Property Overlays The term Property in Graphic Groundwater refers to porous media characteristics or properties that must be set for every active or constant head cell (and in rare circumstances inactive cells), and may include (depending on the model): Initial Head Column Transmissivity Column Conductivity Vertical Leakance Vertical Conductivity Top Elevation Bottom Elevation Primary Storage Factor Secondary Storage Factor Wet/Dry Porosity The Property Overlay window includes a single text box and Set Node, Clear Node, Interpolate, Set, and Finished buttons. Choose a layer from the Slice drop down list contained in the toolbar at the top of the Model window, select a cell or a group of cells within that layer, enter a value in the text box, then click the Set button. The values in the selected cells change. If the scale is large enough (see the View menu) and the Cell Values box is checked (see the View Preferences window), the value will appear within the cell. A dot is displayed in the cell if it is too small to display the entire number. Values can be changed simply by repeating the process. Graphic Groundwater includes features to allow you to interpolate property values in the grid. A node is a cell that has a fixed value in the interpolation algorithm. You set a node or nodes by selecting a cell or group of cells and then click the Set Node button. A node cell is indicated by a blue circle circumscribed in the cell or cells. If you then select a group of cells (this can be a discontinuous block of cells) that includes one or more nodes, then click the Interpolate button, Graphic Groundwater will interpolate values in all the selected cells, based on the node values, by applying an inverse distance algorithm. This feature is particularly useful in setting a property that trends across a grid. A selected node is cleared with the Clear Node button. The inverse distance algorithm has two options that allow you to control the number of nearest nodes and the maximum distance to any node used in interpolation. For example, if you specify three nearest neighbors, and select a block of cells with five nodes, Graphic Groundwater will only use the three nodes closest to a cell in interpolating a value. In addition, Graphic Groundwater will not use any node located at a distance greater than the maximum node distance (in model units) when interpolating a cell value. Maximum node distance and the number of nearest neighbors are set in the View Preferences window. The inverse distance algorithm computes the value for a particular cell with the following equation: