User's guide of Proceessing Modflow 5.0

54 Processing Modflow
3.1 The Grid Editor
The first steps in the groundwater modeling process are to define the goals of the model, select
a computer code (here: MODFLOW), collect the necessary data, develop a conceptual model of
the groundwater system and define the spatial discretization of the model domain. Anderson and
Woessner (1992) discuss the steps in going from aquifer systems to a numerical model grid.
Zheng and Bennett (1995) describe the design of model grids which are intended for use both in
flow and transport simulations. These sources provide valuable general information relating to
spatial discretization and grid design in numerical groundwater modeling.
In the block-centered finite difference method, an aquifer system is replaced by a discretized
domain consisting of an array of nodes and associated finite difference blocks (cells). Fig. 3.1
shows a spatial discretization of an aquifer system with a mesh of cells and nodes at which
hydraulic heads are calculated. The nodal grid forms the framework of the numerical model.
Hydrostratigraphic units can be represented by one or more model layers. The thicknesses of each
model cell and the width of each column and row can be specified. The locations of cells are
described in terms of columns, rows, and layers. PMWIN uses an index notation [J, I, K] for
locating the cells. For example, the cell located in the 2nd column, 6th row, and the first layer
is denoted by [2, 6, 1].
3.1 The Grid Editor
Fig. 3.1 Spatial discretization of an aquifer system and the cell indices
To generate or modify a model grid, choose Mesh Size... from the Grid menu. If a grid does not
exist, a Model Dimension dialog box (Fig. 3.2) will allow you to specify the number of layers
and the numbers and the widths of columns and rows of the model grid. After specifying these