In this article we will discuss about:- 1. Duty of Canal Water at Various Places of Measurement 2. Factors Affecting Duty of Water 3. Methods of Improving Duty 4. Methods of Determining.
Duty of Canal Water at Various Places of Measurement:
Duty of canal water is always expressed as so many hectares of area matured by one cumec of water flowing for base period. But still this definition is not complete. In duty figures it is necessary to state.
1. Base period and,
2. The place of measurement of duty.
If one cumec water can bring to maturity say 2000 hectares of wheat and base period of wheat is say 120 days, then this duty is written in figure as 2000 hectare/ cumec at the field for a base period of 120 days.
Canals are taken from rivers which may be quite far off from the area which they are going to irrigate. River water is diverted to the canal with the help of weir, barrage or dam. This diverted water reaches the fields for irrigation through a network of canals and distributaries.
Some of the flowing canal water is lost due evaporation and percolation in the way before it reaches the fields for irrigation. The water lost in the flow is known as conveyance losses. Conveyance losses do not remain constant but vary with the length of the canal. It is because of variation in the conveyance losses that duty of canal water also undergoes change.
Duty of canal water is maximum at the field channel and goes on decreasing as proceed towards the direction against the flow of water. Duty is minimum at the diversion head works of the main canal. This fact can be explained by considering a network of canals shown in Fig. 3.3.
Let A, B, C, D, be off taking points of main canal, branch distributary, and water course (outlet) respectively. Let E be the point on the water course from where water is flowing to the fields. Let water reaching point E be one cumec. In order to make one cumec water reach point E, some extra water has to be supplied from outlet point D as some water is lost during flow from D to E.
Let conveyance losses between D and E points be say 0.1 cumec. Thus to make available 1 cumec water at point E, 1.1 cumec water will have to be left from D. Similarly let conveyance losses between A and B, B and C, and C and D each again be 0.1 cumec. Then to make one cumec water available at E, 1.1 cumec will have to be left from D, 1.2 cumec from C, 1.3 cumec from B and 1.4 cumec from A.
If one cumec water at E is irrigating 2000 hectare in base period then duties of water at point E, D, C, B and A respectively will be 2000/1, 2000/1.1, 2000/1.2, 2000/1.3 and 2000/1.4 i.e. 2000, 1818.2, 1666.67, 1538.5, 1428.6 hectares. These duty figures clearly reveal that duty is maximum at field and goes on decreasing towards upstream side, and it is minimum at main canal head works.
The duty at the head of main canal is known as gross quantity at head of branch canal as lateral quantity, at outlet as outlet factor and lastly at the head of land being irrigated as net quantity.
Factors Affecting Duty of Water:
The duty of water depends upon following factors:
1. Soil Characteristics through Which Canal Runs:
If canal is unlined and soil through which it flows is coarse grained, seepage and percolation losses will be too much and duty of water will be reduced. If on the other hand canal is either lined or runs through fine grained soil, the losses will be considerably small and duty of water will be more.
2. Soil Characteristic of Fields:
If soil of the field is deep coarse grained, percolation losses will be more. If, however, a hard pan is present at a depth of 1 or 2 m, the percolation losses will be less and duty will be more.
3. Undulated Fields:
If fields which are to be irrigated are quite undulated, the duty of water will be less. Lower portions are filled with more water whereas higher portions get less depth of water.
4. Climatic Conditions:
Temperature, wind, rainfall and humidity are the climatic conditions which affect the duty of water. High winds, high temperature, less rainfall and less humidity conditions, require more of irrigation water and duty is thus reduced. If rainfall is taken into consideration while calculating duty figures, it is known as Duty inclusive of rainfall. If rainfall is not considered it is called Duty exclusive of rainfall.
5. Base Period of the Crop:
The crops having longer base period, require waterings for longer time and thus duty will be less and vice-versa.
6. Depth of Water or Delta:
The crops which require lesser overall depth (Delta) of water have more of duty. Delta is different for different crops. Hence duty of water depends upon type of crop.
7. Method of Application of Irrigation Water:
Method of application of irrigation water affects duty figures. Flood irrigation has lesser duty than Furrow irrigation. Sub-irrigation methods give highest duty.
8. Preparation of Fields:
If fields are prepared by deep ploughs, the water retention capacity of the unsaturated zone of soil gets increased. When such soils are irrigated, although they require more of water for irrigation, but the number of waterings are reduced. This also helps in increasing the duty, to some extent.
9. Position of Fields in Relation to Canal:
If fields are situated very near to the canal supplying water for irrigation, conveyance losses beyond outlet will be small and duty of water will be more.
10. Method of Assessment:
Assessment of irrigation if done on volumetric basis, the duty of water is bound to increase. If assessment is done on irrigated area basis, the farmers do not use water carefully and thus duty becomes low
11. Longitudinal Slope to Fields:
If fields are given a little longitudinal downward slope towards the farther end, duty of water is increased.
12. Awareness of Farmers:
If farmers are made to understand the importance of irrigation water, they use it more judiciously in their fields without allowing water to go waste. It helps in increasing the duty of water. Farmers maintain their water courses free from grass and other weed growths and do not allow water to go waste anywhere.
13. If water courses are lined and levels of water courses for each field are kept as per requirements, the duty of water increases considerably. In lined water courses having regular longitudinal slope, all the water coming out of outlet rushes to fields with more velocity and does not remain stationary anywhere and hence gives increased duty figures.
Methods of Improving Duty of Water:
If these factors are somehow made less effective the duty of water can be considerably increased.
Methods of improving duty have been enumerated as follows:
1. By selecting such method of applying water which would cause least wastage by way of seepage, percolation, and evaporation.
