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Water Requirement of Crops – Delta and Duty of Water

Water requirement of Crop

Water requirement of Crop

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The water requirement of the crop is defined as the total quantity and the mode of water required for a crop from the time it is sown to the time it is harvested.

The water requirement varies from one crop to another and also with the agriculture land. Hence, water requirement differs with crops and the same crop demands different water requirement for different places of cultivation.

Every crop planted demands a certain amount of water at certain intervals of time throughout their growth period. If this demand is met by the rainfall at a sufficient amount, then there is no requirement of irrigation water. If not, irrigation is necessary to meet the demands of the crops. Crops require sufficient water at their required time in order to nourish and yield properly.

Here, certain parameters necessary to determine the water requirement of crops are explained briefly.

Crop Period and Base Period

Crop Period

The time period from the sowing of the crop to the instant of its harvesting is called a crop period.

Base Period

The time period from the first watering of the crop during its sowing to the last watering of the crop before its harvesting is called a base period. It is also called a base of the crop.

Mostly, the crop period is greater than the base period. Practically, both terms are considered same and are expressed in days. In the calculation of water requirements of crops, the crop period, base period and the growth period are considered same and represented by ‘B’ in days.

Duty and Delta of a Crop


It is defined as the total depth of water required by a particular crop to attain its maturity condition. It is represented by delta in cm.

Every crop requires a certain amount of water at certain intervals throughout its period of growth. The time interval between such consecutive watering is called as “Frequency of irrigation” or “Rotation period”. Every time, the water depth required by the crop varies from 5 to 10cm which again varies with the type of crop. The rotation period can also vary in the range of 6 to 15 days depending on the type of crop.

Hence, the total depth of water required during the crop period represents the total quantity of water required for its complete nourishment. This total quantity of water required by the crop measured in hectare-meter or acre-ft or million cubic meters or million cubic ft.

The concept of the delta can be explained by a small example.

If rice requires about 10cm depth of water at an average interval of 10 days and the crop period of the rice is 120 days, Determine delta?

Crop Period, B = 120 days

Interval of watering = 10days

Number of Intervals demanded by the crop = 120/10 = 12 Intervals

The depth of water at each interval = 10cm

Delta = Total depth of water throughout the crop period = 12 x 10 = 120cm

Duty of Water (D)

Duty is defined as the number of hectares of land that is irrigated for the complete growth of a crop by supplying 1-meter cubes per second of water continuously throughout the crop or base period ‘B’ of the respective crop. Duty gives a relation between the volume of water and the area of the crops that are harvested.

For example, 200 hectares per cumec to the base of B days means that the water flowing at a rate of 1 cubic per meter cube per second running continuously for B days, matures 200 hectares of crop.

Relation Between Delta and Duty

The relationship between duty and delta is given by the formula:

Also Read: Water distribution techniques in Irrigation

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