### Equations

The three-dimensional flow of water through the porous material between the cells is solved as a partial differential equation. Where K is the hydraulic conductivity [L/T] along the three axis, S the specific storage and W is the volumentric flux per unit volume in and out of the groundwater system. The hydraulic conductivity between two cells is calculated by using the harmonic mean. The equation is solved using a conjugate gradient approach and an Incomplete LUT preconditioner.

Additional information on the equations can be found in the very detailed MODFLOW documentation: Modflow 2005

### Boundary Conditions

G³M support multiple boundary condition types:

- No-flow boundary
- Static head boundary
- General head boundary
- Groundwater recharge
- Lakes
- Wetlands
- Different river approaches

New flows can be defined in Model/ExternalFlows.hpp. The domain boundary is currently defined implicitly through the input grid as no-flow for grid files and as ocean boundary for irregual grids. This behaviour can be changed in DataProcessing/Neighbouring.hpp.