Time steps¶
Timesteps, under Model Build → General Settings, are used only for transient models. If you are doing a transient model, make sure you uncheck Steady under General Options. Each row of input in the Time Steps table defines a time period, during which all boundary conditions are constant. For example, a model could have three time periods with different recharge rates, river stages, or well discharge rates in each of the three periods, but within each period the values remain constant.
For each time period, you specify the total length of the time period (Period Length), the number of time steps the period is divided into (Steps in Period), and the time step multiplier (Step Multiplier). The multiplier causes the length of successive time steps to grow by a factor equal to the time step multiplier. The following table illustrates the lengths of time four time steps for a period that is 100 time units long, using various time step multipliers.
| Time Step | Multiplier = 1.0 | Multiplier = 1.5 | Multiplier = 2.0 |
|---|---|---|---|
| 1 | 25.00 | 12.31 | 6.67 |
| 2 | 25.00 | 18.46 | 13.33 |
| 3 | 25.00 | 27.69 | 26.67 |
| 4 | 25.00 | 41.54 | 53.33 |
| Total Time: | 100.00 | 100.00 | 100.00 |
In all cases, the total time of the period is 100, but the lengths of the four steps change as the time step multiplier changes. This scheme is the same as employed in MODFLOW. Using a multiplier larger than 1.0 helps concentrate computing power early in the time period when there is more transient change occurring. Transient storage fluxes, which are part of spatially-variable extraction, are computed for each time step using a finite-difference approximation of the governing equation (Equation 6 of Fitts, 2010).
Input of Transient Model Parameters¶
Many parameters in Anaqsim models may vary during a transient simulation. In all cases, the parameters must be constant during each time period, but may change from one time period to another. The following model input parameters may be transient:
- Discharges at discharge-specified wells and discharge-specified line boundaries
- Heads at head-specified wells and head-specified line boundaries
- Normal fluxes at normal flux-specified line boundaries
- Stages at River line boundaries
- The flux rate (top or bottom) at a flux-specified spatially-variable area source/sink
- The head (top or bottom) at a head-dependent flux-specified spatially-variable area source/sink
For each of these parameters, you may enter one value that applies through the entire simulation, or you may enter multiple values separated by commas, one value per time period. If you enter fewer values than the number of time periods, the last value entered applies to all subsequent time periods.