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

• MODFLOW-NWT contains the UPW flow package. The UPW flow package is based on the LPF package but differs in that the rewetting and vertical conductance correction options are not available. It also differs from the LPF package in that it smoothes the horizontal conductance and storage change functions to have a continuous derivatives so they can be solved by the Newton method. Using the UPW package requires using the NWT solver.

2. NWT Solver

• MODFLOW-NWT contains the NWT solver package. Using the NWT solver requires twice the memory of other MODFLOW solvers because using the Newton method results in an asymmetric matrix. Because of the asymmetric matrix a different matrix solver must be used. The NWT package provides an option to use one of two matrix solvers: a generalized-minimum-residual (GMRES) solver and an Orthomin/stabilized conjugate-gradient solver called xMD (chi-MD). 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.