Duty: The term duty means the area of land that can be irrigated with unit volume of irrigation water. Duty represents the irrigating capacity of a unit. It is the relation the between the area of a crop irrigated and the quantity of irrigation water required during the entire period of the growth of that crop. For example, if 3 cumecs of water supply is required for a crop sown in an area of 5100 hectares, the duty of irrigation water will be 5100/3 = 1700 hectares/cumecs, and the discharge of 3 cumecs will be required throughout the base period.

Delta: It is the total depth of the water required by a crop during the entire period the crop is in the field and is denoted by the symbol ∆. For example, if a crop requires about 12 watering at an interval of 10 days, and a water depth of 10 cm. If the area under the crop is A hectares, the total quantity will be 1.20 X A = 1.2 A hectare-metres in a period of 120 days.

Crop Period: Crop period is the time in days, that a crop takes from the instant of its sowing to that of its harvesting.

Base Period: Base for a crop refers to the whole period of cultivation from the time when irrigation water is first issued for preparation of the ground for planting the crop, to its last watering before harvesting.

Ways of Reckoning Duty

The duty of water is reckoned in the following four ways:

1. By the number of hectare that one cumec of water can be irrigated during the base period, i.e. 1700 hectares/cumec.
2. By the total depth of water (or Delta), i.e. 120 metres
3. By the number of hectare that can be irrigated by a million cubic metre of stored water. This system is used for tank irrigation.
4. By the number of hectare metre expended per hectare irrigated. This is also used in tank irrigation

2.5.1 Relation between Duty and Delta

D= duty in hectares/cumec

∆= total depth of water supplied in metres

B= base period in days

i. If we take a field of area D hectares, water supplied to the field corresponding to the water depth ∆ metres will be = ∆ x D hectares-metres = D x ∆ x10$^4$ cubic-metres. .... (1)

ii. Again for the same field of D hectares, one cumec of water is required to flow during the entire base period. Hence, water supplied to this field. = (1) x (B x 24 x 60 x 60) m$^3$ .... (2)

Equating Equations (1) and (2), we get D x ∆ x 10$^4$ = B x 24 x 60 x 60

∆ = $\frac{B \times 24 \times 60 \times 60}{D \times 10^4} = 8.64 \text{ B/D metres}.$

1 hectare = 104 sq metres

cumec-day = 8.64 hectare-metres

Various Terms

1. Commanded area (CA): is defined as the area that can be irrigated by a canal system, the CA may further be classified as under:

2. Gross command area(GCA): This is defined as total area that can be irrigated by a canal system on the perception that unlimited quantity of water is available. It is the total area that may theoretically be served by the irrigation system. But this may include inhibited areas, roads, ponds, uncultivable areas etc which would not be irrigated.

3. Culturable command area (CCA): This is the actually irrigated area within the GCA. Thus G.C.A = C.C.A + Unculturable area. However, the entire CCA is never put under cultivation during any crop season due to the following reasons: The required quantity of water, fertilizer, etc. may not be available to cultivate the entire CCA at a particular point of time. Thus, this is a physical constraint. The land may be kept fallow that is without cultivation for one or more crop seasons to increase the fertility of the soil. This is a cultural decision. Due to high water table in some areas of the CCA irrigated water may not be applied as the crops get enough water from the saturation provide to the surface water table. During any crop season, only a part of the CCA is put under cultivation and this area is termed as culturable cultivated area. The remaining area which is not cultivated during a crop season is conversely termed as culturable uncultivated area.

Intensity of irrigation: is defined as the percentage of the irrigation proposed to be irrigated annually. Usually the areas irrigated during each crop season (Rabi, Kharif, etc) are expressed as a percentage of the CCA which represents the intensity of irrigation for the crop season. By adding the intensities of irrigation for all crop seasons the yearly intensity of irrigation to be obtained. For example, if CCA of an irrigation field is 120 hectares, out of which 90 hectares of the land is cultivated during the kharif season and 60 hectares of the land is cultivated during the rabi season, the intensity of the during kharif season will be = (90/120) /100 = 75% and the intensity during the rabi season will be = (60/120)/100 = 50%. However, the yearly intensity of irrigation will be the sum of the two i.e. equals to 75 + 50 = 120%. Thus, yearly intensity of irrigation can be more than 100%.

Factor Effecting Duty

The duty of water of canal system depends upon varieties of factors. The principal factors are as follows:

Methods and system of irrigation: In the perennial irrigation system, soil is continuously kept moist, and hence water required for initial saturation is less. Also, due to the shallow depth of water table, deep percolation losses are less. In the inundation irrigation, there is wasteful use of water. Hence, the perennial irrigation system has more duty than the inundation irrigation. The flow irrigation system has lower duty due to the conveyance losses in the network of the canals, while the lift irrigation system has higher duty because the command area of each well is very near to it. Tank irrigation gives high duty dur to rigid control.

Mode of Applying Water: The flood irrigation system has lesser duty than the furrow system. Sub-irrigation system gives still higher duty. The ring basin irrigation and uncontrolled flooding give less duty.

Mode of Cultivation: If the land is properly ploughed and made quite loose before irrigating, soil will have high water retention capacity in its unsaturated zone. Thus, the number of waterings can be reduced, increasing the duty. The old and the convention methods of cultivation gives less duty in comparison to modern method.

