GMS provides an interface to the GNC package. The package can be turned on in the MODFLOW Packages dialog, accessible from the MODFLOW Global/Basic Package dialog. The package dialog can be accessed from the main MODFLOW menu or by right-clicking on the " MODFLOW" item in the Project Explorer.
GNC Package Dialog
Enter the ghost node data by hand, or allow GMS to compute the data by clicking the Generate Ghost Node Data button. Clicking the button causes GMS to overwrite whatever is in the GncNodes table with the ghost node data calculated using an algorithm which calculates the locations of ghost nodes and computes the AlphaJ values based on the distances from ghost nodes to adjacent cells. Ghost node locations are calculated using the perpendicular bisector of the face between adjacent cells and its intersection with the lines connecting cell centers as illustrated in the figure below. The algorithm considers only the horizontal relationship of cells when locating ghost nodes. Thus, if the figure above is considered to be in plan view, the ghost node will be located as shown, but if it is considered to be in a front or side view with a subdivided layer on the right, no ghost node will be created.
If the Create points at ghost node locations when generating option is on, GMS will create a new UGrid called "Ghost Nodes" with points at the ghost node locations as calculated by GMS.
GMS does not currently support some GNC package options, including:
- IFALPHAn (always 0 in GMS)
- Integration with the CLN (Connected Linear Network) package to correct for errors due to the CLN cells not passing through the center of the groundwater grid cell
- ^ a b Panday, Sorab, Langevin, C.D., Niswonger, R.G., Ibaraki, Motomu, and Hughes, J.D., (2013), MODFLOW-USG, version 1: An unstructured grid version of MODFLOW for simulating groundwater flow and tightly coupled processes using a control volume finite-difference formulation: U.S. Geological Survey Techniques and Methods, book 6, chap. A45, 66 p., Reston, Virginia, http://water.usgs.gov/ogw/mfusg/
GMS – Groundwater Modeling System
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