Transpiration and Plant Water Uptake
141
0.0 0.2 0.4 0.8 0.8 1.0 1.2
1.4
1.6
1.8 2.0
Dimensionless Leaf or Soil Water Potential
FIGURE 9.8. Dimensionless water uptake and loss.
Equation (9.15) is already in a dimensionless form. The dimensionless
transpiration rate can be defined as E* = Ep/Ep,,. The ratio of the
potentials is the same as the ratio ofthe dimensionless potentials. Equation
(9.15) is also plotted in Fig. 9.8 and it can be seen that the declining part
of Eq. (9.15) is closely approximated by a straight line. The equation of
the line is
The maximum or potential uptake rate for a given soil water potential is
at the intersection of the uptake and loss lines. Solving Eqs. (9.17) and
(9.18) simultaneously to find that point, U*,, gives
The actual rate of uptake cannot be higher than this value, but it can be
lower if the evaporative demand of the atmosphere is lower. The actual
transpiration rate of the plant canopy is therefore equal to the minimum
of the evaporative demand of the atmosphere and Epm U*, .
These ideas can be related to the depletion of the sod moisture by
defining yet another dimensionless quantity, the available water fraction.
The available water fraction can be defined as
where 0 is average water content of the root zone and the subscripts
indicate field capacity and permanent wilting water contents. For the
simplest case, where it is assumed that water content of the root zone is
141
0.0 0.2 0.4 0.8 0.8 1.0 1.2
1.4
1.6
1.8 2.0
Dimensionless Leaf or Soil Water Potential
FIGURE 9.8. Dimensionless water uptake and loss.
Equation (9.15) is already in a dimensionless form. The dimensionless
transpiration rate can be defined as E* = Ep/Ep,,. The ratio of the
potentials is the same as the ratio ofthe dimensionless potentials. Equation
(9.15) is also plotted in Fig. 9.8 and it can be seen that the declining part
of Eq. (9.15) is closely approximated by a straight line. The equation of
the line is
The maximum or potential uptake rate for a given soil water potential is
at the intersection of the uptake and loss lines. Solving Eqs. (9.17) and
(9.18) simultaneously to find that point, U*,, gives
The actual rate of uptake cannot be higher than this value, but it can be
lower if the evaporative demand of the atmosphere is lower. The actual
transpiration rate of the plant canopy is therefore equal to the minimum
of the evaporative demand of the atmosphere and Epm U*, .
These ideas can be related to the depletion of the sod moisture by
defining yet another dimensionless quantity, the available water fraction.
The available water fraction can be defined as
where 0 is average water content of the root zone and the subscripts
indicate field capacity and permanent wilting water contents. For the
simplest case, where it is assumed that water content of the root zone is