Some aquifer systems can be treated as being at steady state with regard to flow. In modeling such systems, a single flow-only simulation is performed at the beginning of the simulation and the flows from that simulation are used for all the transport simulations. The PHAST Steady Flow dialog box is used to specify this option and its associated parameters as listed below. The dialog box is shown by selecting Model|PHAST Steady Flow…
•The steady-flow option is selected by checking the Steady flow checkbox.
•The user also specifies the Head tolerance in the Steady Flow dialog box. The head tolerance is the maximum change in head for determining when flow is at steady state.
•The user also specifies the Flow balance tolerance. The fractional flow balance must be less than the flow balance tolerance for the flow system to be at steady state. The fractional flow balance is the difference between the inflow and outflow rates divided by the average of the inflow and outflow rates for the current time step.
•In the Iterations edit box, the user specifies the maximum number of time steps that will be used in attempting to attain a steady-flow condition.
•The user also specifies the Minimum time step size. It is the initial time step used in calculating steady-state flow and also is the minimum time step allowed for automatic time stepping in the steady-state flow calculation. The default is the time-step length defined in the PHAST Time Control dialog box divided by 1,000.
•The user specifies the Maximum time step size in the related edit box. The maximum time step is the maximum time step used in the steady-state flow calculation. The default value is the time-step length defined in the PHASTTime Control dialog box times 1,000.
•The user specifies the Head change target in the related edit box. The head change target is the amount by which it is desired that the head will change during a single time step in the steady-flow calculation. The time step will be decreased or increased during the steady-flow calculation to try to achieve head changes of target over each time step. The default value is 0.3 times the thickness of the grid region.
