Models in Irrigation andWater Management - Practices of Irrigation and On-farm Water Management - page 410

- Determine maximum yield ( Y m ) of the crop as dictated by climate, under
conditions of full water requirement.
- Calculate the maximum evapotranspiration (ET m ) that prevails when crop water
requirements are fully met by the available supply.
- Quantify the effect of water deficit on the crop yield by the relationship between
relative yield decrease and relative ET deficit:

1 − Y a / Y m = k y (1 − ET a /ET m ) (10.33)

or,

Y a ( i )
Y m ( i )

1 −

k y ( i )

= (10.34)

ET a ( i )
ET m ( i )

1 −

Jensen Model

According to Jensen (1968), the effect of water deficit during certain growth stages
on grain yield is

ET i
ET m

λ i

n

Y
Y m =

(10.35)

i = 1

where

Y grain yield (t/ha)

Y m =

=

maximum yield from the plot without water stress during the growing
season (t/ha)
ET i = actual evapotranspiration during the growing season stage i (mm)

ET m = maximum evapotranspiration corresponding to Y m (mm)

λ i = sensitivity index of crop to water stress (-)

i = growth stage (i = 1ton)

n = total number of growth stages

Determination of Sensitivity Index ( λ i )

The sensitivity index ( λ i ) can be determined as follows:

The above form of production function, expressing the effect of water defi-
ciency on yield at different stages of growth, in the form of power function, can
be expressed as (Tsakiris, 1982):

m

y
y 0 =

w λ i
i

(10.36)

i = 1