MODFLOW Case Study - Agricultural Drain Model, Part 2

In this exercise, we will revisit the agricultural drain model we built for our previous in-class task.

Click here to download a completed version of the previous model. Unzip the model and load it into GMS. Then do the following:

1) Add a well in the interior.

a) Use Q = -2000 ft^3/day.

b) Use Q = -5000 ft^3/day

2) Analyze the flow budget

a) Select the cells on each side and change the zone budget id's. Use 2 on the left and 3 on the right.

b) Look at the flow budget. What percentage of the water goes to the left drain vs. the right drain?

c) Remove the well and re-run the model to get the original solution. Look at the flow budget again.

3) Determine impact of using a 2D model

a) Set the head contours to a fixed interval (1.0) and save a copy of the contours to a CAD layer.

b) Rebuild the model using a multi-layer grid (six layers). Assign head bc to top layer only and re-enter the inputs in the same order used in the first exercise. Let Kv=Kh for now.

c) Compare the contours. Is there a significant difference?

d) Has the flow budget changed?

4) Determine impact of vertical anisotropy

a) Save another copy of head contours.

b) Change Kv=Kh/5=0.8.

c) Save and run the model. Compare the solution.

5) Determine impact of using drains vs. constant head bc for ag drains.

a) Save another copy of head contours. Note the flow budget.

b) Remove fixed head BC and add drains to left and right side. Use a large value for conductance (1e6).

c) Save and run. Compare results.

Note: You may need to reduce the acceleration (relaxation) parameter to get the PCG solver to converge. Try reducing from 1.0 to 0.2 in the PCG Package dialog.

d) Compute a drain conductance value assuming K=0.5, L=50, width=6, thickness=3. Enter CD, save and run again. Compare results.