Solver Options 1

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The Solver Options 1 pane is on the SWR: Surface-Water Routing Process Pane of the MODFLOW Packages and Programs dialog box.

The Solver Options 1 pane is used to specify parameters related to solving the surface-water flow equations.

Solver (ISOLVER): ISOLVER defines the solver used by the SWR1 Process to solve the defined surface-water equations for each reach group.

•ISOLVER = 1, LU decomposition using Crout’s algorithm (Crout, 1941) based on Press and others (1990, 1999).

•ISOLVER = 2, Biconjugate gradient stabilized method (bi-CGSTAB) based on Barrett and others (1994), and Kelley (1995).

•ISOLVER = 3, GMRES method based on Brown and Saad (1990), Jones and Woodward (2001), and Saad and Schultz (1986).

Maximum number of outer iterations (NOUTER): NOUTER defines the maximum number of outer iterations used to resolve non-linearities for all solvers. NOUTER must be > 0 and is generally less than 100.

Maximum number of inner iterations (NINNER): NINNER defines the maximum number of inner iterations used within the preconditioned biconjugate gradient solver (ISOLVER = 2) and GMRES solver (ISOLVER = 3). NINNER also defines the maxi­mum number of iterations to perform before restarting GMRES (ISOLVER = 3) and reduced storage requirements for the method. Typical values for the biconjugate gradient stabilized method range from 10 to 30. Typical values for the GMRES method range from 5 to 30. NINNER is not used for ISOLVER = 1 and can be set to any positive value greater than 0.

Maximum number of line search iterations (IBT): IBT defines whether line search is enabled (IBT > 1) or disabled (IBT ≤ 1). IBT also defines the maximum number of line search iterations to perform (if IBT > 1) at the end of each outer iteration to minimize the ℓ2-norm using Brent’s method (Press and others, 1990). Line search should be disabled (IBT ≤ 1) if DAMPSS and/or DAMPTR values other than 1.0 are specified.

Stage tolerance (TOLS): TOLS—a real value that defines the tolerance level for reach group stages used to evaluate convergence for stage within the selected numerical solver and to trigger increases and decreases in the SWR1 time step when adaptive time step­ping is enabled. Value is in units of length and must be consistent with the MODFLOW length unit. Usually a value of 1.0×10-9 is sufficient when units of feet or meters are used in the model.

Flow tolerance option: This option determines what PTOLR means. There are three options.

•None: PTOLR is not used.

•USE_FRACTIONAL_TOLR: The PTOLR value specified in Item 3 represents a fractional value. Residual convergence is achieved when the ratio of the residual to the average of the inflow and outflow for a reach group is less than or equal to PTOLR.

•USE_L2NORM_TOLR: The PTOLR value specified in Item 3 represents a ℓ2-norm. Residual convergence is achieved when the ℓ2-norm for all active reach groups is less than or equal to PTOLR.

Flow tolerance (TOLR): TOLR defines the tolerance level for reach group flow residuals to evaluate convergence for flow within the selected numerical solver and to trigger increases and decreases in the SWR1 time step when adaptive time stepping is enabled. Value is in units of length cubed per time, and must be consistent with MODFLOW length and time units. Usually a value of 1.0×10-1 is sufficient for the flow-residual criteria when meters and seconds are the defined MODFLOW length and time.

Exchange tolerance option: The exchange tolerance option determines what TOLA means.

•None:

•USE_GLOBAL_TOLA: The TOLA value specified in Item 3 represents a global surface-water/groundwater exchange convergence criterion. Surface-water/groundwater exchange convergence is achieved when the absolute value of sum of the discrepancies between MODFLOW and SWR1 for all active reaches is less than or equal to TOLA.

•USE_ABSOLUTE_TOLA: The TOLA value specified in Item 3 represents an absolute surface-water/groundwater exchange convergence criterion. Surface-water/groundwater exchange convergence is achieved when the absolute value of the discrepancy between MODFLOW and SWR1 for each active reaches is less than or equal to TOLA.

Exchange tolerance (TOLA): TOLA defines the tolerance level for relative differences in surface-water/groundwater exchanges between SWR1 and MODFLOW at the end of each MODFLOW outer iteration (Picard iteration). Convergence of surface-water/groundwater exchanges between successive MODFLOW outer iterations is disabled if TOLA is less than or equal to zero. Disabling the tolerance level for surface-water/groundwater exchanges may be appropriate for models in which groundwater levels do not change noticeably from one MODFLOW time step to another; the validity of this assumption can be evaluated by looking at the differences in SWR1 and MODFLOW calculated aquifer-reach exchanges printed in the MODFLOW listing file. Use of TOLA may improve results in cases where use of lagged heads in SWR1 formulate subroutines (GWF2SWR1FM) allow convergence of MODFLOW’s iterative solvers on the first outer iteration but will increase run-times. Value is relative and unitless. Usually a value of 0.01 is sufficient.

Steady state damping factor (DAMPSS) : DAMPSS defines the steady-state dampening factor that is applied for each inner iteration of the selected linear solver. Acceleration can be applied to the inner iteration of the selected linear solver by specifying DAMPSS values greater than 1.0. DAMPSS values between 0.1 and 0.5 have been found to work well for steady-state stress periods. Dampening/acceleration is not applied if a DAMPSS value of 0.0 is specified (DAMPSS is reset to 1.0). DAMPSS should be set to 1.0 if IBT is greater than 1.

Transient damping factor (DAMPTR): DAMPTR defines the transient dampening factor that is applied for each inner iteration of the selected linear solver. Acceleration can be applied to the inner iteration of the selected linear solver by specifying DAMPTR values greater than 1.0. Dampening/acceleration is not applied if a DAMPTR value less than or equal to 0.0 is specified (DAMPTR is reset to 1.0). DAMPTR should be set to 1.0 if IBT is greater than 1.

Convergence printout interval (IPRSWR): IPRSWR is the printout interval for convergence information for the SWR1 Process. SWR1 convergence information is written to the MODFLOW listing file every IPRSWR MODFLOW time steps. If IPRSWR is less than 1 it is changed to 999. This printout also occurs at the end of each MODFLOW stress period regardless of the value of IPRSWR.

Print convergence (MUTSWR): MUTSWR controls printing of convergence information for the SWR1 Process. There are four choices.

•Print max residual every time step (0): Print tables of the final maximum absolute residual for each SWR1 time step.

•Print number of iterations (1): Print only the total number of iterations in each SWR1 time step.

•Print none (2): No printing.

•Print if convergence fails (3): Print only if convergence fails for at least one SWR1 time step in a MODFLOW time step.