feature request: PM for specific discharge

[cneyens]

I've read your paper with great interest, and have done some tests with this method, which worked very well.

Unless I'm mistaking, it is currently not possible to use specific discharges at cell nodes as performance measures. Since they are a function of hydraulic head, and MODFLOW 6 can readily calculate them (with the SAVE_SPECIFIC_DISCHARGE keyword), would it be possible to implement this? PM_keys could include qx, qy, qz and q (magnitude) at specified cells. Some use-cases include assessing parameter sensitivities on flow between aquifers or zones, groundwater seepage in wetlands, advective movement of a solute at a specified location...

I realize this may not be straightforward to implement, and I completely understand if it is not feasible or falls outside the scope of your project.

[jtwhite79]

I suspect this can be done if we can get the coorespinding derivatives worked out... @mhayek-spec what do you think?

[mhayek-spec]

Yes, this can be done. For each performance measure (qx, qy, qz, or q), we will need to calculate its derivative with respect to the hydraulic head.