Soil water potential equations

[tsutchi]

Dear Jirka,
I am trying to learn about soil water potential through Soil Physics with Hydrus. For gravitational potential it is given as z = zsol - z0 (page 37) but for hydrostatic component it is given as h = z water table - z sol (page 40). Why it is not h = zsol - z water table? I could not find an easy explanation for this.
Thank you

[Jirka]

Note that hydrostatic potential needs to be positive (reflecting the weight of water above the selected position). Also, note that the z-axis is positive upward. As a result, our formula leads to positive values, while your formula would give negative values.

Example:
z water table = -100 cm (100 cm below the soil surface)
z soil = -200 cm (200 cm below the soil surface)
our formula: h = z water table - z sol = -100 – (-200) = 100 cm
your formula: h = zsol - z water table = -200 –(-100) = -100 cm (i.e., tension).