GMS:GNC Package

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MODFLOW
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Packages Supported in GMS
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Unsupported MODFLOW Features
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Packages
Flow: BCF6, HUF, LPF, UPW
Solvers:

DE4, GMG, NWT, PCG,

PCGN, LMG, SIP, SOR,

SMS
Other:

BAS6, BFH, CHD1, CLN,

DRN1, DRT1, EVT1, ETS1,

GAGE, GHB1, GNC, HFB1,

HUF, LAK3, MNW1, MNW2,

OUT1, RCH1, RIV1, SFR2,

STR1, SUB1, SWI2, WEL1,

UZF1

The Ghost Node Correction (GNC) package is an optional package included in MODFLOW-USG. It can be used to correct errors in the model that are created “when a line between two connected nodes does not bisect the shared face at a right angle”.[1]

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 Folder.svg MODFLOW" item in the Project Explorer.

Ghost node conceptualization for nested grids.[1]

GNC Package Dialog

The 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.

Ghost node general case algorithm.

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.

Unsupported Options

GMS does not currently support some GNC package options, including:

  • Parameters
  • 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

Notes

  1. ^ 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/