CXT-FIT and Hydrus-1D.

[vss0015@auburn.edu]

Greetings Dr. Jirka,

I am using CXT-FIT to model tracer transport in a 25 cm column. I am also using Hydrus-1d for the same data. Shouldn't the Dispersivity value or dispersion co-efficient values for both the models be same.

I am aware that CXT-FIT gives us effective dispersion value whereas Hydrus-1D gives us longitudinal dispersivity.

For CXT-fit, I am first calculating Darcy's flow velocity and beta from the data and using them as inputs. Omega and Dispersion co-efficient are optimised. From your paper: "Comparison of CXTFIT and Hydrus-1D projects ", I was able to convert omega to alpha which goes as an input into the hydrus-1d model.
To calculate dispersivity from dispersion co-efficient in cxt-fit, I am using the following equations.

calculations.xlsx

Calculations

(12.5 KiB) Downloaded 819 times

Now, using Darcy's flux in hydraulic parameters, thlimb and alpha in reaction parameters (already calculated from the data), I only optimise for dispersivity.


My cxt-fit and hydrus dispersivity values are close but not the same?

Could you guide me if I am missing out something.

Thank you for your time.

With Regards,
Vishawjot Singh Sandhu

[Jirka]

As you may expect, we have used the analytical solutions (in CXTFIT) to do the verification of HYDRUS in its early stages of development. We have done that for many examples, and posted one of them on the HYDRUS website in the HYDRUS library, under the name CXTFIT:
https://www.pc-progress.com/en/Default.aspx?h1d-library
https://www.pc-progress.com/Downloads/P ... Hydrus.pdf

You need to write the equation solved by HYDRUS and CXTFIT next to each other, look how the parameters are defined, and make sure that you do conversions between these parameters correctly. This may be complicated by the fact that HYDRUS uses real units, while in CXTFIT, you can use either real or dimensional parameters. We provide a detailed description on how this should be done in this example.

Jirka

[vss0015@auburn.edu]

Thank you Dr. Jirka for your time and suggestions. I will start looking into it right away.

With regards,
Vishawjot Singh Sandhu

[vss0015@auburn.edu]

Greetings Dr. Jirka,

I want to optimise only 2 parameters in CXT-FIT MIM model.

Darcy's velocity will be easily calculated using Darcy's Flux. To calculate beta we need mobile water content. To get mobile water content, we first calculate mobile pore velocity. This was done by dividing the time it takes C/Co = 0.5 to reach the bottom of the column/ length of the column.
Dividing the flux by mobile water velocity gives us mobile water content.

Is this approach right?

Thank you sir.

[Jirka]

Only approximately. This also depends on dispersion, length of the column, and upper BC (see van Genuchten and Parker, 1984). J.

[vss0015@auburn.edu]

Thank you sir. That was really helpful.

Have a nice day.

[vss0015@auburn.edu]

Greetings Dr. Jirka,

I have used the Non-dimensional MIM model in CXT-FIT. In this model, mobile Dispersion co-efficient, Omega and Beta were optimized and the Confidence intervals were positive.

Using the formulae in "Comparison of CXTFIT and Hydrus-1D projects", theta mobile was calculated from Beta and alpha was calculated from Omega.
Mobile dispersivity was calculated from the mobile dispersion co-efficient. Using the formulae given by Bear,1972.

These Alpha and ThetaIM (ThetaS-ThetaM) values were used in the Hydrus model and mobile dispersivity was optimised.

1. There is a very small difference in the values of dispersivity from the two models. Is there any reason for that or I am doing anything wrong?
2. Both the MIM models in CXT-FIT and Hydrus, calculate mobile dispersion and mobile dispersivity? I am sure about CXT-FIT (van Genuchten and Wagenet, 1989), what about hydrus MIM model?

I am attaching both the models, I would really appreciate if you can check them and let me know If am doing something in the calculations.

Thank you.

[Jirka]

I do not think that I will find time to look into it. You should be able to figure it out based on the example that I have referred you to. J.