Irrigation requirement of crops id the quantity of water, exclusive of rainfall, required by a crop in a given time period for their normal growth under filed condition. It includes evapotranspiration not met by the rainfall, it also includes surface run off and percolation losses
Consumptive Irrigation Requirement (CRI) :
CRI is the amount of irrigation water that is required in order to meet the evapotranspiration (consumptive use) needs of a crop during its full growth. So, nothing but the consumptive use itself, but exclusive of effective precipitation, stored soil moisture or ground water.
When the last two are ignored, then we can write
CRI = Cu - Re
Where,
Cu = Consumptive use of water
Re = Effective rainfall during growth period of crop
Net Irrigation Requirement (NIR) :
NIR is the amount of irrigation water required to be delivered at the field to meet the evapotranspiration needs of crop as well as other needs like leaching, presowing requirements etc.
NIR = CIR + LR + PSR
Where,
LR = Leaching requirement
PSR = Presowing requirement
Field Irrigation Requirement (FIR) :
FIR is the amount of water required to meet the NIR plus the amount of water lost as surface runoff and deep percolation.
Where,
ᶯa = Water application efficiency
Gross Irrigation Requirement (GIR) :
GIR is the amount of water required to meet the FIR plus the amount of irrigation water lost in conveyance through canal system by evaporation and seepage.
Where,
ᶯa = Water conveyance efficiency
