Model name

Mathematical equation

van Genuchten (1980)

$S_r=\frac{1}{{\left(1+{\left(a\text{ψ}\right)}^n\right)}^m}$

Fredlund and Xing (1994)

$S_r=\frac{1}{{(\text{In}(\text{e}+{\left(\frac{\text{ψ}}{a}\right)}^n))}^m}$