Vegetation: Transport Processes Inside and Outside of Plants - Transport in the Atmosphere-Vegetation-Soil Continuum - page 213

Table 6.2 Critical pressure head values h 1 - h 4 of the reduction factor for root
water uptake α rw ( Figure 6.6 ) for some main crops (Wesseling, 1991)

Crop h 1 h 2 h 3, high h 3, low h 4

Potatoes -10 -25 -320 -600 -16 000

Sugar beet -10 -25 -320 -600 -16 000

Wheat 0 -1 -500 -900 -16 000

Pasture -10 -25 -200 -800 -8000

Corn -15 -30 -325 -600 -8000

The value of h 3.high applies to a high transpiration rate; the value of h 3.low to a low transpiration
rate.

at the microscopic approach, the root water uptake rate is limited by the product of
hydraulic conductivity and gradient of soil water pressure head (Darcy's law). Most
macroscopic root water uptake models simplify the reduction of root water uptake,
by deining a dimensionless reduction factor as function of either soil water con-
tent, soil water pressure head or soil hydraulic conductivity (Feddes and Raats, 2004 ;
Hupet et al., 2004 ). A much used reduction function is the one formulated by Feddes
et al. ( 1978 ), which is shown in Figure 6.7 . At soil water pressure heads above h 2 the
root water uptake is reduced because of oxygen deiciency. Below h 3 water uptake is
reduced because of too dry conditions. When the atmospheric demand for transpira-
tion is higher, the reduction of transpiration rate will start at higher water contents.
Therefore the parameter h 3 depends on T p . For an indication of the parameter values
h 1 - h 4 , see Table 6.2 . Although the parameters h 1 - h 4 are commonly deined with
respect to crop type, they are to a certain extent also affected by soil texture.
In addition to water stress, plants may experience stress due to high salt concen-
trations, which cause a certain osmotic head. Thinking in head differences, it might
seem logical to add the osmotic head to the pressure head in the soil, and derive the
reduction factor as function of the total head. However, pressure head and osmotic
head have entirely different impacts on the soil hydraulic conductivity, which plays a
key role in root water uptake. In the case of a lower pressure head, the soil hydraulic
conductivity will decrease. In the case of a higher osmotic head, the soil hydraulic
conductivity is hardly affected. Therefore the concept based on differences in the sum
of hydraulic and osmotic head can be applied only with additional parameters that
account for the different impact on the hydraulic conductivity (Skaggs et al., 2006 ).
A pragmatic solution to derive root water uptake in the case of both water and salt
stress, is by considering the effects of both stresses separately, and multiply dimen-
sionless reduction factors (Cardon and Letey, 1992 ). We previously discussed the
reduction factor for water stress ( Figure 6.7 ). A simple and much used reduction fac-
tor for salt stress is depicted in Figure 6.8 . Below a critical value for the soil water
electrical conductivity no reduction occurs; above this threshold the reduction factor