An unstructured grid is made up of triangular, rectangular or other polygon shaped geometries individually or in combination to discretize the groundwater model domain. This is in contrast to MODFLOW-2005 which uses structured, rectilinear grids. Unstructured grids require the use of the finite volume method whereas traditional structured grids are used with the finite difference method.

Structured finite difference MODFLOW-2005 grid (left) and an UnStructured grid with Voronoi Cells for MODFLOW-USG grid (right)

Structured finite difference MODFLOW-2005 grid (left) and an UnStructured grid with Voronoi Cells for MODFLOW-USG grid (right)