Pressure-dependent Neumann Boundary condition

Tao,

I'm not sure how to solve your problem exactly, but I'm going to respond here assuming you were attempting to implement an atmospheric boundary condition at the top of a water column or unsaturated zone...

If you made a boundary element for the atmosphere with very large volume, you can change the pressure in that volume over time with a source (or sink) with suitable injection (or production) rates.  See Section 7.2 of the TOUGH 2 User's Guide, starting on page 62.  Once you have that pressure vs. time set up properly in that element, the evaporation occurring in the elements beneath it should take care of itself.

I hope this helps get you started!  If this doesn't describe your problem sufficiently, please send me more details about what you're trying to do.

Thanks!

- Mikey