Pressure scaling parameter with units of pressure.
Corresponds to van Genuchten parameter \rho g / alpha. The inverse of \alpha "corresponds to the water contents lower then saturation" [1].
[1] van Lier, Quirijn de Jong, and Everton Alves Rodrigues Pinheiro. "An alert regarding a common misinterpretation of the van Genuchten α parameter." Revista Brasileira de Ciência do Solo 42 (2018).
double