.

Net and Gross Sown Areas:

Sometimes, two crops in two seasons are grown during a particular year on the same area. Hence, such an area will be sown more than once during a given year. If this area is added to the area which is sown only once (and called the net sown area), then we get what is known as the gross sown area, or the gross cropped area. Hence,

Gross cropped or Gross sown area (during a year) = Net cropped area, i.e. area sown once in a year + Area shown more than once during the same year

Net and Gross Irrigated Areas:

Based on the above analogy, the area which is irrigated once during a year is called· the net irrigated area, and when to this is added the area irrigated more than once, we obtain the gross irrigated area.

Gross irrigated area (in a given year) = Net irrigated area {i.e: area irrigated once in a year) + Area irrigated more than once during the same year .

Area to be Irrigated:

The area proposed to be irrigated in any one crop season or over any given year, is called the area to be irrigated in that season or the year, respectively. It is obtained by multiplying CCA by the seasonal or annual intensity of, irrigation, as the case may be.

The areas to be irrigated are usually worked out separately for each crop season, because the water requirement of the crops of two seasons are quite different.

Time Factor:

To check the dangers of over irrigation, leading to water-logging and salinity, no distributary is allowed to operate on all the days during any crop season.

The ratio of the actual operating period of a distributary to the crop period is the time factor of the distributary.

For the Bhakra canal system, for example, the time factors for Kharif and Rabi seasons are fixed at 0.80 and 0.72, respectively, which means that each distributary would receive its full supply for a period of 0.80 x 180 = 144 days, and 0.72 x 180 = 129 days, respectively, in each crop season of 180 days.

Capacity Factor:

The capacity factor for a canal is the ratio of the mean supply discharge in a canal during a period to its designed foll capacity. Since canals have to run almost to their full designed capacities during Kharif (summer) season, this value usually varies from 0.9 to 0.95 for Kharif season.

However, since water requirement during Rabi (winter) season reduces to about 2/3rd times the full supply, the capacity factor usually varies from 0.60 to 0. 70 for Rabi season. To improve this factor during Rabi season, the cropped area in Rabi season is, hence, usually increased.

Full supply coefficient:

Full supply coefficient is the design duty at the head of the canal. In other words, the number of hectares irrigable per cumec of the canal capacity at its head, is known as the full supply coefficient of the canal. It can hence, be represented by the equation :

This factor is also called the Duty on capacity.

Nominal Duty:

It is the ratio of the area actually irrigated by the cultivators to the mean supply discharge Jet out from the outlet of the distributary over the crop period.

For example, let cumec of water is released daily from the outlet of a distributary for 100 days (says) in a total crop period of 125 days (say). Then, the mean supply discharge over the crop period will be cumec.

Nominal Duty:

If the area of crop irrigated by this discharge is hectares, then the nominal duty
will be given as:

Example:

The gross commanded area for a distributary is 6000 hectares, 80% of which is culturable irrigable. The intensity of irrigation for Rabi season is 50% and that for Kharif season is 25%. If the average duty at the head of the distributary is 2000 hectares!cumec for Rabi season and 900 hectares/cumec for Kharif season, find out the discharge required at the head of the dis tributary from average demand considerations.

G.C.A. = 6000 hectares, C.C.A. =6000 x 0.8=4800 hectares
Area to be irrigated in Rabi season = C.C.A. x Intensity of Irrigation
= 4800x0.5=2400ha
Area to be irrigated in Kharif season = C.C.A. x Intensity of Irrigation
= 4800x0.25=1200ha

Water required at the head of the distributary to irrigate Rabi area =2400/2000==1.20 cumec.
Water required at the head of the distributary to irrigate Rabi area =1200/900=1.33 cumec.

Thus, the requirement in Kharif season IS 1.33 cumec and that in Rabi season is 1.20 cumecs. The required discharge is maximum of the two, i.e. 1.33 cumec. Ans.

Hence, the distributary should be designed for 1.33 cumec discharge at its head, from average demand considerations. The· head regulator should be sufficient to carry 1.33 cumec ; and in Rabi season, only 1.20 cumec will be released.

Example

Determine the discharge required at the head of the distributary in Example given above, for fulfilling maximum crop requirement. Assume suitable values of kor depth and kor period.

Solution: Let us assume a kor period of 4 weeks for Rabi (wheat) and 2.5 weeks
for Kharif crop (rice). Also assume, Kor depth of 13.5 cm for Rabi (wheat) and 19 cm for Kharif (Rice) crop

Now, outlet factor for rabi = ha/cumec

outlet factor for rabi = ha/cumec

Area to be irrigated in Rabi season = 2400 hectares
Area to be irrigated in Kharif season = 1200hectares.

Water required at the head of the distributary to irrigate Rabi area = 2400/1792 = 1.34 cumec

Water required at the head of the distributary to irrigate Kharif area = 1200/796 = 1.51 cumec.

The required discharge is maximum of the two, i.e. 1.51 cumec. Ans.

The required discharge from kor demand considerations have gone up to 1.51 cumec from 1.33 cumec i.e. an increase of about
14%.

End