Campbell (1974) describes the soil water retention function with the following equation:

where ψm (J kg−1) is the soil water pressure head, ψe (J kg−1) is the air entry potential (known as a parameter, too), θ (m3 m−3) is the volumetric water content, θs (m3 m−3) is the soil volumetric water content at saturation and b is a shape parameter related to particles dimension distribution in porous means.

This equation is discontinuous at air entry potential and can be analytically integrated and differentiated only for water content below ψe .

where ψm (J kg−1) is the soil water pressure head, ψe (J kg−1) is the air entry potential (known as a parameter, too), θ  (m3 m−3) is the volumetric water content, θsat (m3 m−3) is the soil volumetric water content at saturation and b is a shape parameter related to particles dimension distribution in porous means.

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