Crop water requirement, Operational water requirement and Irrigation water requirement.

 Crop water requirement is the water used by the crop for its survival. Plants use water for photosynthesis by which it produces carbohydrates which serves as food for the plants. Water is transfered into the leaves through roots of the plants, after utilization of some amount of water for photosynthesis the rest of the water comes at the surface of the leaves and finally it gets vaporized into the atmosphere which is known as transpiration. While transpiration occurs at the same time Evaporation from the surrounding surfaces such as: open water surfaces, bare soil or vegetative cover occurs. These two contributes for the crop water requirement which is generally measured under the name called 'Evapotranspiration' or 'Consumptive use'.

Operational water requirement is the water used for land preparation, percolation losses in flooded paddy fields. For irrigation water requirement, it must be supplied in addition to the crop water requirement.

Field water requirement:  It is the sum of crop water requirement and the Operational water requirement.

Net irrigation water requirement(NIR): It is the amount of water obtained after deducting the Effective rainfall from the Field water requirement. Effective rainfall is the portion of reliable rainfall that contributes to meeting the water requirement of the crop. It is generally taken as the 80% reliable rainfall multiplied by suitable factor.
          P-eff = f* P(80%)
f=0.70 for upland crops
f=0 if p(80%)<5 mm
f = .85 if 5mm < p(80%) < 100mm
f = .70 if p(80%) > 100mm

Gross irrigation water requirement, commonly said as irrigation water requirement, is the amount of water needed at the diversion intake.
 Irrigation system while conveying water losses water in the form of seepage, spillage and evaporation.
From the intake to the field application, water irrigation system has different efficiencies in its different component. It has field applicaton efficiency (E- field), on- farm distribution channel efficiency(E-farm), and the main canal efficiency(E-main).
Gross irrigation water requirement( I-gross)= NIR/ (E-field*E-farm*E-main).