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NoNoBoy

One question about the “Material Properties”

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Hollow, sir.

I have a question about the “Material Properties”. My model use one kind of the soil “silt”. When I input the soil “silt” to simulate, I can get one kind of the “moisture content”, “relation content” and “water capacity” datas. But I use the data in chap 5 of guiding book. (SM:0.46, RM:0.034, α:0.106, β:1.37.) I can’t get the same result. Why? Can you explain this? Thank you!

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Ok you're talking about Soil properties for unsaturated zone in FEMWATER (SP1 card) (next time be more specific)

Yes I agree the preset Silt in GMS:FEMWATER is different from the parameters in Table 5 in the FEMWATER manual.

The parameters in the manual are just examples, maybe they changed the defaults for GMS.

Or there may be an error in the manual or in GMS.

I notice that the alpha parameter is inverse-length-dependent, maybe they don't adjust the default value properly for units in GMS.

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I can confirm thet there seems to be a bug in GMS regarding calculating van Genuchten unsaturated material properties for FEMWATER.

I calculated van Genuchten curves for "Loamy Sand" using the equations and parameters in the FEMWATER manual (page 52-53).

When I compared these to the curves produced by FEMWATER for the same situation, it seems that GMS is not adjusting the alpha parameter for the length scale. The FEMWATER table of alpha values is given in 1/cm, this needs to be changed if a different length scale is being used.

According to the GMS wiki http://www.xmswiki.com/xms/GMS:FEMWATER_Model_Input:

"Note also that the units of the van Genuchten 'alpha' have been 'hard-wired' into GMS. Because of this the values should always be entered in units of 1/cm. GMS will convert to the model units accordingly."

There are two ways to specify van Genuchten release curves. Either use a preset soil type or enter the parameters manually. If you enter the alpha parameter in units of 1/cm both ways give the same result. However, the wiki quote above claims that GMS converts alpha to the appropriate length scale before calculating, but it is not doing this.

You can workaround this bug by entering the parameters manually with alpha in the correct units.

Edited by woodward
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Correct me if I am wrong, but I understand that van Genuchten characteristic curves, while realistic and commonly used, present difficulties in practice. They are highly non-linear, which means that a fine spatial scale is required to achieve convergance and/or good solutions. Vogel & Ippisch (2008) Estimation of a critical spatial discretization limit for solving Richard's Equation at Large Scales. Vadose Zone Journal 7(1) 112-114. So how do people handle this?

Edited by woodward
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