Vegetation: Transport Processes Inside and Outside of Plants - Transport in the Atmosphere-Vegetation-Soil Continuum - page 214

1.0

T low

α rw

T high

0.0
0.0 h 4 h 3l h 3h h 2 h 1

Soil water pressure head

Figure 6.7 Dimensionless reduction coeficient for root water uptake, α rw , as a func-
tion of soil water pressure head h and potential transpiration rate T p (Feddes et al.,
1978 ).

1.0

EC slope

α rs

0.0

EC max

0.0

Soil water electrical conductivity

Figure 6.8 Dimensionless reduction coeficient for root water uptake, α rs , as a func-
tion of soil water electrical conductivity EC (Maas and Hoffman, 1977 ).

declines linearly with the electrical conductivity of the soil water (Maas and Hoff-
man, 1977 ). The actual root water extraction rate S ( z ) (d -1 ) is then derived by multi-
plying the reduction factors for water ( α rw ) and salt stress ( α rs ) with the potential root
water extraction rate:

() = () () ()

Sz αα

rw

z z Sz

(6.17) rs p

Finally, the actual plant transpiration rate T a (mm d -1 ) is derived by integrating S ( z )
over the rooting depth:

∫ 0

= () ∂
−

T S zz (6.18)

a

D

r

Let us take an example of an irrigated crop with combined water and salt stress.
Consider a corn crop with a linear decline of the root density over a rooting depth of
80 cm. On a particular summer day potential transpiration T p = 8.0 mm d -1 . Table 6.3