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MODFLOWT KEY FEATURES
- Fully-compatible with previous versions of MODFLOW.
- Preprocessor support with Groundwater Vistas, ModelCad for Windows,
and other popular graphical preprocessors.
- Simulation of advective-dispersive transport with adsorption and first-order decay implemented in the Basic Transport and Block-Centered Transport packages.
- Central differencing and upstream weighting approximation options for advective terms.
- Fully implicit (backward-in-time) or Crank-Nicolson
(central-in-time) approximation options for time derivative.
- Longitudinal and transverse dispersivities plus an optional vertical dispersivity.
- Variable thickness and elevations of model layers.
- Variable porosities, variable anisotropies, retardation factors, and first-order decay rates.
- Optional expansion of cross products in dispersion tensor.
- Choice of strongly implicit procedure (SIP), slice-successive over-relaxation (SSOR) and enhanced preconditioned conjugate gradient (PCG) solvers for flow.
- Choice of SSOR or ORTHOMIN solvers for transport.
- Automatic time stepping with adaptive controls for transport.
- Options for simulating transient transport with steady-state/transient flow or successive periods of steady-state flow.
- Restart capabilities for flow and transport simulations.
- Addition of new Observation Well Package for recording heads and concentrations at observation nodes.
- Output files (1) recording concentrations and concentration changes in MODFLOW's "head save" format, and (2) mass budget history.
- Dynamic allocation of memory.
MODFLOWT SOLUTION METHOD
MODFLOWT uses an implicit finite-difference discretization scheme for the numerical solution of the partial-differential equations for solute
transport. Both central-differencing and upstream-weighting options are available for solution of the advective terms. In addition, fully implicit (backward-in-time) or Crank-Nicholson (central-in-time)
approximation options for the time derivative are included in MODFLOWT. Solvers for flow include Strongly Implicit Procedure (SIP), Preconditioned Conjugate Gradient (PCG2), Slice Successive Overrelaxation (SSOR),
and the new ORTHOMIN solver (OMN). For transport, the available solvers are Slice Successive Overrelaxation and ORTHOMIN.
MODFLOWT PREPROCESSOR SUPPORT
Data set preparation for MODFLOWT is simple using Groundwater Vistas. The Windows-based graphical interface features full support for creation of
all packages to simulate both groundwater flow and contaminant transport using MODFLOWT. Because it is compatible with ModelCad from Geraghty & Miller and can import MODFLOW data sets, conversion of existing
modeling projects to simulate contaminant transport using MODFLOWT is straightforward. In addition, other popular preprocessors like ModelGIS, Visual MODFLOW, and ModelCad for Windows have the ability to create
MODFLOWT data sets.
ADDITIONAL FEATURES IN MODFLOWT
MODFLOWT allows for full expansion of the cross-product terms of the dispersion tensor or "lumping" of these terms on the main diagonal. Full
expansion of these terms is appropriate in scenarios where the principal direction of groundwater flow is not parallel with the grid. In these cases, lumping of the terms may overestimate transverse dispersion and
cause "fattening" of the plume. Full expansion of these terms in MODFLOWT minimizes this effect. In addition, MODFLOWT can use either backward (implicit) or central (implicit-explicit) differencing techniques of the
time derivative. This allows for larger time steps (and shorter execution times) without sacrificing numerical stability or accuracy.
MODFLOWT FREQUENTLY ASKED QUESTIONS
How do execution times of MODFLOWT compare with other transport models?
One of the principal advantages of MODFLOWT is its execution speed compared to other popular transport models. In benchmark tests against MT3D96,
MODFLOWT was shown to range from two to 20 times faster depending on the nature of the problem. This is particularly true for simulations with complex geology comprised of variable-layer elevation and thicknesses.
The new ORTHOMIN solver in MODFLOWT offers significant advantages over the traditional Slice Successive Overrelaxation (SSOR) solver without sacrificing numerical stability or accuracy.
What are cross-products and why are they important?
In simulations where the principal direction of groundwater flow is not parallel with the finite-difference grid, the dispersion tensor will have
non-zero off-diagonal components. Traditionally, these components are approximated in the numerical solution by "lumping" them along the main diagonal (Dxx, Dyy, Dzz) of the dispersion tensor. MODFLOWT
allows for both "lumping" and full inclusion of the cross products during solution of the dispersion term in the finite-difference equation. For some scenarios, "lumping" the cross products can
greatly overestimate transverse dispersion, unrealistically "fattening" the contaminant plume. In the example, full inclusion of the cross products has minimized this effect.
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