(1) Available moisture for the plant 
(2) Readily available moisture for the plant = FC - Mo
Here FC = field capacity
φ = wilting point or wilting coefficient below plant can’t survive.
Mo = Readily available moisture content
(3) Frequency of Irrigation
(4) 
where, weight of water stored in soil of unit area 
Weight of some soil of unit area 
dw = depth of water stored in root zone.
(5) 
dry unit wt. of soil
(6) Available moisture depth to plant
(7) Readily available moisture depth to plant
(8) 
where, G = specific gravity and n = porosity
Duty and delta
Duty: The duty of water is the relationship between the volume of water and the area of the crop it matures.it is defined as the area irrigated per cumec of discharge running for base period B. the duty is generally represent by D.
Delta: it is the total depth of water required by a crop during the entire base period and is represented by the symbol Δ.
Relation between duty and delta
Where, Δ = Delta in meter
D = Duty in Ha/cumec
B = Base period in days
Also 
Where, Δ = Delta in meter
B = Base period in days
D = Duty in acre/cures
Consumptive use determination by use of equations
The following are some of the commonly used methods.
1. Penman-method
2. Blaney-Criddle method
3. Hargreaves class A pan evaporation method.
Blaney-Criddle method: Blaney-Criddle equation expresses the consumptive use in terms of temperature and day time hours. If CU is monthly consumptive use, its value is given by Cu = K.t.(inches)
Where, k = crop factor to be determined for
Each crop; its value depends upon
Certain environmental conditions
f= monthly consumptive use factor
= t×(p/100)
t = mean temperature in oF.
p = percentage of day time hours of the year, occurring during the period.
If Expressed in metric units, the above formula becomes:
Where, t = temperature in oC
Cu = monthly consumptive use in cm
Irrigation Efficiencies
(1) Water conveyance Efficiency (ηc)
where, wf = water delivered to the filed.
wr = water delivered from the reservoir.
(2) Water Application Efficiency (ηa)
Where, ws = water stored in the root Zone.
wf = water delivered to the filed.
(3) Water use Efficiency (ηu)
Where, wu = water use consumptively
wf = water delivered to the field.
(4) Water Storage Efficiency (ηs)
Where, ws’ = Actual water stored in the root Zone.
wn = water needed to store to bring the water content up to field capacity.
wn = FC – Available moisture
(5) Water Distribution Efficiency, (ηd)
Where, y = Average numerical deviation in the depth of water stored from the average depth of irrigation stored.
d = Average depth during irrigation
(6) Consumptive use Efficiency (ηcu)
Where, Wcu or Cu = Water used by plant consumptive.
wd = Net amount of water depleted from root Zone.
Irrigation Requirements of crops
(1) Consumptive irrigation requirement (CIR)
CIR = Cu - Peff
Where, Cu = total consumptive use requirement
Peff = Effective rainfall.
(2) net irrigation requirement (NIR)
NIR = CIR + Leaching requirement
(3) Field irrigation requirement (FIR)
(4) Gross irrigation requirement, (GIR)
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