Time Step Options

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The Time Step Options pane is on the SWR: Surface-Water Routing Process Pane of the MODFLOW Packages and Programs dialog box.

The Time Step Options pane is used to specify the variables that control the lengths of time steps in the SWR process. The time steps in the SWR process can be shorter than those in the groundwater flow process.

Initial time step length (RTINI): This is the length of the first SWR time step in the first stress period.

Minimum time step length (RTMIN): This is the minimum SWR1 time-step length to use. RTMIN should be > 0.0. Typical values of RTMIN are between 1 and 30 seconds but is dependent on the magnitude of streamflow.

Maximum time step length (RTMAX): This is the maximum SWR1 time-step length to use. RTMAX should be ≥ RTMIN and ≤ the minimum MODFLOW time-step length. Adaptive time stepping is disabled if RTMAX = RTMIN. Typical values of RTMAX are between 1 and 24 hours but are dependent on the magnitude of streamflow.

Time step multiplier (RTMULT): This is the multiplier used to increase the SWR1 time-step length. Adaptive time stepping is disabled if RTMULT = 1.0. Typical values of RTMULT are between 1.0 and 1.2. RTMULT should be ≥ 1.

Frequency with which time step increases (NTMULT): This defines the frequency of SWR1 time-step length increases. The SWR1 time-step length is increased only when SWR1 solver convergence is achieved, the SWR1 time-step length is less than the maximum time-step length for the stress period defined by DMAXRAI, the maximum stage change in any reach are less than DMAXSTG, and the maximum inflow to any reach is less than DMAXINF on NTMULT consecutive SWR1 time steps within a single MODFLOW time step. NTMULT values ranging from 2 to 10 have been found to work reasonably well in the test cases evaluated. NTMULT should be > 1.

Minimum gradient for diffusive flow (DMINGRAD): This is used to define the water-surface gradient that must be exceeded for surface-water flow to be simulated in reach groups or structures using the diffusive-wave approximation (IROUTETYPE = 3). DMINGRAD can be set to any real number if all reach groups in a simulations use the reservoir-routing approximation (IROUTETYPE = 1 or 2). A non-zero value should only be used in cases where small gradients are causing numerical instabilities or with SWR1 datasets with a large number of reach groups to reduce numerical overhead; in these cases DMINGRAD values ranging between 1.0×10-9 and 1.0×10-12 have been found to be reasonable.

Minimum depth for outflow (DMINDPTH): This is used to define the minimum reach depth where outflows (evaporation, aquifer-reach exchanges, and diffusive wave flow) are permitted. Sigmoid-depth scaling is the default depth scaling approach used. Scaling is not applied if DMINDPTH = 0.0. Value is in units of length and must be consistent with the MODFLOW length unit. A value of 1×10-3 m has been found to work well in the test simulations evaluated.

Maximum rainfall for step adjustment (DMAXRAI): This is used to define the maximum rainfall rate for SWR1 reaches in the current MODFLOW stress period. The SWR1 time step is reduced using equation 57 of Hughes and others (2012) if the rainfall in any reach exceeds DMAXRAI. Rainfall is not used to adjust SWR1 time steps if DMAXRAI ≤ 0.0. DMAXRAI is only specified for SWR1 simulations with RTMIN < RTMAX and RTMULT > 1.0.

Maximum stage change per step (DMAXSTG): This is used to define the maximum stage change permitted in a SWR1 time step. The SWR1 time step is reduced using equation 59 of Hughes and others (2012) if the stage change in any reach exceeds DMAXSTG. Simulated stage changes are not used to adjust SWR1 time steps if DMAXSTG ≤ 0.0. DMAXSTG is only specified for SWR1 simulations with RTMIN < RTMAX and RTMULT > 1.0.

Maximum inflow change per step (DMAXINF): This is used to define the maximum net inflow change permitted in a SWR1 time step. The SWR1 time step is reduced using equation 60 if the net inflow change in any reach exceeds DMAXINF. Simulated net inflow changes are not used to adjust SWR1 time steps if DMAXINF ≤ 0.0. DMAXINF is only specified for SWR1 simu­lations with RTMIN < RTMAX and RTMULT > 1.0. Value is in units of cubic length per time (L3/T)