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Duty of Irrigation | Formula and Numericals

Although many irrigation methods are available there are some terms like Duty, Delta and Base period. From the sowing to the harvest of a crop, the irrigation demands keeps varying. Now let's see those basic terms related to irrigation.

  • Delta: Depth of water required to raise a crop.

  • Duty: Area of land that can be irrigated with the unit volume of irrigation water.

  • Base period: The duration between the first watering (after sowing) to the last watering (before harvesting) represented in days.

  • Crop period: is the duration between sowing to harvesting represented in days. In calculation, we assume crop period and base period as the same.

  • Kor watering: The supply of water when the plant is still in the young or earlier stage.

  • Paleo Irrigation: Before the complete growth of the plant if the amount of moisture in the soil gets reduced/ dried, we resupply the water to a field, known as paleo irrigation.

For solving numerical, the formula mainly used to calculate base period, duty or delta is given.

Duty in ha/cumec, D = 8.64 x B / Δ

Here, B - the base period in days and Δ - delta in m


Now, let's see about the types of areas used for irrigation.


Gross Command area (GCA)


It is the total area that can be irrigated by the canal.


Culturable command area(CCA)


The actual area of land that can be irrigated among the GCA is called the CCA. CCA is of two types.

  • Culturable Cultivated Area: The effective or particular land of the Culturable Command Area that can be irrigated.

  • Culturable Uncultivated Area: The particular land of the Culturable Command Area, that has remained uncultivated.

There are some factors that affect the duty of a crop and let's see what they are.


Factors affecting Duty


Duty depends upon the following factors-

  • Type of crop

  • Type of soil

  • Cultivation methods

Duty varies from place to place, yet the Duty at the outlet is the main consideration in irrigation. The term which is used to compare the duty at the outlet and canal is the discharge factor. The formula for the discharge factor is


Discharge factor = Discharge at oultlet / Discharge at canal


As the duty varies from place to place, some methods have to be practised to improve the duty.


Methods to improve duty


Some of the methods to improve duty is given below.

  • Level land - It results in less seepage and so high duty.

  • Lined canal, Proper irrigation methods

  • Crop rotation - More yield with less water supply for the same crop, if we apply the crop rotation method.

  • Optimum utilization of Irrigation water - Supplying an adequate amount of water.








This diagram shows how crop yield varies according to the water depth adopted.


Irrigation efficiencies


There are three types of irrigation efficiencies.

  • Water conveyance efficiency: Due to transmission losses from channel to outlet (water gets absorbed by the soil, seepage losses, etc.).

  • Water application efficiency: Root zone held water vs Applied water.

  • Water storage efficiency: Water stored on rootzone vs Water required to bring it to the Field capacity.

Now, let's look into the various types of crops and crop seasons.


Crop seasons


Mainly there are two types of crop seasons - Kharif and Rabi

  • Kharif (Summer season): It is the season that requires more water. Some of the crops grown in this season include Rice, Cotton and Maize.

  • Rabi