The duty of water is the relationship between the volume of water and the area of land it matures. It may be defined as the area of land (in hectares) irrigated for full growth of given crop by a supply of 1 m3lsec of water continuously during the entire base period of the crop. Duty of water( duty in irrigation) is generally expressed in hectare cumecs and is denoted by letter D `mathematically,
Where D=Duty in hectare/cumec
A=Area in hectares
Q=Discharge in m3/s
Duty can also be expressed as D=8.44 B/Δ
Types of duty in Irrigation:
Duty can be classified as:
In direct irrigation, duty is always expressed in hectare cumec, it is then called flow duty.
2. Quantity duty or storage duty
In storage irrigation, duty may sometimes be expressed in hectares /million cubic meters of water available in a reservoir. It means that every million cubic meter of water available to the reservoir will mature so many hectares of a particular crop. Hence the irrigating capacity of the reservoir is directly known, When duty is expressed in this manner, then it is called quantity duty.
2. Factors affecting the duty of water in Irrigation
Duty of irrigation water depends upon the subsequent factor:
1. Type of crop
The water demand for numerous crops is completely different than the duty varies from crop to crop. The duty is going to be less for a crop requiring additional water and the other way around.
2. Climate & Season
The duty varies from season to season and also time to time in the same season. Since duty also includes the water lost in evaporation and percolation. Losses due to evaporation and percolation vary with climatic condition. The values of duties which are generally expressed are their average values considered in the entire crop period.
3. Useful rainfall
More the helpful precipitation less is the need for irrigation water, and hence additional is the duty of irrigation water.
4. Soil type
The duty of irrigation water will be less for permeable soil and vice versa.
In coarse-grained soil; percolation is high, so less duty. In fined grained soil; percolation is less, so high duty. So the duty of clayey soil is more than the duty of sandy soil.
5. Method of ploughing
Proper ploughing of land reduces the number of watering & increases duty. But for faulty cultivation & improper ploughing, duty is decreased.
6. Method of irrigation
The duty of water is high just in case of the perennial irrigation system as a compared inundation irrigation system.
7. The topography of agricultural land
If the agricultural land is uneven, the water requirement will be more and hence duty will below. As the ground slope will increase, the duty decreases as a result of there’s wastage of water.
8. Base period
If the base period is longer, the water requirement will be more, duty will be low and vice versa.
9. Skill of cultivation
A skilled & properly trained cultivator can make judicious use of water and therefore duty will be high.
Importance of duty
The Knowledge of duty helps engineers in designing efficient canal irrigation system. Knowing the duties of different crops and their areas, the discharge required for designing the irrigation canal can be estimated.
Methods for improving duty/ Measures for improving duty
1. Ploughing, the land to proper depth increases the moisture-retaining capacity of the soil. Hence duty gets improved.
2. Reduction in percolation loss. The canal should be lined to reduce percolation losses.
3. Land used for cultivation should be levelled as far as possible.
4. Manure fertilizers should be added to increase the water-holding capacity of the soil.
5. Rotation of crops will ensure increased crop yield with minimum use of water.
6. Method of irrigation should be efficient.
7. The cultivators should be trained & skilful.
8. The land should be regularly cultivated.
9. Reduction of transmission losses Canal should be taken close to the irrigable land to minimize transmission losses.