Contents preface (VII) introduction 1—37

Table 5.2 Conveyance losses in canals (1)
		Loss in m^3/s per million square
	Material	metres of wetted perimeter (or water
		surface)
	Impervious clay loam	0.88 to	1.24
	Medium clay loam underlaid with hard pan at	1.24 to	1.76
	depth of not over 0.60 to 0.90 m below bed
	Ordinary clay loam, silty soil or lava ash loam	1.76 to	2.65
	Gravelly or sandy clay loam, cemented gravel,	2.65 to	3.53
	sand and clay
	Sandy loam	3.53 to	5.29
	Loose sand	5.29 to	6.17
	Gravel sand	7.06 to	8.82
	Porous gravel soil	8.82 to 10.58
	Gravels	10.58 to 21.17

In this relation, q_l is the loss expressed in m³/s per kilometre length of canal and B and h are, respectively, canal bed width and depth of flow in metres.

5.8. ESTIMATION OF DESIGN DISCHARGE OF A CANAL
The amount of water needed for the growth of a crop during its entire crop-growing period is known as the water requirement of the crop, and is measured in terms of depth of water spread over the irrigated area. This requirement varies at different stages of the growth of the plant. The peak requirement must be obtained for the period of the keenest demand. One of the methods to decide the water requirement is on the basis of kor watering.
When the plant is only a few centimetres high, it must be given its first watering, called the kor watering, in a limited period of time which is known as the kor period. If the plants do not receive water during the kor period, their growth is retarded and the crop yield reduces considerably. The kor watering depth and the kor period vary depending upon the crop and the climatic factors of the region. In UP, the kor watering depth for wheat is 13.5 cm and the kor period varies from 8 weeks in north-east UP (a relatively dry region) to 3 weeks in the hilly region (which is relatively humid). For rice, the kor watering depth is 19 cm and the kor period varies from 2 to 3 weeks.
If D represents the duty (measured in hectares/m³/s) then, by definition,

1 m³/s of water flowing for b (i.e., base period in days) days irrigates D hectares.

∴ 1 m³/s of water flowing for 1 day (i.e., 86400 m³ of water) irrigates D/b hectares This volume (i.e., 86400 m³) of water spread over D/b hectares gives the water depth, ∆.

∴	∆ =	86400	= 8.64 b/D (metres)	(5.2)
( D / b) × 10^4

For the purpose of designing on the basis of the keenest demand (i.e., the kor period requirement) the base period b and the water depth ∆ are replaced by the kor period and kor water depth, respectively.

CANAL IRRIGATION

175

Example 5.1 The culturable command area for a distributary channel is 10,000 hectares. The intensity of irrigation is 30 per cent for wheat and 15 per cent for rice. The kor period for wheat is 4 weeks, and for rice 3 weeks. Kor watering depths for wheat and rice are 135 mm and 190 mm, respectively. Estimate the outlet discharge.
Solution:

Quantity		Wheat		Rice
Area to be irrigated (hectares)	0.30 × 10,000 = 3000	0.15 × 10,000 = 1500
Outer factor D = 8.64 b/∆		8.64(4 × 7)	= 1792		8.64(3 × 7)	= 954.95
0.135	0.19
		(in hectares/m^3/s)
Outlet discharge (m^3/s)	3000/1792 = 1.674 ≈ 1.7		1500/954.95 = 1.571 ≈ 1.6

Since the water demands for wheat and rice are at different times, these are not cumulative. Therefore, the distributary channel should be designed for the larger of the two discharges, viz., 1.7 m³/s. The above calculations exclude channel losses and the water requirement of other major crops during their kor period.

The kor period for a given crop in a region depends on the duration during which there is likelihood of the rainfall being smaller than the corresponding water requirement. Accordingly, the kor period is least in humid regions and more in dryer regions. The kor depth requirement must be met within the kor period. As such, the channel capacity designed on the basis of kor period would be large in humid regions and small in dry regions. Obviously, this method of determining the channel capacity is, therefore, not rational, and is not used in practice.
A more rational method to determine the channel capacity would be to compare evapotranspiration and corresponding effective rainfall for, say, 10-day (or 15-day) periods of the entire year and determine the water requirement for each of these periods. The channel capacity can then be determined on the basis of the peak water requirement of the 10-day (or 15-day) periods. This method has already been explained in Sec. 3.8.
5.9. CANAL OUTLETS
When the canal water has reached near the fields to be irrigated, it has to be transferred to the watercourses. At the junction of the watercourse and the distributary, an outlet is provided. An outlet is a masonry structure through which water is admitted from the distributary into a watercourse. It also acts as a discharge measuring device. The discharge though an outlet is usually less than 0.085 m³/s (3). It plays a vital role in the warabandi system (see Sec. 5.11) of distributing water. Thus, an outlet is like a head regulator for the watercourse.
The main objective of providing an outlet is to provide ample supply of water to the fields, whenever needed. If the total available supply is insufficient, the outlets must be such that equitable distribution can be ensured. The efficiency of an irrigation system depends on the proper functioning of canal outlets which should satisfy the following requirements (3):
(i) The outlets must be strong and simple with no moving parts which would require periodic attention and maintenance.
(ii) The outlets should be tamper-proof and if there is any interference in the function-ing of the outlet, it should be easily detectable.

176 IRRIGATION AND WATER RESOURCES ENGINEERING
(iii) The cost of outlets should be less as a large number of these have to be installed in an irrigation network.
(iv) The outlet should be able to draw sediment in proportion to the amount of water withdrawn so that there is no silting or scouring problem in the distributary down-stream of the outlet.
(v) The outlets should be able to function efficiently even at low heads.
The choice of type of an outlet and its design are governed by factors such as water distribution policy, water distribution method, method of water assessment, sources of supply, and the working of the distributary channel.
Water may be distributed on the basis of either the actual area irrigated in the previous year or the actual culturable command area. The discharge from the outlet should be capable of being varied in the first case, but, can remain fixed in the second. The method of water distribution may be such that each cultivator successively receives water for a duration in proportion to his area. Or, alternatively, all the cultivators share the outlet discharge simultaneously. The first system is better as it results in less loss of water. The outlet capacity is decided keeping in view the method of water distribution.
If the assessment is by volume, the outlet discharge should remain constant and not change with variation in the water levels of the distributary and the watercourse. On the other hand, if water charges are decided on the basis of area, the variation in the outlet capacity with water levels of the distributary and watercourse is relatively immaterial.
With a reservoir as supply source, the cultivators can be provided water whenever needed and, hence, the outlets should be capable of being opened or closed. The outlets generally remain open if the supply source is a canal without storage so that water is diverted to the field when the canal is running.
At times, the amount of water in the main canal may not be sufficient to feed all the channels simultaneously to their full capacity. As such, either all the channels may run with low discharge or groups of channels may be supplied their full capacity by rotation. In the first case, the outlets must be able to take their proportionate share even with large variations in the discharge of the distributary channel. In the second case, the outlets must be such that the required amount of water is available for all the channels being fed with their full capacity.
It should be noted that whereas the cultivator prefers to have outlets capable of supplying constant discharge, the canal management would prefer that the outlets supply variable discharge depending upon the discharge in the distributary channel so that the tail end of the channel is neither flooded nor dried. Obviously, both these requirements cannot be fulfilled simultaneously.