Soil water retention models¶
The soil tool fits volumetric water content θ as a function of matric
potential ψ. All five models share two parameters:
- θr: the residual water content.
- θs: the saturated water content.
PhytKit starts them at the driest and wettest measured θ. ψ is in MPa unless
you say otherwise with --units (kPa, bar, m or cm of water).
Suction given as positive is treated as negative ψ, and water content given as
a percentage is converted to a fraction.
| Model | --model |
Shape |
|---|---|---|
| van Genuchten (1980) | vanGenuchten (default) |
\(\theta = \theta_r + (\theta_s - \theta_r)\,[1 + (\alpha\lvert\psi\rvert)^n]^{-m}\), with \(m = 1 - 1/n\) |
| Modified van Genuchten (Troch 1993) | modifiedVanGenuchten |
The same form with \(m = 1 + 1/n_1\) |
| Raats superclass (Raats 1992; Heinen & Bakker 2016) | raats |
\(\lvert\psi\rvert = b^*\,[(1 - S^\alpha)/S^\beta]^\gamma\), solved numerically for \(S\) |
| Brooks–Corey (1964) | brooksCorey |
\(S_e = (\psi_b/\lvert\psi\rvert)^\lambda\) drier than air entry, saturated wetter |
| Fractal (Li et al. 2025) | fractalSWRC |
Suction from fractal pore structure, with an adsorbed film |
The Raats superclass contains van Genuchten, Brooks–Corey and others as
special cases. The fractal model has a fractal dimension D between 1 and 2,
a maximum pore radius r_max in µm, and an adsorbed film fraction δ.
Some models also carry conductivity parameters that retention data alone
can't determine. Examples are α2 and n2 in the modified van Genuchten
model, λ in the Raats model, and the tortuosity L. These are fixed by default.
See Fit soil water retention and every parameter in Models and parameters.