duty in irrigation( delta duty relationship)

2 Types of Duty In Irrigation : How to Improve Duty? (Numerical, Importance & Factors Affecting Duty)


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 a given crop by a supply of 1 m³/sec of water continuously during the entire base period of the crop.

duty in irrigation

Duty of water( duty in irrigation) is generally expressed in hectare/cumec and is denoted by the letter D.


✔ Mathematically,



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 


B = Base period of crop

Δ = Delta of crop



  1. Types Of Duty In Irrigation  

Duty can be classified as:


a. Flow duty

✔ In direct irrigation; water from the source is directly supplied to the irrigating field without storing it. Hence, the duty of water in direct irrigation is called flow duty.

✔ It is expressed in hectare/cumec.


b. Quantity duty or Storage duty

✔ In storage irrigation, water from the source is supplied to the reservoir or storage unit and then the required amount of water is supplied to the irrigation field from the storage unit. Hence, the duty of water in storage irrigation is Quantity Duty / Storage Duty.

✔ Quantity duty is expressed in hectares /millions of cubic meters of water available in a reservoir. It means that every million cubic meters of water available to the reservoir will mature so many hectares of a particular crop.

✔ The irrigating capacity of the reservoir is directly known which helps in determining the duty.



  2. Factors Affecting The Duty In Irrigation  

Duty in  irrigation depends upon the following subsequent factor:


duty in irrigation


1. Type of crop

The water demand for numerous crops is completely different. So, the duty varies from crop to crop.

The duty is less for a crop requiring a high amount of water and vice versa.


2. Climate & Season 

The duty varies from season to season and also from time to time in the same season. The duty also includes the water lost in evaporation and percolation.

Losses due to evaporation and percolation vary with climatic conditions. The values of duties which are generally expressed are their average values considered in the entire crop period.

Duty in the winter season & rainy season is more and less in the summer season.


3. Rainfall

More rainfall leads to less requirement for irrigation water.

This means, that if rain falls during the crop period then duty will be more and vice versa.


4. Soil type 

The duty in irrigation will be less for permeable soil and vice versa.

In coarse-grained soil; percolation is high, so duty is low.

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 the 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 in the case of the perennial irrigation system as compared to the 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 be low and vice versa.

As the ground slope increases, the duty decreases as a result of the wastage of water.


8. Base period

If the base period is longer, the water requirement will be more and 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.



  3. Importance of Duty  

The knowledge of duty helps engineers in designing efficient canal irrigation systems.

Knowing the duty of different crops and their areas helps in estimating the discharge required for designing the irrigation canal.



  4. Methods 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 leveled 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 & skillful.

8. The land should be regularly cultivated.

9. Reduction of transmission losses. The canal should be taken close to the irrigable land to minimize transmission losses.



  5. Relationship of Duty with Delta and Base Period  

Let us consider a crop having a base period of B days. Assume that 1 cumec of water is supplied to this crop for B days to provide delta Δ  m depth of water.

Now the volume of water supplied by unit discharge  to this crop during B days is given by:

V=Q (m³/s)*B (days)

V= 1*24*60*60*B m³   { B days = 24*60*60 Seconds  & Q = 1 m³/s  }

V=8.64*10000 B m³   …(1)

The volume of water applied to the field can also be calculated as:

V=Area of land irrigated by unit discharge * depth of water applied

V=D(ham)*Δ (m)

V=D *Δ*10000(m³)   ….(2)

From Eqn 1 & 2 , we get

8.864*10000*B= D*Δ*10000




  6. Duty – Numerical Example  

Calculate duty based on the following data

Base peroid=110 days


Delta(Δ)=68cm = 0.68m


From delta duty relationship;


D= 8.64* 110/0.68

∴ D = 1398 ha/cumec


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