RETC fitting

A discussion forum for RETC users. RETC is a program which can be used to analyze the soil water retention and hydraulic conductivity functions of unsaturated soils.
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TimoSamuel
Posts: 11
Joined: Mon Feb 27, 2017 1:48 pm
Location: United Kingdom

RETC fitting

Post by TimoSamuel » Mon Feb 27, 2017 2:41 pm

Hi Jirka,

I am trying to refit three curves that have been derived from HYPROP-fit. With the Dual Porosity soil hydraulic model and the following settings:
ThetaR ThetaS Alpha n l Ks w2 Alpha2 n2
0.002 0.89 0.2823 1.937 0.5 669 0.288 0.0594 1.224
which are derived from the an averaged curve over the three samples that I am refitting.
I have reduced the actual HYPROP output (of 501 rows) since RETC was not providing the final results for the nonlinear least squared final results due to the large datasize.
So I manage to get a relatively good fit with the data after quite some trials, see the following pictures. However, as you can see the Ksat amounts to unrealistic high numbers.
In order to try and find out what caused this, I adjusted the Ksat values. The differences in settings for fit one is that I had 332 cm/day as an Ksat estimate, and for fit two 669 cm/day. (the value of the averaged curve).
I got these fits with unchecking theta r, theta s, w2 and n2 within the soil hydraulic parameters section.
Would you have an idea where I go wrong? I left the rows weight open, perhaps here? Also the relative weight of K vs. theta is quite untransparent to me.
Looking forward to your reply.

Timo

(I wanted to upload the same graphs for fit two, but I was restricted by the limit of 3 attachments)
Attachments
DualPorosityFitSWRC.png
Fit one SWRC graph
DualPorosityFitSWRC.png (24.21 KiB) Viewed 3467 times
DualPorosityFitK.png
Fit one K graph
DualPorosityFitK.png (20.19 KiB) Viewed 3467 times
DualPorosityFit.png
Fit one values
DualPorosityFit.png (8.6 KiB) Viewed 3467 times
Last edited by TimoSamuel on Tue Feb 28, 2017 12:11 pm, edited 5 times in total.

TimoSamuel
Posts: 11
Joined: Mon Feb 27, 2017 1:48 pm
Location: United Kingdom

Re: RETC fitting

Post by TimoSamuel » Mon Feb 27, 2017 2:43 pm

Here, the additional graphs on fit two.
Attachments
DualPorosityFitIISWRC.png
Fit two SWRC graph
DualPorosityFitIISWRC.png (22.08 KiB) Viewed 3466 times
DualPorosityFitIIK.png
Fit two K graph
DualPorosityFitIIK.png (19.64 KiB) Viewed 3466 times
DualPorosityFitII.png
Fit two results
DualPorosityFitII.png (8.62 KiB) Viewed 3466 times

TimoSamuel
Posts: 11
Joined: Mon Feb 27, 2017 1:48 pm
Location: United Kingdom

Re: RETC fitting

Post by TimoSamuel » Mon Feb 27, 2017 3:08 pm

Ok stupid mistake, I have figured it out already. Being relatively new in specifically soil physics I have some trouble with the units from time to time.
I do wonder still when you should/shouldn't use w2 and n2 parameters , since the smaller amount of parameters fitted increases the reliability of the model right?
In fact, the best fit occurs when I leave theta r, theta s, w2 and n2 unchecked within the soil hydraulic model and assigning a weighting factor between K and theta data of 1 (as in the example for retention/conductivity fit)
However, this leads to a noticeable higher value for Ks (compared to checking all dual porosity parameters, which gives a lower fit and lower Ksat value). Which leads to the following question, if I were to use the first fit, what implications would this have when using these parameter values as input within Hydrus, as I will not be able to fit in those parameters while it has been fitted with a dual porosity model?

Regards,

Timo

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