The options that are required if Options is set to "Specified" are on the Additional Options tab of the NWT: Newton Solver pane of the the MODFLOW Packages and Programs dialog box under Solvers.
DBDTHETA is a coefficient used to reduce the weight applied to the head change between nonlinear iterations (symbol θ in equation 21). DBDTHETA is used to control oscillations in head. Values range between 0.0 and 1.0, and larger values increase the weight (decrease under-relaxation) applied to the head change.
DBDKAPPA is a coefficient used to increase the weight applied to the head change between nonlinear iterations (symbol κ in equation 22). DBDKAPPA is used to control oscillations in head. Values range between 0.0 and 1.0, and larger values increase the weight applied to the head change.
DBDGAMMA is a factor (symbol γ in equation 19) used to weight the head change for iterations n–1 and n. Values range between 0.0 and 1.0, and greater values apply more weight to the head change calculated during iteration n.
MOMFACT is the momentum coefficient m of equation 20 and ranges between 0.0 and 1.0. Greater values apply more weight to the head change for iteration n.
BACKFLAG is a flag used to specify whether residual control will be used. .
MAXBACKITER is the maximum number of reductions (backtracks) in the head change between nonlinear iterations. A value between 10 and 50 works well.
BACKTOL is the proportional decrease in the root-mean-squared error of the groundwater-flow equation used to determine if residual control is required at the end of a nonlinear iteration, as applied in equation 23 .
BACKREDUCE is a reduction factor (symbol Br in equation 23) used for residual control that reduces the head change between nonlinear iterations (real). Values should be between 0.0 and 1.0, where smaller values result in smaller head-change values.
