How to use xt3d_off and xt3d_rhs correctly?

[mbakker7]

In the mf6.modflow.ModflowGwtdsp package you can specify xt3d_off and xt3d_rhs.
The first one is clear. When I specify xt3d_off=True then xt3d is turned off, which may be a good idea when you are working on a regular grid as it speeds up the code by about a factor of two (thanks telling us, martijn!).
But the second one, xt3d_rhs is unclear to me. The docs state 'xt3d_rhs (boolean) add xt3d terms to right-hand side, when possible. This option uses less memory, but may require more iterations.' So to turn this off, I guess I should type xt3d_rhs=False, which is the opposite of the xt3d_off term. Am I correct? Or is xt3d_rhs ignored when xt3d_off=True and it doesn't matter?

Incidentally, it is unclear what the default values (None) are for these parameters (i.e., what happens when a user doesn't specify these), as explained in Discussion #2453.

[christianlangevin]

@mbakker7, two different sources of confusion here: (1) how do these keywords work in MODFLOW, and (2) what flopy syntax is needed to set the keywords.

Regarding the MODFLOW side, these keywords are specified in the options block. They are either included in the options block or not. I can see why they might be confusing.

• Neither keyword is specified --- this means XT3D is on with the full left-hand-side and right-hand-side implementation. This is the default and should generally be used for most models.
• XT3D_OFF is specified --- The simplified dispersion with limiting assumptions is used. (BTW, it's more than just a regular grid that justifies this quick alternative. There is a pretty thorough description in the GWT modeling report.)
• XT3D_RHS is specified --- XT3D is on, but it is implemented only as right-hand-side terms.
• Both XT3D_OFF and XT3D_RHS are specified. MODFLOW 6 will apparently allow this. It uses the last one specified. We should probably terminate with an error if both are specified.

Regarding the flopy side for specifying these options, as @wpbonelli mentions, it's tricky and confusing but will work once the magic syntax is discovered. Keep in mind that the keyword is either written or not written to the options block, so it's confusing to have them set to none, true, or false. We are hoping to improve this in the next major release.

[mbakker7]

Thanks for the clear explanations, @langevin-usgs
I didn't realize that turning XT3D off uses a simplified dispersion approach. For our case, turning it off has little effect on the outcome, but significant effect on the runtime. I will definitely read up on it with the supplied reference.

Regarding translation of Flopy to MODFLOW, I am starting to understand the difference you are talking about. In Python we are used to set a variable to True or False. But in MODFLOW a parameter is included or not. So if something is not set in Flopy (i.e., the default None is used), that means that it is not written to the MODFLOW input file, and MODFLOW uses whatever the default is. I guess from a tranparancy point of view it would be desirable to always set all variables, also in the MODFLOW input file (so they are True or False, or whatever the option is). But that may require quite some changes in MODFLOW that you may not want to make.