Tuzet Soil Moisture (Water Potential) Stress
This page documents the Tuzet stress function used to scale leaf net assimilation and stomatal conductance as a function of plant water potential. It provides a smooth, sigmoidal decline in conductance with decreasing (more negative) leaf/xylem water potential. See Tuzet et al. [28] and Duursma and Medlyn [29]. The ClimaLand code can be found here.
Summary
- Purpose: compute a water‑status stress multiplier $\beta \in [0,1]$ from leaf (or xylem) water potential.
- Inputs: leaf/xylem water potential $\psi$ (Pa), parameters controlling the midpoint and slope.
- Outputs: scalar stress factor $\beta$ applied within stomatal/photosynthesis sub‑models.
Model formulation
We compute the stress factor using the following formulation:
\[\beta = \min\Bigg(1, \frac{1 + e^{s_c \, p_c}}{1 + e^{s_c \, (p_c - p_{leaf})}}\Bigg)\]
with
The leaf water pressure $p_{leaf}$ (Pa),
The reference water pressure $p_{c}$ (Pa),
The sensitivity to low water pressure $s_{c}$ (Pa$^{-1}$).
Variables & units
| Quantity | Symbol | Units | Notes |
|---|---|---|---|
| Leaf water pressure | $p_{leaf}$ | Pa | typically ≤ 0 (tension) |
| Reference potential | $p_{c}$ | Pa | location of curve midpoint |
| Sensitivity | $s_{c}$ | Pa⁻¹ | slope/steepness of response |
| Stress multiplier | $\beta$ | – | 0 (full stress) … 1 (no stress) |