Once the MODFLOW simulation has been initialized, the next step is to enter the data required by the Global Options/Basic package. This includes data defining fundamental program options such as the computational time intervals (stress periods), an array defining which cells are inactive and which cells have constant heads, an array of starting head values for a transient simulation, and a set of flags defining which of the other packages are to be used. The input data for this package should be entered before editing any of the other packages. The MODFLOW Global Options\Basic Package Dialog is accessed through the Global Options command in the MODFLOW menu. The options in the dialog are as follows:
The two headings are optional text strings which are written to the MODFLOW text output file.
The run options are not parts of the actual MODFLOW input files, but present different ways that MODFLOW 2000 can be run.
A forward run is a normal MODFLOW simulation. If parameters have been defined for either a stochastic or parameter estimation run, the mean or starting values of the parameters are used for the forward run.
The sensitivity analysis option is used to perform a sensitivity analysis on a set of selected parameters. The results can be presented with a set of plots or by contouring data sets representing grid sensitivities. This option is only available with the LPF and HUF flow packages.
In the parameter estimation mode, a set of parameters is found that minimizes the residual between observed and simulated heads and flows. These options are described in more detail on the parameter estimation page. This option is only available with the LPF and HUF flow packages.
The stochastic simulation option is used to perform a stochastic simulation using the Monte Carlo or Latin Hypercube methods for randomizing a selected set of parameters. Stochastic simulations can also be performed using a material set approach (indicator simulations). A risk analysis wizard can be used to analyze the results for a probabilistic threshold analysis or a probabilistic capture zone delineation. These options are described in more detail on the stochastic simulation page.
If the material set option is used for stochastic simulations, the "stochastic inverse" option can be used to perform automated parameter estimation on each of the N candidate model instances in the stochastic. The weighted RMS errors for each of the resulting optimized solutions can be used to weight the computations performed by the risk analysis wizard. This option is only available with the LPF and HUF flow packages.
The simulation can be designated as either steady state or transient. If a steady state simulation is specified, certain portions of the MODFLOW interface such as the Stress Period dialog are inactivated since they are not relevant.
This is the value of head to be assigned to all inactive (no flow) cells (IBOUND=0) throughout the simulation.
The Stress Periods button is used to bring up the Stress Period dialog. The stress periods should be defined before any of the sources/sinks are entered. If the steady state option is selected, the Stress Periods button is dimmed.
The Packages button brings up the Packages dialog.
The Units button the Basic Package dialog brings up the Units dialog.
The IBOUND button is used to enter the values of the IBOUND array.
The Starting Heads button on the left side of the Basic Package dialog is used to enter the values of the starting heads array. Selecting the Starting Heads button brings up the Starting Heads dialog.
The Top Elevation button brings up an array editor that allows you to edit the top elevation of each layer. GMS requires that the top and bottom elevations for adjacent layers be the same. This means that when you edit the bottom of one layer, move down to the next layer, and edit the top of that layer, the bottom of the previous layer will also be changed. Note that the top and bottom layer elevations must be entered regardless of the which layer data package (BCF, LPF, HUF) is chosen even though these values may not be used because of certain package options.
The Bottom Elevation button brings up an array editor that allows you to edit the bottom elevation of each layer. Note that the top and bottom layer elevations must be entered regardless of the which layer data package (BCF, LPF, HUF) is chosen even though these values may not be used because of certain package options.
Related Links:
Building a MODFLOW Model
Packages Supported in GMS