where qupt 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:
where pexcess is a parameter determining the flow rate in excess of the potential demand from the atmosphere and fpmax 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.
where Sp is the actual active plant water storage and fpmax is a function that gives the maximal plant water storage as a function of LAI of the plant (if “f(LAI)” has been chosen):
where ppsl is a parameter. Alternatively the plant height may also be included in the function as (if “f(LAI, height)”has been chosen):
where ppslh is a parameter similar to ppsl.
Salt is treated analogous to the steady-state SPAC approach.