Crop water requirement

• It can be defined as the quantity of water regardless of its source, required by a crop in a given period of time for its normal growth under field conditions.
• Water requirement includes the losses due to evapotranspiration (ET) or consumptive use (CU) plus losses during the application of irrigation water and the quantity of water required for special operations such as land preparation, transplanting, leaching, surface runoff, percolation, etc.

It may be formulated as

WR= ET or CU + application losses + Special needs

• Water requirement is therefore a demand and the supply would consists of contribution from any of the source of water; the major source being the irrigation water (IW), effective rainfall (ER) and soil profile contribution (S) including that from shallow water tables.

Numerically, water requirement (WR) = IR + ER + S

• The field irrigation requirement of a crop therefore, refers to the water requirement of crops, exclusive of effective rainfall and the contribution from soil profile and it may be given as

IR= WR- (ER + s)

• IR depends upon the irrigation needs of individual crops, their area and the losses in the farm water distribution systems, mainly by seepage.
• The IR of an outlet command area also includes the irrigation requirement of individual farm holding and the losses in the conveyance and distribution system.