|Groundwater modelling with MicroFEM • Lesson 10A: Water divide and Capture zone|
When we created the grid in Lesson 8, we added a Fixed node at the location of a well at (2620, 3025). The well discharges 120 m3/hour somewhere in the centre of our unconfined triangular island.
Click "Lesson 8.fpr" to load the model we saved at the end of Lesson 9
Start the Project manager. Note that the model consists of 7 files, including the batch-file
Close the Project Manager, save the model as "Lesson 10"
Drawing mode / Label 1 / [F7] / [OK]
The small square in the centre is drawn around the node with Label "Fixed node 7".
Zoom in and click on this node / Input mode / enter the text "well" / Enter / Q1 / 120*24 / Enter
The batch-file editor shows the following text
Check the box "Phreatic top aquifer" or (and) change the Run command: RUN phreat=1.
Run the model ([Calculate] / [Stop] / [OK]).
Check the water balance.
Note that 88% of all precipitation (21,120 m3/d) is boundary outflow. Since the well is now labelled, we can specify any well discharge in the batch-file:
The discharge of 120*24 m3/d is assigned to all nodes with label "well", which is only one node in this case.
Run the model.
There is a well-defined capture zone where all infiltrated precipitation flows toward the well. All other precipitation in this model flows to the model boundary. To find this capture zone we can draw the contours of the water table and see the shape of the drawdown cone.
Right-click / Fit to scale / Drawing mode / H1 / [F7] / Choose a small interval = 0.1 / [OK]
Drawdown at the well is found to be 14.186 m. W, NNE, E and SSE of the well are locations where the contour lines are relatively far apart, indicating areas of low head gradient. They are on the water divide at the boundary of the drawdown cone. The four points give an impression of the shape of the drawdown cone. A more accurate view can be obtained with 3D flowlines.
Walking mode / [F12] / Next label / select "well" / [Jump] / [Close]
Zoom somewhat in on the well (+ + +).
Drawing mode / [F9] / Backward / n lines = 20 / [OK]
When you cannot specify the number of lines, you are not in the well node. The yellow flowlines do not reach up to the groundwater divide.
[F9] / Backward / n lines = 20 / levels = 1 / [OK]
This second set of 20 flowlines starts at the base of the well screen and flowlines are computed backwards until an area with no gradient is reached, the two highest points of the water divide. The shape of the capture area becomes clear.
When you have doubts about the open parts in the NNE and SSE direction, increase the number of lines (e.g. 200) and try again.
When you draw a section ([F10] / [F12]) you will only see the first set of 20 flowlines starting at a height of about 12 m. When levels was set at 1, flowlines start at the base of the fully penetrating well, which in this case is also the impervious base of the aquifer. Since the vertical flow component is zero here, these flowlines will remain at the base. To display these flowlines set the Minimum of the vertical scale lower than 10 m (click in the Section / Scale / Min = 9 / [OK]). For a better view of the capture zone in the Section, try levels = 0.999.
In the section right through the drawdown cone, several flowlines starting at Level = 0.5 appear above the water table. Can you explain that?
We can also approach the water divide around the capture zone from the outside. First label all boundary nodes.
Walking mode / [Del] / [F7]
Input mode / Label 1 / "boundary" / [F3]
Drawing mode / [Del] / [F2] / H1 / Click a boundary node / [F9] / Levels = 0.999) / [OK]
Draw a 500 m map grid [F6] to estimate the capture zone area.
Make your own water balance of the capture zone and find that its surface area must be 2.88 km2 (24*120 m3/d / 0.001 m/d).