What is it?
Users can select/input a number of model grid options. This includes: the number of model nodes in the horizontal (west-east) direction (NX), the number of sub-layers in the vertical direction (i.e., into the earth), and matrix solver settings.How to use it:
Horizontal Grid
The default grid size is 35, but this may be increased.The number of nodes in the north-south direction is automatically computed based on NX and the shape of the model domain.
Click the 'Estimated NY,DX,DY' button to auto-calculate and display NY [number of cells in Y-directoin), DX [cell size in X-direction], and DY [cell size in Y-direction].
The maximum number of grid cells in a model in the free version of IGW-NET is 1225.
Suggestion: Begin modeling with a 2D, coarse "big picture" model, using the default grid (NX=40). Then refine the grid or utilize a submodeling approach (See the 'Nested Flow Modeling' and 'Model Hierarchy' Quick Tutorials).
In fact, a vast majority of problems encountered in hydrogeology can be solved with smaller grids, especially when taking advantage of IGW-NET's hierarchical modeling capabilities.
Vertical Layers
IGW-NET allows for vertical discretization of geological (conceptual) layers into desired number of computational layers. (NOTE: For the free version of IGW-NET, the total number of vertical numerical layers is limited to 1).Subdividing can be done based on 1) the land surface as the top boundary surface (this is the default setting); or 2) with a water table solved by a previously simulation within the study area (check the box next to ‘SubLayers=’ to use this setting). In the latter case, the previous water table used for sub-dividing must cover the entire model domain, but need not be the same size. However, what is most common is to:
- first simulate the model domain without sub-dividing the aquifer in the vertical direction;
- then check the box next to ‘Water Table as Top’ in the Default Parameters and Options menu, save the changes (click ‘OK’), and re-run the simulation.
The latest water table can also be saved by clicking the ‘SaveModel’ button along the left side of the IGW-NET Modeling Environment, then selecting ‘Latest Water Table’ from the submenu. To load this water table, check the box next to ‘Import’, then click Import to browse to and select the saved water table file.
Minimum Thickness
It may be necessary to enforce a minimum aquifer thickness to ensure numerical convergence (e.g., if an aquifer layer "pinches out").Use the text input box next to 'Min. Aquifer Thickness' to assign a minimum thickness of the aquifer (conceptual) layer (based on percentage of maximum aquifer thickness). Any cells with a thickness below the threshold will have the thickness modified to the threshold value. (Users can check the box next to 'Treat these cells as inactive cells' to remove them from the model.)
If sub-dividing is being applied, use the text input box next to 'Min Sub Lyr Thickness' to assign a minimum sub-layer thickness based on a specified percentage of the maximum aquifer thickness.