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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


where,

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


Now,


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


Therefore,

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.


Crop

Base period (days)

Delta (cm)

Duty (ha/cumec)

Rice

120

90 to 120

735

Wheat

135

40

1500

Sugarcane

300 to 360

120

750


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?


Solution:


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