Time and frequency of Tilling: Frequency of cultivation reduces the loss of moisture through weeds. Soil structure affects the plant growth to a very great extent. A structure (i.e. the good arrangement of soil particles in relation to one another) is called good tilth of soil. When the soil is in good tilth, evaporation losses from the surface of soil is less, soil becomes properly aerated, and hence yield of the crop is better.

Type of the crop: The duty varies from crop to crop.

Base period of the crop: If the base period of the crop is more, the amount of water required is high, hence duty will be low.

Climatic condition of area: The climatic condition which effects the duty are temperature, wind, humidity and rainfall. Due the high temperature and wind, evaporation losses will be more and the duty will be less. A humid atmosphere reduces the losses. Rainfall during the crop period will reduce the irrigation water requirement and the duty will thus be higher.

Quality of water: If harmful salt content and alkali content of the water is more, water will to be applied liberally so that the salts are leached off. This will, in turn, reduce the duty.

Method of assessment of water: Volumetric method of assessment always leads to higher duty. This is because the farmer will use the water economically. If, however, the method of assessment is based on area under cultivation, the farmer has a tendency to use more water, and the duty will be low.

Canal Conditions: In an earthen canal, seepage and percolation losses will be high, resulting in low duty. If, however, the canal is lined, the losses will be less, and the duty will be high.

Character of soil and sub soil of the canal: If the canal is unlined and if it flows through coarse grained, permeable soils, the seepage and the percolation losses will be high. If the canal flows through fine grained soil, such losses will be less and the duty will be higher.

Character of soil and sub soil of the irrigation fields: If the soil and the sub-soil of the fields will be coarse grained, percolation losses will be high. However, if there is hard pan at depth 1 to 2 metre below the surface, the percolation loss reduces. The duty is also affected by the topography of the land. If the field is not level, the lower portions get more water, while the higher portions may remain drier. In order to supply water to the higher places, more water will be used, and the duty will be reduced.

Methods of Improving Duty

When once the various factors affecting duty are properly understood, the duty can be improved by making those factors less effective which tends to reduce the duty.

1. Suitable method of applying water to the crops should be used.
2. The land should be properly ploughed and levelled before sowing the crops. It should be given good tilth.
3. The land should be cultivated frequently, since the frequent cultivation reduces loss of moisture specially when the ground is within the capillary reach of ground surface.
4. The canals should be lined. This reduces percolation and seepage losses. Also, water can be conveyed quickly, thus reducing evaporation losses.
5. Parallel canals should be constructed. If there are two canals running side by side, the F.S.L will be lowered, and the losses will be reduced.
6. The idle length of canal should be reduced.
7. The alignment of the canal in sandy soil and fissured rock should be avoided.
8. The canals should be so aligned that the area to be cultivated are concentrated along it.
9. The source of the supply should be such that it gives good quality of water.
10. The rotation of the crop must be practised.
11. Volumetric method of assessment should be used.
12. The farmers must be trained in the proper use of water, so that they apply correct quantity of water at the correct timing.
13. The land should be redistributed to the farmers so that they get only as much land as they are capable of managing it.
14. Research station should be established in various localities to study the soil, the seed and the conservation of moisture.
15. The canals administrative staff should be efficient, responsible and honest. The operation of canals should be such that the farmers both at the head as well as at the tail end get water as and when they need.

Some Definitions

1. Kor Period and Kor Depth: The distribution of water during the period is however, nor uniformed. Crops require maximum water during first watering after the crops have grown few centimetres. During the subsequent watering the quantity of water needed by the crops gradually decreases and is least when the crop gains maturity. The first watering is known as kor watering, and the depth applied is known as kor depth. The portion of the base period in which kor watering is need is known as kor period. While designing the capacity of channel, kor water must be taken into account since discharge in the canal has to be maximum during this time.
2. Outlet Factor: It is defined as the duty at the outlet.
3. Time Factor: The time factor of the canal is the ratio between the number of days the canal has actually run to the number days of irrigation period.
4. Capacity Factor: This is the ratio between mean supply and full supply of the canal
5. Cumec Day: The quantity of water flowing for one day at a rate of 1 cumec is known as a cumec-day. It is equal to 8.64 hectare-metres.
6. Paleo: It is the first watering before sowing the crop. This is done to add sufficient moisture to the unsaturated zone of the soil and is required for the initial growth of the crop.
7. Full Supply Coefficient: It is defined as the areaestimated to be irrigated during the base period divided by the design full supply discharge of the channel at its head during maximum demand. This is also known as duty on capacity.
8. Nominal Duty: In some states, cultivators have to submit an application for water required for their land under irrigation, to the department in advance of every season. After a permit is granted, they have to pay full amount for the water irrespective of the area they actually irrigated. Nominal duty is, therefore, the ratio of the area of which the permit has been granted for the period divided by the mean supply of the base period.
9. Open Discharge: It is the ratio of number of cumec days to the number of days the canal has actually been used for irrigation.
10. Root Zone Depth: Root zone depth is the maximum depth of the soil strata in which the crop spreads its roots system and derives water from the soil.