Each model needs to include one Solver package. Each solver package defines a different algorithm used to solve the model equations.
The Preconditioned Conjugate Gradient (PCG) package is the most commonly used solver package because it is fast and does not require excessive memory.
The Preconditioned Conjugate Gradient Solver with Improved Nonlinear Control (PCGN) package provides additional controls that can be useful with difficult problems.
The Geometric Multigrid (GMG) package is sometimes preferable to the PCG solver for large, steady state models because it can be faster and in some cases it can reach a satisfactory solution when other solvers do not. It requires more memory than the PCG package.
The Strongly Implicit Procedure (SIP) package is another solver commonly used in MODFLOW models.
The Direct Solver (DE4) package is another solver used in MODFLOW models. It is best suited to linear models. (Linear models must not have any convertible layers and must not use any of the boundary conditions in which the flow is a non-linear function of head. Of the head-dependant flux packages, only the General Head Boundary package has a linear relationship between floe and head.)
The Newton Solver (NWT) package can only be used in MODFLOW-NWT and MODFLOW-OWHM. It can only be used with the Upstream Weighting UPW) package. and when the Upstream Weighting package is used, The Newton Solver package must be used.
The Iterative Model Solution package can only be used in MODFLOW 6
Modelers are advised to investigate how varying the solver parameters affect the predictions of interest in the model. If making the convergence criteria stricter causes a significant change in the important model predictions, the less strict criteria were not strict enough.
