The Buoyancy package in MODFLOW 6 is used to model variable density flow in a single fluid in MODFLOW 6 models. The BUY: Buoyancy Package pane is on the MODFLOW Packages and Programs dialog box under Flow Packages.
If the Buoyancy package is used, the Groundwater Transport process must also be used. In addition, the flow and transport models must be part of the same simulation. These options will be selected automatically when the Buoyancy package is selected if they are not selected already.
The Buoyancy package simulates the effects of varying fluid density on groundwater flow. Typically, the density depends on the solute concentration. The user must specify a fluid reference density used in the equation of state. In ModelMuse, it is set to 997.08 by default which is the density of fresh water at 25° C.
For each chemical species, the user specifies a reference concentration (crhoref) and drhodc. Drhodc is the slope of the line relating concentration to fluid density.
Instead of calculating the density of water entering the model through boundary conditions, the user can specify the density directly by checking the Specify density checkbox. If this is done, a special chemical species named "Density" will be include in the Chem Species Pane. It is used to specify the fluid density of water entering through the boundary condition in the General Head Boundary (GHB), River (RIV), Lake (LAK), Streamflow Routing (SFR), and Multiaquifer Well (MAW) packages.
The HHFORMULATION_RHS option in the Buoyancy package causes off-diagonal terms to be included in the right-hand side of the equations that MODFLOW solves instead of the left hand side. This option will prevent the BUY Package from adding asymmetric terms to the flow matrix. Thus activating he HHFORMULATION_RHS option can promote model stability.
If the Write density option is selected, MODFLOW will write a binary file with the fluid density at each cell each time model heads are written. The file has the same format as the binary head file. ModelMuse can import and display the density file.
