Van Genuchten (1980) describes the soil water retention function with the following equation:

where Se is the effective degree of saturation (0-1), θ is the volumetric soil water content (m3 m−3), α, n, m are empirical constants affecting the shape of the retention curve and ψ is the soil water pressure head (J kg−1). θr and θs are respectively, the residual volumetric soil water content and the volumetric soil water content at saturation.

where θr is residual volumetric soil water content and θs is the soil volumetric water content at saturation.

Frequently a restriction for n and m values, setting m=1-1/n or m=1, allows for an easier formulation of unsaturated water conductivity.

The component SoilWater calculates the water potential from the water content, so it uses the inverse function.

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