What is Duty and Delta in irrigation | Factors affection on Duty

In this article, we explained what is duty and delta in irrigation, the relation between duty and delta, factors affecting the duty of crops, values of delta for various crops. And so more.

Please read the article till the end.

What is Duty and Delta in Irrigation?

Before understanding the duty and delta of crops, we understand what is crop period and base period in irrigation.

Crop period:

The time period between instant sowing of crops to instant harvesting of crops is known as crop period.

Base period:

The base period is defined as a time period between the first watering of crops at the time of its sowing to the last watering before its harvesting. It is also known as the base of the crop.

What is Delta in Irrigation?

Each crops required such amount of water during its period of growth in some fixed interval of time.

Generally, 5 to 6 cm of water required by crops in every interval of time during their base period, But it is also dependent on climatic conditions, type of crops, and soil.

The time interval between two such consecutive watering is called the frequency of irrigation or rotation period.

The summation of the total water depth supplied in the various intervals of time during its base period for the full growth of crops is known as a delta of crops.


Delta is defined as the total amount of water depth is required by the various crops for the full growth (maturity) during the base period.

Definition of delta of crops

Example of delta in irrigation:

If rice requires about 10cm depth of water at an average interval of about 10 days, and the rice is 120days, find out the delta for rice.


Water is required at an interval of 10days for a period of 120days. Evidently means that 12 NOS of watering are required, and each time, 10cm depth of water is required. Therefore, the total depth of water required

Delta (∆) = 12x10cm = 120cm

Hence delta for rice is 120cm.

Average Approximate values of Delta in India

Sr. NoCropDelta on Field
4.Garden Fruits60cm

What is Duty in irrigation?

Duty is defined as the number of hectares of land that should be irrigated for full growth of crops by supplying 1m3/sec of water continuously to the base period of that crop.

Definition of duty of crops

Lets understand duty of crops by one simple example.

If water flowing at rate of 100m3/sec, runs continuously to the base period, and matures 1500 hectare land of crops, then the duty of water for for that particular cop will be defined as 1500 hectares per cumec.

Relation between Duty and Delta of crops:

Let B = base period,

Let, One cumec of water supplied to the crop on the field to the base period.

So the volume of water is equal to

V = (1x60x60x24xB)m3

= 86400B

By the definition of duty, the D hectares of land mature by supplying 1m3/s water during the base period of crops.

This quantity of water (V) matures D hectares of land or 104 D square meter of area.

Total depth of water applied on this land

= Volume /Area

= 86400B/104 D

= 8.64B/D meters

This total depth of water in known as delta

∆ = 864B/D


  • ∆ is in cm
  • B is in days
  • D is duty in Hectares/cumec

Example of Duty:

Find the delta for a crop when its duty is 864Hectares /cumec on the field, the base period of this crop is 120days.


  • ∆ = 864B/D
  • B = 120days
  • D = 864hectares/cumec

∆ = 864×120/864

∆ = 120cm

Average Approximate values of Duty in India

CropDuty in hectares/cumec
Other Kharif1500
Hot Fodder2000

Factors affecting Duty of crops:

  • Type of crop
  • Climate and season
  • Useful rainfall
  • Type of soil
  • The efficiency of cultivation method

1. Type of crop

The duty is dependent on the type of crops because the various types of crops require different amounts of water. a crop requiring more water has a less flourishing area as compared to that requiring fewer water crops for the same supply of water.

2. Climate and Season:

Duty is also affected by climatic conditions because various factors like evaporation and percolation are causing water loss, which means loss of duty. And evaporation and percolation are varied from season to season.

Also, the duty is numbers of a hectare of land of crops are matures by supplying one cumec water during its base period. The water requirement of crops also varies from season to season, which means the duty is also varied from season to season. In winter the duty is more as compared to summer.

3. Useful Rainfall:

If the rain is directly falling over the irrigation land, is useful for the growth of the crops, which ultimately helps to reduce the irrigation water requirement by the crops for its full growth. This means, the more useful rainfall, less water requirement by the crops, and hence more will be the duty of irrigation water.

4. Type of Soil:

If the permeability of soil under the crops is high, the water lost due to percolation is more that is decreases the duty of irrigation water. so the soil permeability is directly inverse to the duty of irrigation water.

 If the permeability of soil is high the duty of irrigation water is less and the permeability of soil less the duty of irrigation water is more.

5. Efficiency of cultivation method:

If the cultivation method faulty or less efficient causes the wastage of irrigation water, due to that the duty of the irrigation water decreases. If the irrigation water is used economically, then the duty of water will improve, as the same quantity of water would be able to irrigate more area.

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