The MODFLOW Time dialog box is used to specify the stress periods in MODFLOW simulations and related information in MT3DMS. To display it, the user selects Model|MODFLOW Time...
MODFLOW
In MODFLOW, stress periods are used to define time intervals during which the inputs for the model remain constant. For example, if the pumping rate on a well were to change on a monthly basis, the stress periods would need to be one month in length or less. The ModelMuse, however, simplifies things by automatically setting up stress periods whenever the boundary conditions change. Thus, the user may choose to only specify one or two stress periods in the MODFLOW Time dialog box and let ModelMuse figure out the rest of them.
For each stress period that the user defines several items must be specified. These include:
•the Starting time and Ending time of the stress period,
•the Length of the stress period,
•the Maximum length of the first time step in the stress period (Max length first time step)
•the time step Multiplier,
•whether the model is Steady state or Transient, and
•whether the heads computed at the end of the stress period should be used as reference head for computing drawdown (Drawdown Reference). (MODFLOW 6 does not save drawdowns so this option has no effect in MODFLOW 6.)
If you specify a time for a boundary condition that does not correspond to a time in the MODFLOW Time Dialog box, ModelMuse will use the times for the boundary to define additional stress periods when exporting the model input files. For more information, see the explanation for the MODFLOW Features Tab of the Object Properties Dialog Box.
If there is more than one time step in a stress period, the Multiplier and Max length first time step can be used to begin with a short time step and gradually increase it. This can be beneficial if there is an abrupt change in the boundary conditions. It can be necessary to use short time steps to model such abrupt changes adequately but in the middle of the stress period when the boundary conditions are not changing abruptly, longer time steps can be used to make the model run faster while still maintaining adequate accuracy. Based on the Length of the stress period, the Max length first time step and the Multiplier, ModelMuse will determine the number of time steps needed in the stress period to make sure that the first time step in the stress period is less than or equal to Max length first time step. Subsequent time steps will each be increased over the previous one by a factor of Multiplier. If the user wishes to specify the number of times steps in a stress period rather than the length of the first time step, the Time Step Length Calculator can be used. To display it, select a cell in the table for Max length first time step and click the "F()" button.
The Length of the stress period is just the Ending time minus the Starting time. However, the user can edit all three of them. ModelMuse will adjust the ones that the user doesn't edit to be consistent with the changed data.
In many cases, it is advantageous to use a Steady state stress period as the first stress period in an otherwise Transient model in order to start the transient simulation from steady state conditions. However, it is also convenient for the beginning of the transient simulation to begin at time = 0. By default, ModelMuse makes the first stress period be a steady-state stress period ending at time = 0 thereby achieving both goals. If the SWI2 package is used, it is generally necessary to have multiple time steps in a steady-state stress period. See the SWI2 package for more information.
It is possible to select several cells in the table in which the stress period data are specified and copy it to the clipboard, modify it in an external program and then paste it back into the table. To select a block of cells, first click on one of the cells and then hold down the Shift key while clicking on another cell. The selected cells will be highlighted in blue. The user can also click on other cells while holding down the Ctrl key to select a group of cells that are not all in a block. To copy the selected cells to the clipboard, type "C" on the keyboard while holding down the Ctrl key. To paste data into the table, double-click on a cell and then type "V" while holding down the Ctrl key. The cell that the user selected will be in the upper left hand corner of the block of cells whose data will be overwritten by the contents of the clipboard. If the data on the clipboard has more rows than are in the table, the table will expand to accommodate the clipboard data.
If the user wishes to specify the same data for several cells at once, this is also possible, the user selects cells in the table as before and then types the new values in the controls above the table.
New rows can be inserted into the table by selecting a row and clicking the Insert button. Similarly, a row can be deleted by selecting a row and clicking the Delete button
The Time units can also be specified on the MODFLOW Time dialog box. (The time units can also be specified on the MODFLOW Options dialog box.)
The Time Units Converter dialog box can be used to convert values specified in a unit different from that used in the model to values in the time units specified in the model. Click the Convert time units button to display the Time Units Converter dialog box.
