GMS:MODFLOW-NWT

MODFLOW-NWT is a version of MODFLOW based on MODFLOW-2005 that provides a different formulation of the groundwater flow equation designed to solve models that are non-linear due to unconfined cells or non-linear boundary conditions.

Choosing MODFLOW Version

Choosing to use MODFLOW 2000, MODFLOW 2005, or MODFLOW-NWT is done in the MODFLOW Global Options Dialog. At any time you can switch from one to the other.

Changes to MODFLOW 2005

1. UPW Package

• The Upstream Weighting package (UPW) is the flow package used with MODFLOW-NWT. The UPW package is only used with the NWT Solver Package.

2. NWT Solver

• MODFLOW-NWT contains the NWT solver package. The NWT solver must be used when using the UPW flow package.

3. WEL Package

• The WEL package was modified to add an option to decrease the pumping rate as the head drops below a user-specified percentage of cell thickness. The new option is called PHIRAMP.

4. SFR Package

• The SFR package is modified to avoid removing water from dry cells and to calculate derivatives for the Newton formulation.

5. UZF Package

• The UZF package is modified to avoid removing water from dry cells and to calculate derivatives for the Newton formulation.

6. Other Packages

• Poor input values for other packages can result in convergence failure. For example, an EVT extinction depth below the cell bottom elevation can result water being improperly removed from a dry cell.