Duty: The term duty means the area of land that can be irrigated with the unit volume of irrigation water. Duty represents the irrigating capacity of a unit.

Delta: It is the total depth of the water required by a crop during the entire period the crop is in the field and is denoted by the symbol ?.

Base Period: Base for a crop refers to the whole period of cultivation from the time when irrigation water is first issued for preparation of the ground for planting the crop, to its last watering before harvesting.

Relation between Duty and Delta

? =8.64 B/D meters.

D = duty in hectares/cumec

? = total depth of water supplied in meters

B= base period in days

= (8.64 × 100)/400

? = 2.16 m

Converting into mm

? = 2.16 × 1000 = 2160 mm