MODFLOW Case Study - Solver Problems with a Thin Aquifer

This exercise involves a variation of the Fremont River model. The following changes have been made to the original model:

• The hydraulic conductivity has been increased to 1.5 m/d.
• The two wells have been turned off
• Rather than using a flat bottom, a new set of bottom elevations has been interpolated from a set of scatter points. This results in a surface that slopes up to the west side.

1) Download and unzip the thinaq.zip file.

2) Save and run the model. Read the solution.

a) What do the red symbols mean?

b) Why did this happen?

3) Try fixing the problem by adjusting the starting heads to a larger value. Turn on the Starting heads equal grid top elevations option in the Global/Basic package dialog. Or just change the starting heads to 400.

4) Try fixing the problem by adjusting the acceleration/damping* parameter and/or trying a different solver.

5) Try changing the starting head value to be just above the bottom elevation. You will to turn off the Starting heads equal grid top elevations option before you can edit the starting heads again.

6) Change the starting heads to a constant value such as 400. Switch to MODFLOW-NWT and turn on the Newton solver.

*For the PCG2 solver, the acceleration parameter is called the damping parameter.