Actually there is a diagram included osmotic potential and saturation percentage in Handbook no. 60 and the relationship is linear and one can obtain the empirical equation from it , however this relationship depend on the companion of salts which is largely variable which mean it is more complicated to obtain an equation.

I characterized the real availability of water for plants through a curve of humidity liberation from the substrate that I used in my experiment, the curve was generated accordind to the method proposed by Campell et al. (2001) the title is "An improved method for creating moisture release curves in dry soils to determine meneralogical and physical properties", converting the content water (in percentage) into water potential (MPa), check it out and good luck.

Thank you very much......