Below is a list of Frequently Asked Questions (FAQ’s) for MODHMS® and MODFLOW-SURFACT.

Q: When or why would you use MODFLOW-SURFACT rather than use MODFLOW?

A: MODFLOW-SURFACT overcomes limitations and eliminates restrictions associated with MODFLOW and its current transport counterparts. Examples include rewetting of drained cells, handling of pumping wells, solute mass balance problems, numerical dispersion and oscillations, and the implicit assumption of the negligible impact of transient flow storage effects on transport.

Q: Is there a user manual available for MODFLOW-SURFACT / MODHMS®?

A: The user manual for MODFLOW-SURFACT / MODHMS® is provided in the form of a PDF file in the installation CD.

Q: What are the units of length and time that MODFLOW-SURFACT uses?

A: The time is user specified through the BAS file and the length is not specified, so all of the parameters and data must be consistent.

Q: Is the layer thickness sensitive to convergence? In other words, by increasing the layer thickness would you minimize convergence problems?

A: MODFLOW-SURFACT is very robust and was initially designed to overcome the convergence problems associated with MODFLOW; therefore, artificially increasing the layer thickness is not necessary unless non-physical zero or negative values are used in the model.

Q: Can I use any solver with transport in MODFLOW-SURFACT?

A: You must use the PCG4 solver or the PCG5 solver package with transport; although you may use any solver for the associated flow problem solution. It is, however, recommended to use the PCG4 solver or the PCG5 solver with Newton-Raphson linearization for the flow solution as well.

Q: When would I need to use the NEWTON-RAPHSON package in MODFLOW-SURFACT / MODHMS®?

A: The Newton-Raphson scheme providing quadratic convergence is used for nonlinear situations or when convergence difficulties are encountered. For unconfined or unsaturated flow problems, the use of the Newton-Raphson scheme is always recommended.

Q: I have a 64-bit machine; can I use MODFLOW-SURFACT / MODHMS® on this machine?

A: Yes. Both MODFLOW-SURFACT / MODHMS® can be used on 64-bit machines. 32-bit and 64-bit versions of the code are provided with each license of the software.

Q: Is it necessary to use the PCG5 solver?

A: No, but we suggest using the PCG5 solver because it is robust and much faster than other solvers. Some users have reported the PCG5 solver to be 10 times faster than other solvers.

Q: The suggested defaults for viscosity and gravity have the time unit in seconds. If my model is using time in days, do I have to convert the units for viscosity and gravity, or is this already assumed in the model?

A: The model only assumes a consistent set of units. Thus, you will need to convert all parameters to a consistent set of units before using them as input to the model.

Q: Can MODFLOW-SURFACT / MODHMS® simulate flow and transport in the unsaturated zone?

A: MODFLOW-SURFACT / MODHMS® is capable of simulating flow and transport in the unsaturated zone using pseudo-soil or real soil relationships. It is capable of modeling flow and transport for the following:

1) Advective and dispersive transport of chemical species.

2) Linear and nonlinear retardation for each species.

3) First-order decay and biochemical degradation of contaminants in water and soil.

4) Generation of transformation products.

5) Mass partitioning of a single or multicomponent contaminant with diffusive mass movement in the inactive phase.

6) Mass partitioning of a single or multicomponent contaminant from a depleting, immobile NAPL phase with advective and dispersive transport in the active phase and diffusive transport in the inactive phase.

Q: Does MODFLOW-SURFACT / MODHMS® incorporate interaction between the unsaturated zone and rivers and lakes?

A: Yes. MODFLOW-SURFACT / MODHMS® does incorporate interaction between the unsaturated zone and rivers and lakes; however, for a rigorous treatment of surface water-groundwater interactions the integrated surface water-groundwater code MODHMS® is recommended.

Q: What is the difference between the RECHARGE-SEEPAGE FACE BOUNDARY CONDITION package vs. the RECHARGE package?

