|Groundwater modelling with MicroFEM • Lesson 8B: Water table aquifer|
When the spacing of the segments is chosen as 150 and 90, while keeping the region spacings at 60 and 120, there will be 2203 nodes in the grid. The lake level is at 20 above reference level. The horizontal impervious base of the unconfined aquifer is 10 m below the lake level. We assume a hydraulic conductivity of 25 m/d. Average recharge by net precipitation is 0.001 m/d. The well is not yet producing.
[Save data file (F9)] / Save FEN-file / [Create aquifers (F10)] / accept 1 / [OK]
We don't know yet the saturated thickness of the aquifer. When the water table would be at lake level, thickness is 10 m and transmissivity is 250 m2/d.
Input mode / T1 / 250 / [F5]
There is no aquitard at the top of the aquifer, so there is no "head-dependent recharge" at the top: we keep the vertical resistance C1 at 0 (infinite resistance: see Lesson 5A, step 4). The effective precipitation (precipitation minus evaporation) causes a constant recharge of 0.001 m/d at the top of the model.
Menu bar: Project / Project manager ... / add unit to project [green +] / Select Precipitation / New / [OK] / Close project manager
A new tab is added to the Table "Top". Click on it. The parameter code for Precipitation is PPN.
Input mode / PPN / 0.001 / [F5]
Menu bar: Files / Save all
Run the model.
Check the model water balance.
When the water balance shows an error, adjust the stopping criterion and rerun the model (Lesson 4B).
Drawing mode / H1 / [F7] / Write the highest head on a peace of paper / [OK]
The displayed head shows the level of the groundwater table. The highest point is computed at 25.864 m, almost 6 m above lake level. That implies that the saturated thickness is almost 16 m in the centre part of the island. The transmissivity must be much larger than the assumed 250 m2/d. The saturated thickness can be computed as: H1–10, and the transmissivity as: (H1–10)*25.
Input mode / T1 / (H1-10)*25 / [F5]
Run the model.
Draw the contours of the water table.
The increased transmissivity makes that the highest water table level is now somewhat lower: 24.614 m. When we repeat the above three steps (enter a new T1, run and draw contours) we find 24.752 m. Repeated calculations show: 24.740 m, 24.741 m and again 24,741 m.
Our iterative procedure apparently requires 5 or 6 steps to find a solution.
In the lesson 9B we will learn a more automatic way to account for head-dependent transmissivities.
We will now add the thickness unit (as in Lesson 6B).
Menu bar: Project / Project manager ... / add unit to project [green +] / New / [OK] / Close project manager
The "top level" is the water table height above reference level. This is different for all nodes and computed by our model as H1. The aquitard thickness is zero (not present). The aquifer thickness is also different for all nodes, dependent on H1, and equal to (H1–10).
Table: Thick tab / top level / H1 / [F5] / aquifer / H1–10 / [F5]
Show the water table in a West-East profile.
Drawing mode / [F10] / [F11]
The vertical scale ranges between 0 and 30, while the water table is at and above 20 m. This is because not only H1 is displayed in the profile, but also H0, while all H0 values are still zero. The H0 parameter plays no role in this model, since we have unconfined conditions (no top aquitard).
Left click in the Profile / Uncheck the H0 box / [OK]
Now show the saturated aquifer in a West-East profile.
Drawing mode / [F12]
The vertical scale ranges from 10 to 30 m. When we also want to see the impervious base.
Left click in the Section / Scale / Uncheck Vertical automatic scale / Min = 8 / [OK]
Save your model.
We will use the same model in Lesson 9 to draw 3D flowlines and introduce the batch-file editor. In Lesson 10 we will continue with the Batch-file editor and start with transient modelling.