#### Dynamic SPAC approach

In this approach the change of water storage in the plant,
*S*_{p}, is calculated
during the day. The change of plant water storage is defined as:

(3.43)

where *q*_{upt} is the water uptake rate
calculated with an equation similar to the steady-state SPAC approach, eq. (3.37), but now
without the direct connection to the potential demand:

(3.44)

where *p*_{excess} is a parameter determining the flow rate in excess
of the potential demand from the atmosphere and *f*_{pmax} is a
function that gives the maximal plant water storage as a function of LAI of the
plant (see below). This parameter corresponds to the compensatory uptake rate from a single layer.

Note that in this approach the additional compensatory
uptake mechanism that was included in the previous two more simplistic
approaches are not applicable since the uptake rate is governed by a potential
gradient and not a flux as in the previous approaches.

Since the SPAC-based formula is now used to calculate water
uptake, the
transpiration is instead given as:

(3.45)

where *f(**ψ*_{l}) is a function that controls
the opening of the stomata as a function of the leaf water potential, *ψ*_{l}:

(3.46)

where *ψ*_{min}_{ }and *ψ*_{th}* *are parameters.

The leaf water potential is a linear function of the plant
water storage given as:

(3.47)

where *S*_{p}_{}_{} is the actual active
plant water storage and *f*_{pmax} is a function that gives the
maximal plant water storage as a function of LAI of the plant (if “f(LAI)” has
been chosen):

(3.48)

where *p*_{psl}_{}_{}_{ }_{}_{}is a parameter.
Alternatively the plant height may also be included in the function as (if
“f(LAI, height)”has been chosen):

(3.49)

where *p*_{pslh}_{}_{} is a parameter similar
to *p*_{psl}_{}_{}*.*

Salt is treated analogous to the steady-state SPAC
approach.