The Polyline Boundary Junction Package: A Novel Method for Incorporating Polylinear Boundary Conditions in Voronoi Grids

Abstract:

Groundwater modeling codes supporting unstructured grids (such as MODFLOW-USG and MODFLOW 6) allow for flexible discretization which can incorporate the locations of wells, streams, and other boundary features while varying cell sizes around these features and other areas of interest. However, in some modeling applications (for instance, at the regional scale), it is not always possible or advantageous to explicitly incorporate all features within the model grid discretization. Additionally, linear features like streams often still are simplified (i.e., straightened) to reduce grid complexity. We introduce a new method of incorporating complex polylinear boundary conditions in Voronoi or standard rectangular grids that has been implemented in a MODFLOW-USG package and a related pre-processing R package. The method evaluates boundary conditions along individual segment locations within the grid’s dual Delaunay triangulation and geometrically distributes the resultant fluxes to the appropriate Voronoi cells. Groundwater heads are interpolated along the segment to handle head-dependent flux calculations. The primary boundary condition modeled by the package is the drain condition, which is a suitable approximation for groundwater-fed streams and drainage channels, but external water elevations (e.g., river stage) can also be used to simulate variably connected streams and couple the package to an external surface water model. The method is shown to appropriately handle the impact of fine resolution polylinear surface water features on groundwater heads and groundwater-surface water fluxes, even when using a coarse numerical grid. The package and pre-processor are open-source and available on GitHub.