Soil water potential equations

A discussion forum for book "Soil Physics with HYDRUS"
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tsutchi
Posts: 26
Joined: Fri Mar 07, 2008 12:18 pm
Location: Japan

Soil water potential equations

Post by tsutchi » Thu Sep 19, 2024 12:40 pm

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
Posts: 6151
Joined: Sat Mar 16, 2002 3:47 pm
Location: USA
Location: Riverside, CA

Re: Soil water potential equations

Post by Jirka » Thu Sep 19, 2024 4:13 pm

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).

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