Basic Options Tab: Newton Solver

The options in the Newton Solver that are always required are on the Basic Options tab of the NWT: Newton Solver pane of the the MODFLOW Packages and Programs dialog box under Solvers.

HEADTOL is the maximum head change between outer iterations for solution of the nonlinear problem. The recommended default value is 10-4 meters. The value should be adjusted to match the units used in the model.

FLUXTOL is the maximum root-mean-squared flux difference between outer iterations for solution of the nonlinear problem. The recommended default value is 500 meters3/day. The value should be adjusted to match the units used in the model.

MAXITEROUT is the maximum number of iterations to be allowed for solution of the outer (nonlinear) problem. If the Lake package is used, more iterations may be required.

THICKFACT is the portion of the cell thickness (length) used for smoothly adjusting storage and conductance coefficients to zero (symbol Ω in equation 9).

LINMETH is a flag that determines which matrix solver will be used. A value of 1 indicates GMRES will be used and a value of 2 indicates ChiMD will be used (integer).

IPRNWT is a flag that indicates whether additional information about solver convergence will be printed to the main listing file.

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 (integer). A value of 1 indicates that a correction will be made and a value of 0 indicates no correction will be made. This input variable is problem specific and both options should be tested.

OPTIONS is used to activate one of the following options.

•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 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 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 sw/gw interaction. The “COMPLEX” option should be used when the “MODERATE” option does not result in successful convergence. (See table 2 for the solver input values that will be used for this option.)

•SPECIFIED indicates that the optional solver input values listed on the other tabs will be specified in the NWT input file by the user.

CONTINUE allows MODFLOW to continue to run even if it fails to converge. See also: the STOPERROR option in MODFLOW Options dialog box.