Groundwater modelling with MicroFEM • Lesson 3B: Labels and Water balances


Start MicroFEM and load the "Lesson 3" model by clicking the project file "Lesson 3.fpr"

Of all 817 nodes, there is only one node that represents the well. Since the well is exactly in the centre of the model, the current node at the start is also the well node. We recognize the well node by its well discharge (Q1 in the Table) of 1000 m3/d. It is often useful to assign names to nodes. We will label this well node as "well". Make sure that the current node (the blinking node in Walking mode) is the well node.



Step 14: Assigning labels


Input mode / Make "Label 1" the active cell.

The "Label 1" cell can be found at the lower right hand side, just above the four Mode buttons. The text in this cell is "fixed node 4" which was inherited from the grid generation procedure and not important at the moment.


Open the cell / type "well" / press this time the Enter-key on the keyboard.

Note that a click on a cell makes that cell the "active cell". Clicking on an active (=selected) cell in Input mode (the cell is blue) opens that cell for input. When you prefer the keyboard, use Tab key and the Up and Down arrow keys to select a cell, and use the Enter key to open a cell. By pressing the Enter-key the text "well" is assigned to the current node only.



Step 15: Water balance


A water balance is a simple, yet powerful way to look at (and check) model results.


Walking mode / [Water balance all nodes (F5)]

The model on the Map is shaded. The pop-up window shows the model water balance in three columns: "Inflow", "outflow" and "In-Out" (Inflow minus Outflow). The Inflow is Boundary flow: this should be a value close to 1000 m3/d, but slightly different, for example 999.58 m3/d. The outflow is Well flow: 1000 m3/d. For a steady state model the Inflow must be equal to the Outflow: any difference is an error. Small errors (< 1 per mille of the Inflow) are usually acceptable.

The reason for this small water balance error is that the heads of the nodes have small errors. The present nodal heads are obtained from the file when we loaded the "Lesson 3" model. Heads were saved in the fem-file with a limited accuracy. When we want to improve the accuracy, we have to run the model (again).


Menu bar: Calculate / Options ... / [Calculate] / [Stop]

In the black window "Calculation of Heads" the iterative calculation process is displayed. Within a few milliseconds 20 iterations are performed. When we close the calculation windows and check the water balance, it is clear that the error is reduced, but still larger than zero.




Walking mode / [Water balance all nodes (F5)]

Apparently we need to do more iterations to obtain a more accurate result.


Menu bar: Calculate / Options ... / Minimum number of iteration = 200 (just add a zero) / [Calculate] / [Stop] / [OK]

Walking mode / [Water balance all nodes (F5)]

The water balance error is now given as 0.00 m3/d. Since all values in the water balance table are rounded off, this implies that the error is now less than 0.005 m3/d.


Save the model.


Menu bar: Files / Save as ... / "Lesson 3" / [Save]



Step 16: On-line Help system


Menu bar: Help / Help F1 / Contents tab

Many years ago, when MicroFEM was still DOS software (before 1998) we used to supply MicroFEM with a 190-page Manual. All this information (and more) has been reworked for the present MicroFEM on-line Help system. It works like a book with Chapters and Sections. Also, on many pop-up windows you will find a [Help] button. For example:


Menu bar: Project / Information ...




Menu bar: Calculate / Options ...


When you click such [Help] buttons, a Help screen is displayed related to the information on that window, the so called "Context-sensitive help".


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