MT3DMS or MT3D-USGS
If MT3DMS or MT3D-USGS is selected, additional data are specified on the MT3DMS or MT3D-USGS tab. The times specified for these data do not need to correspond to the times used to define the MODFLOW stress periods. However, all the times for MT3DMS or MT3D-USGS, must be between the beginning of the MODFLOW model and the end of the MODFLOW model. If the last time that is specified here is earlier than the last time specified for MODFLOW, when you export the MT3DMS or MT3D-USGS input files you will be given a choice between aborting the export of the input files, only including the times for MT3DMS or MT3D-USGS in the MT3DMS or MT3D-USGS input files, or including all the times for MT3DMS or MT3D-USGS and MODFLOW in the MT3DMS or MT3D-USGS input files.
DT0 is the initial transport step size. If it is specified as zero, the model-calculated value of DT0, based on the user specified Courant number in the Advection Package, will be used. The subsequent transport step size may increase or remain constant depending on the user-specified transport step size multiplier TTSMULT and the solution scheme for the advection term.
MXSTRN is the maximum number of transport steps allowed within one time step of the flow solution. If the number of transport steps within a flow time-step exceeds MXSTRN, the simulation is terminated.
TTSMULT is the multiplier for successive transport steps within a flow time-step if the GCG solver is used and the solution option for the advection term is the standard finite-difference method. A value between 1.0 and 2.0 is generally adequate. Note that for the particle-tracking-based solution options and the third order TVD scheme, TTSMULT does not apply.
TTSMAX is the maximum transport step size allowed when transport step size multiplier TTSMULT > 1.0. Setting TTSMAX=0 imposes no maximum limit.
SSFlag is an optional flag to indicate whether the steady-state transport option should be activated. This option is only available if MIXELM in the Advection package is set to Standard (0).
Adaptive Time Stepping
In MODFLOW 6, Adaptive Time Stepping is an alternative method for allowing MODFLOW to determine the appropriate time step length. If MODFLOW fails to converge in a time step, MODFLOW will attempt to correct the problem by reducing the time step and trying to solve the time step again. MODFLOW can also increase a time step length if a time step is solved very easily. Adaptive Time Stepping can be applied to none, some or all of the stress periods.
Adaptive time stepping should not be used when the groundwater transport process of MODFLOW 6 is used in a separate simulation because the time stepping in the flow and transport models will not be identical.
Use ATS activates Adaptive Time Stepping for a stress period.
Initial Time Step Length (dt0)—is the initial time step length for the stress period. If dt0 is zero, then the final step from the previous stress period will be used as the initial time step. If dt0 is zero for the first stress period, dtmin will be used as the initial time step.
Minimum Time Step Length (dtmin)—is the minimum time step length for this period. This value must be greater than zero and less than dtmax. dtmin must be a small value in order to ensure that simulation times end at the end of stress periods and the end of the simulation. A small value, such as 1.e-5, is recommended.
Maximum Time Step Length (dtmax)—is the maximum time step length for this period. This value must be greater than dtmin.
Time Step Multiplier Factor (dtadj)—is the time step multiplier factor for this period. If the number of outer solver iterations are less than the product of the maximum number of outer iterations (OUTER MAXIMUM) and ATS OUTER MAXIMUM FRACTION (an optional variable in the IMS input file with a default value of 1/3), then the time step length is multipled by dtadj. If the number of outer solver iterations are greater than the product of the maximum number of outer iterations and ATS OUTER MAXIMUM FRACTION, then the time step length is divided by dtadj. dtadj must be zero, one, or greater than one. If dtadj is zero or one, then it has no effect on the simulation. A value between 2.0 and 5.0 can be used as an initial estimate.
Time Step Divisor on Failure (dtfailadj)—is the divisor of the time step length when a time step fails to converge. If there is solver failure, then the time step will be tried again with a shorter time step length calculated as the previous time step length divided by dtfailadj. dtfailadj must be zero, one, or greater than one. If dtfailadj is zero or one, then time steps will not be retried with shorter lengths. In this case, the program will terminate with an error, or it will continue of the CONTINUE option is set in the simulation name file. Initial tests with this variable should be set to 5.0 or larger to determine if convergence can be achieved.
GWT
The GWT tab is only visible if the Use separate time discretization option is enabled the Groundwater Transport Process.
The table on the GWT tab is used to to specify the number of times steps and the time-step multiplier for each chemical species in each stress period. When the groundwater transport process is used, it can be helpful to increase the number of time steps used for solute transport. However, if the number of time steps in a flow stress period is greater than one, the time steps in the groundwater transport process must be the same as in the flow model. Adaptive time stepping should not be used when groundwater transport is used in a separate simulation because the time stepping in the flow and transport models will not be identical.
