The NWT Solver parameters are shown in the image below. Detailed information about MODFLOW-NWT can be found:

http://water.usgs.gov/ogw/modflow-nwt/

NOTE: MODFLOW-NWT includes an upstream-weighting (UPW) intercell conductance package as a replacement internal flow package to those provided by the BCF, LPF, and HUF Packages. When MODFLOW-NWT is selected as the flow engine, the UPW property package will be generated during translation and used in the model run.

VMOD Flex provides options for the most-commonly used parameters (head change criterion, flux change criterion, number of iterations, etc.) and to choose the solver OPTIONS (Automate Solver Options). MODFLOW-NWT provides flexibility to specify the values for each of the parameters, or to choose from one of the pre-defined solver “schemes”. When you select one of the pre-defined schemes, then the solver parameters are fixed and cannot be modified (all fields will be non-visible). If choose the "SPECIFIED" option, then the full set of solver parameters will be defined in the .NWT file (using defaults) and you will have the ability to change these parameters in the .NWT file after Translation. When you choose one of the pre-defined schemes (Simple, Moderate, or Complex) then you will see only an abbreviated set of parameters.

•SPECIFIED indicates that the optional solver input values listed for items 1 and 2 will be specified in the NWT input file by the user.

•SIMPLE indicates that default solver input values will be defined that work well for nearly linear models. This would be used for models that do not include nonlinear stress packages, and models that are either confined or consist of a single unconfined layer that is thick enough to contain the water table within a single layer. (See Table 2 of package documentation for the solver input values that will be used for this option.)

•MODERATE indicates that default solver input values will be defined that work well for moderately nonlinear models. This would be used for models that include nonlinear stress packages, and models that consist of one or more unconfined layers. The “MODERATE” option should be used when the “SIMPLE” option does not result in successful convergence. (See Table 2 of package documentation for the solver input values that will be used for this option.)

•COMPLEX indicates that default solver input values will be defined that work well for highly nonlinear models. This would be used for models that include nonlinear stress packages, and models that consist of one or more unconfined layers representing complex geology and surface water/groundwater interaction. The “COMPLEX” option should be used when the “MODERATE” option does not result in successful convergence. (See Table 2 of package documentation for the solver input values that will be used for this option.)

HEADTOL (units of length)—is the maximum head change between outer iterations for solution of the nonlinear problem.

FLUXTOL (units of length cubed per time)—is the maximum root-mean-squared flux difference between outer iterations for solution of the nonlinear problem.

MAXITEROUT is the maximum number of iterations to be allowed for solution of the outer (nonlinear) problem.

THICKFACT is the portion of the cell thickness (length) used for smoothly adjusting storage and conductance coefficients to zero.

LINMETH is a flag that determines which matrix solver will be used.

•1 = GMRES will be used

•2 = xMD will be used

IBOTAV is a flag that indicates whether corrections will be made to groundwater head relative to the cell-bottom altitude if the cell is surrounded by dewatered cells. Checked = a correction will be made; Not-checked = no correction will be made. This input variable is problem specific and both options (IBOTAV selected/not-selected) should be tested.

NOTE: The modification to the .WEL package for variable PHIRAMP is currently not supported in Visual MODFLOW

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