Irrigation Water Requirement of Crops | Relation Between Duty, Delta and Base Period

Water requirement of crops is the total quantity of water required or demanded by a crop from the time it is sown till it is harvested. Irrigation is the process by which the water is artificially supplied to irrigation fields in order to meet crop demands. In this blog, the water requirement of crops is discussed in detail.

Crop Period

The crop period is the time interval between the instant of sowing the crop to the instant of harvesting the crop. It depends on the crop type.

Base Period (B)

The base period is the time between the first watering of the crop and the last watering of the crop before harvesting. The base period is always lesser than the crop period for a particular crop.

Rotation Period

Crops require water at different intervals of times throughout their growth period. The time interval between such watering is called the rotation period or frequency of irrigation. This depends on the type of crop.

Delta (Δ)

Delta is the total depth of water supplied to the crop during its base period. It is expressed in cm. It can be found by using the formula below.

Delta = Total volume of water supplied / Total area of the land

Duty (D)

Duty is the ratio of irrigated area to the discharge of water supplied. It is also called the outlet factor of the canal. It is usually expressed in hectares per cumec. There are different names given to duty based on the units in which it is expressed.

  • Flow duty or Canal duty - when duty is expressed in hectares per cumec

  • Storage duty, Quantity duty or Reservoir duty - when duty is expressed in hectares per million cubic metre

Duty is mathematically expressed as,

Duty = Area of land / Discharge of water

Duty is lowest at the head of the main canal as the discharge is higher at the main canal. Duty is the highest in the field i.e., discharge is much lower on the field.

Relation Between Delta, Duty and Base period

The relation between the three important terms i.e., delta (Δ), duty (D) and the base period (B) is expressed as shown below.

Δ= (8.64 * B) / D


Δ is the delta in metres

D is the duty of water in hectare/cumec

B is the base period in days

The relationship can be derived as follows.

Let 1 cumec of water be supplied for 'B' days i.e., base period, then the total volume of water supplied is given as,

V = 1 * B * 24 * 60 *60 = 86400 * B cubic metres

Let this amount of water be used for an area of D hectare (ha) of land.

D ha of land = D * (10^4) m^2


Δ = V / A = (86400 * B) / (D * (10^4))


Δ= (8.64 * B) / D, Δ in metres

Δ= (8.64 * B) / D, Δ in centimetres

Base period, delta and duty for some of the common crops are tabulated below.


Base period (days)

Delta (cm)

Duty (ha/cumec)



90 to 120







300 to 360



Example Problem

Question: The total irrigation depth of water required by a certain crop in its entire growth period of 150 days is 25.92 cm. The culturable command area for a distributor channel is 10,000 hectares. The discharge for which the canal is to be designed is?


Delta is given as, Δ = 25.92 cm = 0.2592 m

Base period is the growth period, B = 150 days

Duty is found as D = (8.64 * 150) / (0.2592

Therefore, D = 5000 ha/cumec

This implies that one cumec of water can irrigate up to 5000 hectares of land. Therefore, the discharge required to cultivate 10,000 cumecs of field is given as,

Minimum discharge = 100,000/5000 = 20 cumecs

But the canal is to be designed for higher capacity as the obtained discharge is assumed to be uniform without any peaks and losses. Therefore, the canal should be designed for a discharge of greater than 20 cumecs.

Practise Problem

For more insights do check out the video lecture below.

That's all for the day. There are still some important topics in irrigation engineering like field capacity, losses, gross command area etc. If you want us to cover those comment "Irrigation" in the form below. Or if you want us to cover any other topic do comment on the topic and we will cover it in our next blog.