A: The RECHARGE-SEEPAGE FACE BOUNDARY CONDITION package allows supplied recharge into the groundwater system if the water table is below a user-specified ponding elevation. This elevation corresponds to ground elevation when there is no ponding. If the water table reaches ponding elevation, the simulation allows only as much recharge to occur as is required to maintain the prescribed ponding elevations. The rest of the recharge is shed off as surface runoff. Thus recharge to the system is reduced from prescribed levels to meet maximum ponding elevation constraints. When recharge is reduced to below zero, it indicates that water is discharging the model system under seepage conditions. When the ponding elevation is above land surface, the storage term is augmented by the additional volume of ponded water.

Additionally, the RECHARGE-SEEPAGE FACE BOUNDARY CONDITION package can be used to simulate a seepage face boundary condition by prescribing the elevation of a seepage face boundary node. If the water table is below this elevation, zero flux occurs at the boundary. If the water table reaches this elevation, the aquifer drains to maintain the seepage face elevation.

Q: What is the difference between the FWL package and the WEL package? If you are using the FWL4 / FWL5 package, can you also use the standard WEL package?

A: The FWL4/FWL5 package should be used instead of the WEL package, even for wells that lie within a single cell, especially for extraction, because the FWL package checks for unphysical drawdowns, and reduces the pumping accordingly, to maintain bottom-hole conditions. Both may be used in a simulation, but the WEL package should be used only in locations where the well is not expected to go dry since this package does not check for unphysical conditions.

Q: What are the options in MODFLOW-SURFACT / MODHMS® to solve the variably saturated water flow equations?

A: Three options are available:

a) Pseudo-soil relations with no need of further soil parameters – this is the gravity segregated, vertically integrated solution.

b) Van Genuchten for both Kr and retention, with need of further soil parameters alpha, beta and residual saturation.

c) Brooks and Corey for Kr but van Genuchten for moisture retention with need of even one more soil parameter, the Brooks-Corey N. This last gives further freedom to the Kr term for calibration or for numerical purposes.

Q: What head value does MODFLOW-SURFACT / MODHMS® write for a grid cell above the water table? As I understand, there are no dry cells in MODFLOW-SURFACT / MODHMS®, so what does it do with grid cells above the water table?

A: MODFLOW-SURFACT / MODHMS® does have dry cells, only those cells are not inactive. In dry cells, it writes the heads calculated for the dry cell, which will be equal to the water table head with no recharge. With recharge, it will be slightly higher than that, to allow for the recharge to go down to the water table. So, in essence, it is a pseudo-head, but it may be used to see where the first water table lies since it should not be much different unless confinement at the surface is large.

Q: Does MODFLOW-SURFACT interpret the head assigned with constant heads as being the pressure head boundary condition for a vapor flow simulation? If yes, then how does it treat river boundaries and general head boundaries?

A: This is a very interesting point regarding river boundary conditions, etc. The heads for a vapor phase simulation are the equivalent freshwater heads for the air — be it on a boundary or internal to the system. For air phase simulations, therefore, the river package, etc., should not be used (or should be used with caution and full understanding if you want to conceptualize something that way). You can then think of such boundaries as applying a general head condition (head-dependent flux condition, with limitations on inflow or outflow) which moves air into or out of the system under that prescribed “air-head.”

Q: There was a power outage while my simulation was running. How can I restart my simulation from where it stopped?

A: MODFLOW-SURFACT / MODHMS® offers the capability to restart a simulation that has stopped before the completion of total simulation time (for any reason: power outage, simulation failed).


A: First you need to run your model by adding a flag in the BAS file (Basic file used by MODFLOW-SURFACT). An MPH file is produced when the flag MPHFLG is turned on in the BAS file. This flag is located on the eighth position (between columns 71 and 80) in the BAS file.

Q: Is a density-dependent module available with MODFLOW-SURFACT / MODHMS®?

A: Yes. The density-dependent module is available for an additional fee with MODFLOW-SURFACT / MODHMS®.