In this article we will discuss about:- 1. Definition of Duty of Canal Water 2. Factors Affecting Duty of Canal Water 3. Improvement.
Definition of Duty of Canal Water:
Duty shows the degree of efficient utilization of irrigation water supplied from a canal to any crop.
Precisely “Duty of the canal water is the number of hectares under a particular crop, brought to the state of maturity by a constant supply of one cubic metre of water per second flowing continuously for the base period”. It is expressed in hectares per cubic metre per second.
Base Period:
It is the time elapsed from first watering at the time of sowing to the last watering when the crop matures.
Relation between Delta, Base Period and Duty for a Crop:
Suppose discharge of a channel is one cubic metre per second and the channel water is continuously flooding the land for irrigation purpose. If this rate of applying water is continued for a base period of a particular crop then the total amount of water applied in the base period will cover certain area (in hectares) to give a total depth of water equal to a delta of the crop. This area measured in hectares is called the duty of the canal water.
Let D be duty of the canal water in hectares per cubic metre per second.
B be the base period in days.
Δ be delta or depth of water required for a crop in metres.
From the definitions of delta, duty, and base period, 1 cubic metre per second continuously for B days mature D hectares of land under the crop or 1 cubic metre per second of flow continuously for B days gives a depth Δ over D heatares.
So, 1 m3/sec for 1 day gives depth Δ over D/B hectares.
Or
1 m3/sec for 1 day gives (Δ × D)/B hectare-metres of water …. (1)
(N.B. 1 hectare = 10,000 m2)
Now 1 m3/sec flowing continuously for 1 day gives:
24 × 60 × 60 × 1 = 864000, cubic metres of water.
This volume is equal to 86,400/10,000 = 8.64 hectare-metres … (2)
Since 1 hectare-metre = 10,000 × 1 cubic metres.
Equating (1) and (2)
8.64 = Δ × (D/B)
or Δ = 8.64 B/D metres
Here D is in hectares per cubic metre per second.
High Duty:
When small amount of water matures comparatively large area under a crop duty realised in high.
Low Duty:
Conversely when large amount of water matures comparatively small area under the crop duty is said to be low.
Duty Inclusive and Exclusive of Rainfall:
During the base period of a crop there may be rainfall quite a number of times. The rainfall also fulfills part of the total water requirement of the crop. As such while calculating duty when rainfall is also taken in consideration, comparatively less amount of water is applied to the same area because rainfall has already contributed part of the water requirement. So duty inclusive of rainfall will be high. Conversely duty of the water calculated without taking rainfall into account is said to be duty exclusive of rainfall. Duty exclusive of rainfall will be low.
Duty of the Canal Water at Specific Points:
Duty of canal water can be calculated at all the important control points of a canal system. For example, at headworks where the canal takes off from the source; at a junction of a canal and a branch canal; at an outlet through which water is finally supplied to the cultivators, etc.
Duty of the canal water measured at the headworks is low. The reason is that huge loss of water takes place before finally water reaches the fields. The water lost in conveyance is also assumed to be used up by the crops. As we proceed towards the tail of the canal system duty goes on increasing.
Duty calculated at the outlet is highest of all because at this point the difference between the water actually supplied and the water actually utilised on the field is considerably small. Duty of the canal water calculated at an outlet is called outlet factor. Duty calculated at the headworks is called gross duty.
Table 3.2 gives average duties in different States. It may, however, be clarified that the duty is never constant. It depends on climatic, agricultural and topographical factors. It is seen that the duty even varies for the same crop from region to region.
Factors Affecting Duty of Canal Water:
The factors which have some influence on duty of canal water can be summarised as given below:
(1) Length of Base Period of a Crop:
If the length of base period is small then amount of water required will be less and duty will be high.
(2) Rainfall:
If the amount of rainfall is more in base period of a crop the amount of water required will be less and duty will be high.
(3) Evaporation:
If climate of the area is hot, evaporation will be more. Land is deprived of the water because of the evaporation and hence when evaporation loss is more duty is below.
(4) Soil Characteristics:
(a) Physical Factor:
Close textured soils hold more water in unsaturated zone than sandy soils.
(b) Chemical Factor:
Alkali and saline soils require more amount of water, firstly to leach the salts and secondly to fulfill the crop water requirement.
(5) Defective Handling of Water:
If storage, conveyance, and distribution systems are faulty or neglected, water losses will be more and duty will be low.
(6) Kind of Crop:
Water requirement of each crop is different on the same field.
(7) Cultivation Method:
If land is tilled and made quite loose before irrigating, more water can be retained in the unsaturated zone of soil for satisfying future water requirement. Consequently the number of waterings can be reduced to increase the duty of water.
(8) Quality of Water:
If the water contains alkali or other harmful salts it is essential to use water liberally because by excess water the salts will be leached off and only good water will remain in the available zone of soil. This, of course lowers duty as more water is used for the same land.
(9) Topography of Land:
If the field is not level, low portion receives more water and higher patches receive less amount of water. This inequitable distribution necessitates more use of water to provide the required delta over higher portions of the field and hence duty will be low.
(10) Skill of Irrigator:
The efficient method of distribution requires comparatively small quantity of water on the same area.
(11) Departmental Practices:
(a) Continuous running of a canal tend irrigators to use water carelessly as the water is at their disposal all the time.
(b) Method of assessing the water supply. If it is based on actual quantity of water used duty will be high. But if it is based on the area irrigated tendency of the careless cultivators is to allow watering in excess and hence duty will be low.
Improvement of Duty of Canal Water:
The duty of the irrigation water can be improved considerably by adopting correct practices.
Following points are note-worthy:
(i) If the soil is sandy or if it contains alkali or saline salts the soil should be cured before sowing.
(ii) Field should be properly tilled and levelled before sowing.
(iii) Water of good quality should be used for irrigation.
(iv) Depth of each watering should be scientifically determined.
(v) Irrigation water should be assessed in such a way that the cultivators are prevented from using the water carelessly.
(vi) Water losses in storage, conveyance and distribution should be reduced to minimum. This can be achieved by properly designing and constructing hydraulic structures, by lining the canals etc.
(vii) Crops should be grown in rotation in such a way that the soil moisture is used to a maximum extent.
(viii) Cultivators should be trained to use irrigation water economically to derive maximum benefit.
(ix) Trained irrigation staff should be employed.
(x) The issues involving economical use of water should be studied in research stations.