Overlays Menu You create the grid for your model within Grid mode, but in reality a groundwater model consists of many grids (or arrays), each containing information on a particular hydrologic property or properties. For example, a model must include an array of values on hydraulic conductivity or transmissivity, with one value corresponding to every constant head or active cell in the grid. We refer to this array of values as an overlay. The Overlays menu helps you access the arrays. Only relevant overlays as determined with the Basic Information or the Layer Qualities windows are enabled in the Overlays menu. For example, the commands for Primary Storage Factor and Secondary Storage Factor are only enabled in transient models, and the Transmissivity command is only enabled under layer options where saturated thickness does not change (LAYCON 0 and 2) and if you did not check homogeneous conditions for this property in the Layer Qualities window. The following commands may be visible under the Overlays menu: Overlays Initial Head Row Transmissivity Column Transmissivity (not implemented) Row Conductivity Column Conductivity (not implemented) Vertical Leakance Vertical Conductivity Top Elevation Bottom Elevation Primary Storage Factor Secondary Storage Factor Wet/Dry Porosity Confining Bed Conductivity Confining Bed Porosity Evapotranspiration Recharge Drain General Head Boundary River Well Targets Particle Distribution Ring Cell Faces Cell Volume Each Overlays command also displays a variety of the Overlays window for entering or changing values in selected cells or the status of a cell (see the section on Graphic Groundwater windows later in this chapter for a general discussion of overlays).