2. Land should be properly ploughed and levelled so that a thin sheet of water may evenly be spread over it.
3. The fields should be frequently ploughed. This measure reduces loss of moisture specially when ground water is within capillary reach of the ground surface.
4. Fields should be divided into small kiaries. Smaller the size of kiaries more will be the duty of water.
5. Area to be irrigated should be located as near the canal as possible. This reduces coveyance losses and duty is increased. In other words idle length of the canal should be minimum.
6. Volumetric method of assessment should be adopted.
7. Only that much land should be given to the farmers as they can carefully look after.
8. Farmers should be given training for judicious use of water.
9. All the canals, water courses should be lined.
10. The alignment of canal either in sandy soil or in fissured rocks should be avoided.
11. Severe punishments should be awarded for those who get irrigation water by unlawful means. Deliberate canal cuts should be prevented by resorting to effective patrolling of canals by the staff.
12. Water tax should be increased. This measure will force farmers to use water more economically.
13. Research stations should be established in adequate number and research in regard to moisture conservation, and economical use of water should be continuously carried out.
Methods of Determining the Duty of Water:
The duty of water can be found out by any of the following three methods:
1. Duty by Inductive Method:
This method is based mostly on past experiments and experiences. Various states have developed standard duty tables depending upon the methods of irrigation adopted by them. These tables are prepared usually not by considering chemical properties of soils and climatic conditions. Methods adopted by various states to determine discharge in the channels to meet the crop requirements vary.
Following are the methods mostly adopted:
(a) In this method the percentages of areas under different crops are worked out and water allowance for the entire C.C.A. for a particular channel fixed. Example of this method may be 0.17 cumec per 1000 hectares. This method of fixing the duty is practiced in Punjab, Haryana, and Rajasthan.
(b) In the method outlet discharge factors for different crops of the region are standardised. Based on outlet discharge factor, and crops grown in each season, discharges required for Kharif and Rabi seasons are separately worked out. The channel is designed for the maximum of the two discharges.
(c) In this method, based on the standard duties of various crops and area under cultivation in each crop, month-wise required discharges are worked out. The channel is designed to meet the maximum requirements of discharge, required in a month.
Example:
Determine the maximum discharge at the outlet of a distributary having following cropped areas and duties of water –
Solution:
Discharges required in cumec for maturing different crops are as follows:
This discharges required during different seasons are as follows:
It is seen from this table that maximum discharge of 10 cumecs is required during Kharif. Hence distributary should be designed for this discharge.
2. Critical Growth Period Basis:
This method is based on the concept that crops require different quantities of water at various stages of their growth. The water requirements of all the crops are usually maximum during initial growth and then at flowering period. It is minimum during maturing period.
Out of maximum demand periods, Kor period generally requires the maximum discharge. This is because Kor watering itself requires more water and more so this watering has to be supplied to the crop during the limited Kor period.
If crop plants do not receive adequate water during Kor period, yield of the crop is affected significantly. Depth of Kor watering U.P., Punjab and Haryana at outlet head, is usually taken as 13.3 cm, 16.50 cm and 19.10 cm respectively for wheat, sugarcane and rice.
The Kor periods for wheat, sugarcane and rice are specified as follows:
If any channel is capable to satisfy the needs of water during Kor period, it will satisfy the needs of all other periods of the crops.
Example:
Areas to be irrigated in Kharif and Rati are respectively 2000 and 3000 hectares. Kor depths for Kharif and Rabi crops are 19 cm and 13.5 cm and Kor periods are respectively 2 
weeks and 4 weeks. Find out the outlet discharge.
3. Consumptive Use of Water:
Consumptive use of water is also known as evapo-transpiration use of water by the crop. It is the depth of water consumed by the crop by evaporation and transpiration during the crop period. This depth of water also includes the water consumed by accompanying weed growth in the field.
Water or moisture supplied by dew or rainfall and which subsequently evaporates away without entering the plant system is also considered as part of the consumptive use. Thus consumptive use of water is the sum total of the quantity of water applied to the field by irrigation plus the quantity of water supplied by rainfall minus the quantity of water removed as surface run off and plus or minus the quantity of water absorbed by the root zone soil moisture neglecting the deep percolation losses.
The consumptive use expresses the sum total of the volume of water in evaporation and transpiration and can be expressed as hectare-metres or hectar-cm. for the growth of the plant. Once the consumptive use of water is known it is very easy to fix the duty of water.
Consumptive use of water by the crop is primarily governed by meteorological factors. As a consequence, the water requirements of different crops are found out largely by the length of their growth periods and seasonal changes in climate. The nature of soil does not affect the consumptive use of water by the crop but affects only the frequency and depth of irrigation.
Shallow soils require relatively more number of waterings of lesser depth, whereas deep soils require lesser number of waterings but of larger depths. But in both the cases, the amount of water required by the crops remains the same provided the climatic conditions are identical. Evapo-transpiration rate curve for potato crop has been shown in Fig. 3.4. Once the consumptive use of the crop is known, optimum requirements of water of large units can be easily found out.
i. Evaporation:
It is transfer of water from liquid state to vapour state. The rate of evaporation from the surface is proportional to the difference between the vapour pressure at the surface and the vapour pressure in the air over lying. When irrigation water is supplied to the field by flooding method, large amount of water is lost by direct evaporation from the surface of the soil without going into the life cycle of the plants.
ii. Transpiration:
It is the process by which plants dissipate water from the surface of leaves, stalks and trunks, during the period of their growth.
iii. Transpiration Ratio:
It is ratio of the weight of water transpired by the plant during its growth period to the weight of dry matter produced by the plant. When we talk of the dry matter produced by the plant, matter in roots is excluded from it. The average values of transpiration ratio for rice vary between 650 and 700 and that for wheat between 550 and 575.