Groundwater modelling with MicroFEM • Lesson 2B: Finite element head computation 
Start MicroFEM and load the "Lesson 1" model by clicking the project file "Lesson 1.fpr" To make sure that all heads are computed, we will run the model first.
Menu bar: Calculate / Go calculate
Step 10: Water balance
[Walking mode] / [Draw grid (Ins)] Make the centre node the current node. [Waterbalance node (F2)] An area around the current node is shaded. This area intersects the nodal connections of the current node exactly in the middle. This area is also exactly one third of each neighbouring element of the current node. It is called the "Nodal area".
Also a popup window with a water balance is displayed. The size of the shaded area appears to be 21650650 m^{2}. The outflow (by the well) is 1000 m^{3}/d and the lateral inflow is the same. The well outflow was given (we entered a well discharge of 1000 m^{3}/d). The lateral inflow is computed, based on the location of the nodes, the transmissivity and the nodal heads.
[Water balance node (F2)] or Close the Water Balance popup window.
Step 11: Head computation based on water balance of nodal area
Table: Make the "head [m]" cell (4th cell from the top) the active cell. From now on we will use codes for all cells in the table. From top to bottom the codes of these cells are: H0, C1, T1, H1, Q1. The "C" and "Q" are the often used notations for "vertical resistance (days)" and "well discharge (m^{3}/day)" respectively. The "1" is used here because these codes refer to the uppermost aquifer (aquifers are numbered topdown).
Toolbar [Drawing mode] / [Blue] / [Draw grid] / [Yellow] / [Draw contours (F7)] / Maket Interval = 0.072 / [OK] In this way we created a yellow sixsided polygon around the centre node. This polygon connects the middles of the nodal connections.
Make the centre node the current node. [Walking mode] / [Water balance node (F2)]
The lateral inflow into the shaded area is the same as the flow over the yellow polygon. The net lateral inflow (inflow minus outflow) of the shaded areas outside the yellow polygon is zero for each element (because these are closed areas within an area with uniform flow). The length of each side of the polygon is 2500 m. The gradient is perpendicular to these sides (in this model). Using Darcy the inflow over each side = Length of the side * Transmissivity * Gradient. Total inflow of the nodal area = 6 * 2500 * 2000 * delta_h/(2500*√3). Total outflow = Well discharge = 1000 m^{3}/d. With a higher head (less negative) the gradient in all elements would be lower and the lateral inflow would be less that the well discharge. Similarly, a more negative head and associated higher gradient would produce more lateral inflow than the well discharges. There is only one head for the centre node that makes the total inflow equal to the outflow. Please note: We will continue with this in the next lesson.
Menu bar: Files / Save all This last command is routine only. Actually we did not have to save the model, because we did not change the model data.
