Contents preface (VII) introduction 1—37
Table 5.4 Discharge characteristics of parshall flumes
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- 5.13. ASSESSMENT OF CHARGES FOR IRRIGATION WATER
| Table 5.4 Discharge characteristics of parshall flumes Q = kh n (9)
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1 |
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Throat |
Discharge range |
k |
n |
Head range, m |
Modular | ||
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width b |
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limit |
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minimum maximum |
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minimum |
maximum |
h2/h1 | |
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(l/s) |
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1 in |
0.09 |
5.4 |
0.0604 |
1.55 |
0.015 |
0.21 |
0.50 |
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2 in |
0.18 |
13.2 |
0.1207 |
1.55 |
0.015 |
0.24 |
0.50 |
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3 in |
0.77 |
32.1 |
0.1771 |
1.55 |
0.03 |
0.33 |
0.50 |
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6 in |
1.50 |
111 |
0.3812 |
1.58 |
0.03 |
0.45 |
0.60 |
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9 in |
2.50 |
251 |
0.5354 |
1.53 |
0.03 |
0.61 |
0.60 |
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1 ft |
3.32 |
457 |
0.6909 |
1.52 |
0.03 |
0.76 |
0.70 |
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1 ft 6 in |
4.80 |
695 |
1.056 |
1.538 |
0.03 |
0.76 |
0.70 |
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2 ft |
12.1 |
937 |
1.428 |
1.550 |
0.046 |
0.76 |
0.70 |
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3 ft |
17.6 |
1427 |
2.184 |
1.566 |
0.046 |
0.76 |
0.70 |
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4 ft |
35.8 |
1923 |
2.953 |
1.578 |
0.06 |
0.76 |
0.70 |
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5 ft |
44.1 |
2424 |
3.732 |
1.587 |
0.06 |
0.76 |
0.70 |
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6 ft |
74.1 |
2929 |
4.519 |
1.595 |
0.076 |
0.76 |
0.70 |
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7 ft |
85.8 |
3438 |
5.312 |
1.601 |
0.076 |
0.76 |
0.70 |
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8 ft |
97.2 |
3949 |
6.112 |
1.607 |
0.076 |
0.76 |
0.70 |
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m3/s |
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10 ft |
0.16 |
8.28 |
7.463 |
1.60 |
0.09 |
1.07 |
0.80 |
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12 ft |
0.19 |
14.68 |
8.859 |
1.60 |
0.09 |
1.37 |
0.80 |
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15 ft |
0.23 |
25.04 |
10.96 |
1.60 |
0.09 |
1.67 |
0.80 |
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20 ft |
0.31 |
37.97 |
14.45 |
1.60 |
0.09 |
1.83 |
0.80 |
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25 ft |
0.38 |
47.14 |
17.94 |
1.60 |
0.09 |
1.83 |
0.80 |
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30 ft |
0.46 |
56.33 |
21.44 |
1.60 |
0.09 |
1.83 |
0.80 |
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40 ft |
0.60 |
74.70 |
28.43 |
1.60 |
0.09 |
1.83 |
0.80 |
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50 ft |
0.75 |
93.04 |
35.41 |
1.60 |
0.09 |
1.83 |
0.80 |
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For measurements, the current meter is mounted on a rod and moved vertically to measure the velocity at different points. The speed of rotation of cups or blades depends on the velocity of flow. The instrument has an automatic counter with which the number of rotations in a given duration is determined.
The current meter is calibrated by moving it with a known speed in still water and noting the number of revolutions per unit of time. During measurement, the current meter is held stationary in running water. Using the appropriate calibration (supplied by the manufacturer) the velocity can be predicted. By this method one can obtain velocity distribution and, hence, the discharge. Or, alternatively, one can measure the velocity at 0.2 h and 0.8 h (here, h is the depth of flow) below the free surface and the mean of the two values gives the average velocity of flow. Sometimes, velocity at 0.6 h is taken as the average velocity of flow.
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CANAL IRRIGATION |
207 |
5.12.4. Other Methods
Mean velocities in open channels can, alternatively, be determined by measuring surface velocities using surface floats. The surface float is an easily visible object lighter than water, but sufficiently heavy not to be affected by wind. The surface velocity is measured by noting down the time the surface float takes in covering a specified distance which is generally not less than 30 metres and 15 metres for large and small channels, respectively. The surface velocity is multiplied by a suitable coefficient (less than unity) to get the average velocity of flow.
A double float consists of a surface float to which is attached a hollow metallic sphere heavier than water. Obviously, the observed velocity of the double float would be the mean of the surface velocity and the velocity at the level of the metallic sphere. By adjusting the metallic sphere at a depth nearly equal to 0.2 h above the bed, the observed velocity will be approximately equal to the mean velocity of flow.
Alternatively, velocity rods can be used for the measurement of average velocity of flow. Velocity rods are straight wooden rods or hollow tin tubes of 25 mm to 50 mm diameter and weighted down at the bottom so that these remain vertical and fully immersed except for a small portion at the top while moving in running water. These rods are either telescopic-type or are available in varying lengths so that they can be used for different depths of flow. As the rod floats vertically from the surface to very near the bed, its observed velocity equals the mean velocity of flow in that vertical plane.
For measuring discharge in a pipeline, one may employ either orifice meter or venturi meter or bend meter or any other suitable method.
5.13. ASSESSMENT OF CHARGES FOR IRRIGATION WATER
Irrigation projects involve huge expenditure for their construction. The operation and maintenance of these projects also require finances. With the introduction of irrigation facilities in an area, farmers of the area are immensely benefited. Hence, it is only appropriate that they are suitably charged for the irrigation water supplied to them.
The assessment of irrigation water charges can be done in one of the following ways: (i) Assessment on area basis,
(ii) Volumetric assessment,
(iii) Assessment based on outlet capacity, (iv) Permanent assessment, and
(v) Consolidated assessment.
In the area basis method of assessment, water charges are fixed per unit area of land irrigated for each of the crops grown. The rates of water charges depend on the cash value of crop, water requirement of crop, and the time of water demand with respect to the available supplies in the source. Since the water charges are not related to the actual quantity of water used, the farmers (particularly those whose holdings are in the head reaches of the canal) tend to overirrigate their land. This results in uneconomical use of available irrigation water besides depriving the cultivators in the tail reaches of the canal of their due share of irrigation water. However, this method of assessment, being simple and convenient, is generally used for almost all irrigation projects in India.
208 IRRIGATION AND WATER RESOURCES ENGINEERING
In volumetric assessment, the charges are in proportion to the actual amount of water received by the cultivator. This method, therefore, requires installation of water meters at all the outlets of the irrigation system. Alternatively, modular outlets may be provided to supply a specified discharge of water. This method results in economical use of irrigation water and is, therefore, an ideal method of assessment. However, it has several drawbacks. This method requires the installation and maintenance of suitable devices for measurement of water supplied. These devices require adequate head at the outlet. Further, there is a possibility of water theft by cutting of banks or siphoning over the bank through a flexible hose pipe. Also, the distribution of charges among the farmers, whose holdings are served by a common outlet, may be difficult. Because of these drawbacks, this method has not been adopted in India.
The assessment of canal water charges based on outlet capacity is a simple method and is workable if the outlets are rigid or semi-modular and the channel may run within their modular range.
In some regions, artificial irrigation, though not essential, has been provided to meet the water demand only in drought years. Every farmer of such a region has to pay a fixed amount. The farmers have to pay these charges even for the years for which they do not take any water. A farmer has also to pay a tax on the land owned by him. In the consolidated assessment method, both the land revenue and the water charges are combined and the cultivators are accordingly charged.
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